Mobile communication system, master base station, secondary base station and mobile terminal

Abstract

To provide technology in which a mobile terminal device during dual connectivity can perform handover.SOLUTION: A communication system comprises: a communication terminal (UE); and at least one base station (S-MeNB, T-MeNB, SeNB) that constructs a plurality of cells for performing wireless communication with the communication terminal. The plurality of cells includes: a first cell (cell based on SeNB); a second cell (cell based on S-MeNB) that provides first dual connectivity to the communication terminal in cooperation with the first cell; and a third cell (cell based on T-MeNB) that provides second dual connectivity to the communication terminal in cooperation with the first cell. The communication terminal maintains connection to the first cell (cell based on SeNB) while handover from the second cell (cell based on S-MeNB) to the third cell (cell based on T-MeNB) is performed to transition from the first dual connectivity to the second dual connectivity.SELECTED DRAWING: Figure 17

Description

[Technical Field]

[0001] The present disclosure relates to wireless communication technology. [Background technology]

[0002] 3GPP (3rd Generation Partnership Project), a standardization organization for mobile communication systems, is considering a new communication method called Long Term Evolution (LTE) for the wireless section and System Architecture Evolution (SAE) for the overall system configuration including the core network and radio access network (hereinafter collectively referred to as the network) (see, for example, Non-Patent Documents 1 to 10). This communication method is also called the 3.9G (3.9 Generation) system.

[0003] LTE uses OFDM (Orthogonal Frequency Division Multiplexing) for downlink and SC-FDMA (Single Carrier Frequency Division Multiple Access) for uplink as its access method. Unlike W-CDMA (Wideband Code Division Multiple Access), LTE does not include circuit switching and is only a packet communication method.

[0004] The decisions made by 3GPP regarding the frame structure in the LTE system, as described in Non-Patent Document 1 (Chapter 5), will be explained using Figure 1. Figure 1 is an explanatory diagram showing the structure of a radio frame used in an LTE communication system. In Figure 1, one radio frame is 10 ms. The radio frame is divided into 10 equally sized subframes. Each subframe is divided into two equally sized slots. The first and sixth subframes of each radio frame include a downlink synchronization signal (SS). The synchronization signals include a primary synchronization signal (P-SS) and a secondary synchronization signal (S-SS).

[0005] The decisions made by 3GPP regarding the channel configuration in the LTE system are described in Non-Patent Document 1 (Chapter 5). It is assumed that the same channel configuration as that of a non-CSG cell is used in a CSG (Closed Subscriber Group) cell.

[0006] The Physical Broadcast Channel (PBCH) is a channel for downlink transmission from a base station to a mobile terminal. A BCH transport block is mapped to four subframes within a 40 ms interval. There is no explicit signaling of the 40 ms timing.

[0007] The Physical Control Format Indicator Channel (PCFICH) is a channel for downlink transmission from a base station to a mobile terminal. The PCFICH notifies the mobile terminal of the number of Orthogonal Frequency Division Multiplexing (OFDM) symbols used for PDCCHs. The PCFICH is transmitted every subframe.

[0008] The Physical Downlink Control Channel (PDCCH) is a channel for downlink transmission from a base station to a mobile terminal. The PDCCH reports resource allocation information for a Downlink Shared Channel (DL-SCH), which is one of the transport channels described below, resource allocation information for a Paging Channel (PCH), which is also one of the transport channels described below, and Hybrid Automatic Repeat reQuest (HARQ) information for the DL-SCH. The PDCCH carries an uplink scheduling grant. The PDCCH carries Acknowledgement (Ack) / Negative Acknowledgement (Nack), which are response signals to uplink transmissions. The PDCCH is also called an L1 / L2 control signal.

[0009] The Physical Downlink Shared Channel (PDSCH) is a channel for downlink transmission from a base station to a mobile terminal. A Downlink Shared Channel (DL-SCH), which is a transport channel, and a PCH, which is also a transport channel, are mapped to the PDSCH.

[0010] A physical multicast channel (PMCH) is a channel for downlink transmission from a base station to a mobile terminal, and a multicast channel (MCH), which is a transport channel, is mapped to the PMCH.

[0011] The Physical Uplink Control Channel (PUCCH) is a channel for uplink transmission from a mobile terminal to a base station. The PUCCH carries Ack / Nack, which are response signals to downlink transmissions. The PUCCH also carries CQI (Channel Quality Indicator) reports. CQI is quality information that indicates the quality of received data or the quality of the communication path. The PUCCH also carries Scheduling Requests (SR).

[0012] The Physical Uplink Shared Channel (PUSCH) is a channel for uplink transmission from a mobile terminal to a base station. The Uplink Shared Channel (UL-SCH), which is one of the transport channels, is mapped to the PUSCH.

[0013] The Physical Hybrid ARQ Indicator Channel (PHICH) is a channel for downlink transmission from a base station to a mobile terminal. The PHICH carries Ack / Nack, which are response signals to uplink transmissions. The Physical Random Access Channel (PRACH) is a channel for uplink transmission from a mobile terminal to a base station. The PRACH carries a random access preamble.

[0014] Downlink reference signals (RS) are symbols known in LTE communication systems. The following five types of downlink reference signals are defined: Cell-specific Reference Signals (CRSs), MBSFN reference signals, UE-specific reference signals such as Demodulation Reference Signals (DM-RSs), Positioning Reference Signals (PRSs), and Channel-State Information Reference Signals (CSI-RSs). Reference signal received power (RSRP) measurements are available as a physical layer measurement for mobile terminals.

[0015] The transport channels described in Non-Patent Document 1 (Chapter 5) will be explained below. Among the downlink transport channels, a broadcast channel (BCH) is broadcast to the entire coverage of the base station (cell). The BCH is mapped to a physical broadcast channel (PBCH).

[0016] The Downlink Shared Channel (DL-SCH) is subject to retransmission control using Hybrid ARQ (HARQ). DL-SCH can be broadcast to the entire coverage of a base station (cell). DL-SCH supports dynamic or semi-static resource allocation. Semi-static resource allocation is also called persistent scheduling. DL-SCH supports discontinuous reception (DRX) for mobile terminals to reduce power consumption. DL-SCH is mapped to the Physical Downlink Shared Channel (PDSCH).

[0017] The Paging Channel (PCH) supports DRX for mobile terminals to enable low power consumption. The PCH is required to broadcast to the entire coverage of the base station (cell). The PCH is mapped to physical resources such as the Physical Downlink Shared Channel (PDSCH) that can be dynamically used for traffic.

[0018] The Multicast Channel (MCH) is used for broadcasting to the entire coverage of a base station (cell). The MCH supports SFN combining of MBMS (Multimedia Broadcast Multicast Service) services (MTCH and MCCH) in multi-cell transmission. The MCH supports semi-static resource allocation. The MCH is mapped to the PMCH.

[0019] Among the uplink transport channels, the Uplink Shared Channel (UL-SCH) is subject to retransmission control using HARQ (Hybrid ARQ). The UL-SCH supports dynamic or semi-static resource allocation. The UL-SCH is mapped to the Physical Uplink Shared Channel (PUSCH).

[0020] The Random Access Channel (RACH) is limited to control information. The RACH is subject to collision risk. The RACH is mapped to the Physical Random Access Channel (PRACH).

[0021] We will explain HARQ. HARQ is a technology that improves the communication quality of a transmission channel by combining Automatic Repeat reQuest (ARQ) and Forward Error Correction. HARQ has the advantage that error correction works effectively through retransmission even on transmission channels where communication quality varies. In particular, by combining the reception results of the initial transmission and the retransmission when retransmitting, it is possible to achieve further quality improvement.

[0022] An example of a retransmission method will be explained below. If the receiving side is unable to decode the received data correctly, in other words, if a CRC (Cyclic Redundancy Check) error occurs (CRC=NG), the receiving side will send a "Nack" to the sending side. The sending side, having received the "Nack," will retransmit the data. If the receiving side is able to decode the received data correctly, in other words, if no CRC error occurs (CRC=OK), the receiving side will send an "Ack" to the sending side. The sending side, having received the "Ack," will send the next data.

[0023] The logical channels described in Non-Patent Document 1 (Chapter 6) will be explained below. The Broadcast Control Channel (BCCH) is a downlink channel for broadcast system control information. The BCCH, which is a logical channel, is mapped to the broadcast channel (BCH) or the downlink shared channel (DL-SCH), which are transport channels.

[0024] The Paging Control Channel (PCCH) is a downlink channel for transmitting paging information and system information changes. The PCCH is used when the cell location of the mobile terminal is unknown to the network. The PCCH, which is a logical channel, is mapped to the Paging Channel (PCH), which is a transport channel.

[0025] The Common Control Channel (CCCH) is a channel for transmitting control information between a mobile terminal and a base station. The CCCH is used when the mobile terminal does not have an RRC connection with the network. In the downlink direction, the CCCH is mapped to the Downlink Shared Channel (DL-SCH), which is a transport channel. In the uplink direction, the CCCH is mapped to the Uplink Shared Channel (UL-SCH), which is a transport channel.

[0026] The Multicast Control Channel (MCCH) is a downlink channel for point-to-multipoint transmission. The MCCH is used to transmit MBMS control information for one or several MTCHs from the network to mobile terminals. The MCCH is used only by mobile terminals receiving MBMS. The MCCH is mapped to the Multicast Channel (MCH), which is a transport channel.

[0027] A Dedicated Control Channel (DCCH) is a channel that transmits dedicated control information between a mobile terminal and a network on a one-to-one basis. The DCCH is used when the mobile terminal is in an RRC connection. The DCCH is mapped to an uplink shared channel (UL-SCH) in the uplink and to a downlink shared channel (DL-SCH) in the downlink.

[0028] A Dedicated Traffic Channel (DTCH) is a point-to-point communication channel for transmitting user information to an individual mobile terminal. DTCH exists in both uplink and downlink. In uplink, DTCH is mapped to an uplink shared channel (UL-SCH) and in downlink, it is mapped to a downlink shared channel (DL-SCH).

[0029] The Multicast Traffic Channel (MTCH) is a downlink channel for transmitting traffic data from the network to mobile terminals. The MTCH is a channel used only by mobile terminals receiving MBMS. The MTCH is mapped to the Multicast Channel (MCH).

[0030] CGI stands for Cell Global Identifier. ECGI stands for E-UTRAN Cell Global Identifier. Closed Subscriber Group (CSG) cells are introduced in LTE, LTE-A (Long Term Evolution Advanced) (described below), and UMTS (Universal Mobile Telecommunication System).

[0031] A CSG (Closed Subscriber Group) cell is a cell for which an operator has identified available subscribers (hereinafter referred to as a "specific subscriber cell"). The identified subscribers are permitted to access one or more cells in a PLMN (Public Land Mobile Network). The one or more cells to which the identified subscribers are permitted to access are called "CSG cell(s)." However, there are access restrictions within the PLMN.

[0032] A CSG cell is part of a PLMN that broadcasts a unique CSG identity (CSG ID; CSG-ID) and broadcasts a CSG indication of "TRUE." Members of a pre-registered and authorized subscriber group access the CSG cell using the CSG-ID, which is access permission information.

[0033] The CSG-ID is broadcast by a CSG cell or cells. There are multiple CSG-IDs in an LTE communication system. The CSG-ID is used by a mobile terminal (UE) to facilitate access to CSG-related members.

[0034] The location of a mobile terminal is tracked in units of an area consisting of one or more cells. Location tracking is performed to track the location of a mobile terminal even when it is in standby mode and to enable the mobile terminal to be called, in other words, to allow the mobile terminal to receive calls. The area used for tracking the location of a mobile terminal is called a tracking area.

[0035] 3GPP is studying base stations called Home-NodeB (Home-NB; HNB) and Home-eNodeB (Home-eNB; HeNB). HNB in UTRAN and HeNB in E-UTRAN are base stations for access services for homes, businesses, and businesses, for example. Non-Patent Document 3 discloses three different modes of access to HeNB and HNB. Specifically, it discloses an open access mode, a closed access mode, and a hybrid access mode.

[0036] Each mode has the following characteristics: In the open access mode, the HeNB and HNB are operated as normal cells of a regular operator. In the closed access mode, the HeNB and HNB are operated as CSG cells, which are accessible only by CSG members. In the hybrid access mode, the HeNB and HNB are operated as CSG cells, which are simultaneously accessible by non-CSG members. In other words, a cell in the hybrid access mode (also called a hybrid cell) is a cell that supports both the open access mode and the closed access mode.

[0037] In 3GPP, among all physical cell identities (PCIs), there is a PCI range reserved by the network for use in CSG cells (see Non-Patent Document 1, Chapter 10.5.1.1). Dividing a PCI range is sometimes called PCI split. Information about PCI splits (also called PCI split information) is broadcast from a base station to mobile terminals under its control by system information. Being under the control of a base station means that the base station is the serving cell.

[0038] Non-Patent Document 4 discloses the basic operation of a mobile terminal using PCI split. A mobile terminal that does not have PCI split information must use all PCIs, for example, all 504 codes, to perform a cell search. On the other hand, a mobile terminal that has PCI split information can perform a cell search using the PCI split information.

[0039] 3GPP is also working on the development of the Long Term Evolution Advanced (LTE-A) standard as Release 10 (see Non-Patent Documents 5 and 6). LTE-A is based on the LTE wireless communication system, and is configured by adding several new technologies to it.

[0040] In the LTE-A system, carrier aggregation (CA) is being considered, which aggregates two or more component carriers (CCs) (also called "aggregation") to support wider frequency bandwidths (transmission bandwidths) up to 100 MHz.

[0041] When CA is configured, the UE has only one RRC connection with the network (NW). In the RRC connection, one serving cell provides NAS mobility information and security input. This cell is called the primary cell (PCell). In the downlink, the carrier corresponding to the PCell is the downlink primary component carrier (DL PCC). In the uplink, the carrier corresponding to the PCell is the uplink primary component carrier (UL PCC).

[0042] Depending on the UE's capabilities, a secondary cell (SCell) is configured to form a pair of a PCell and a serving cell. In the downlink, the carrier corresponding to the SCell is a downlink secondary component carrier (DL SCC). In the uplink, the carrier corresponding to the SCell is an uplink secondary component carrier (UL SCC).

[0043] For one UE, a set of one PCell and a serving cell consisting of one or more SCells is configured.

[0044] New technologies for LTE-A include wider bandwidth extension and Coordinated Multiple Point transmission and reception (CoMP). CoMP, which is being considered for LTE-A by 3GPP, is described in Non-Patent Document 7.

[0045] Mobile network traffic volume is on the rise, and communication speeds are also increasing. When LTE and LTE-A begin full-scale operation, communication speeds are expected to increase even further, leading to an increase in traffic volume.

[0046] Furthermore, the widespread use of smartphones and tablet devices has led to an explosive increase in traffic volume via cellular wireless communications, raising concerns about a shortage of wireless resources around the world.

[0047] To address the problem of increasing traffic volume, 3GPP is currently formulating Release 12 of the specifications. The Release 12 specifications consider the use of small eNBs to handle the massive traffic volumes that will be expected in the future. For example, technologies are being considered that would increase communication capacity by installing a large number of small eNBs to form a large number of small cells, thereby improving frequency utilization efficiency.

[0048] Among these, dual connectivity is being discussed as a technology that allows a mobile terminal to connect to both a macro cell and a small cell when the macro cell and the small cell overlap (see Non-Patent Document 11). [Prior art documents] [Non-patent literature]

[0049] [Non-Patent Document 1] 3GPP TS36.300 V11.7.0 [Non-patent document 2] 3GPP TS36.304 V11.2.0 [Non-patent document 3] 3GPP S1-083461 [Non-patent document 4] 3GPP R2-082899 [Non-patent document 5] 3GPP TR 36.814 V9.0.0 [Non-patent document 6] 3GPP TR 36.912 V10.0.0 [Non-Patent Document 7] 3GPP TR 36.819 V11.1.0 [Non-patent document 8] 3GPP TS 36.141 V11.1.0 [Non-Patent Document 9] 3GPP R1-134496 [Non-Patent Document 10] 3GPP R1-132236 [Non-Patent Document 11] 3GPP TR36.842 V0.2.0 Summary of the Invention [Problem to be solved by the invention]

[0050] As mentioned above, Non-Patent Document 11 discloses dual connectivity as a technology for connecting a mobile terminal to both a macro cell and a small cell when the macro cell and the small cell overlap.

[0051] However, Non-Patent Document 11 does not disclose anything about how to handle a mobile terminal in dual connectivity when it performs a handover. In conventional handover methods, a mobile terminal is connected to only one cell, and does not take into consideration connections to both a macro cell and a small cell in dual connectivity.

[0052] Therefore, it is impossible to apply the conventional handover method to a mobile terminal in dual connectivity without any ingenuity.

[0053] An object of the present disclosure is to provide a technology that enables a mobile terminal device in dual connectivity to perform a handover. [Means for solving the problem]

[0054] In a mobile communication system according to the present disclosure, a master base station and a secondary base station concurrently communicate wirelessly with a mobile terminal, and measurement results relating to the secondary base station are transmitted from the mobile terminal to the master base station. [Effects of the Invention]

[0055] According to the present disclosure, a mobile terminal device in dual connectivity can perform handover.

[0056] The objects, features, aspects, and advantages of the present disclosure will become more apparent from the following detailed description and the accompanying drawings. [Brief explanation of the drawings]

[0057] [Figure 1] FIG. 1 is an explanatory diagram showing the configuration of a radio frame used in an LTE communication system. [Figure 2] FIG. 7 is a block diagram showing the overall configuration of an LTE communication system 700 being discussed in 3GPP. [Figure 3] FIG. 3 is a block diagram showing the configuration of a mobile terminal 71 shown in FIG. [Figure 4] FIG. 3 is a block diagram showing the configuration of a base station 72 shown in FIG. [Figure 5] FIG. 2 is a block diagram illustrating the configuration of an MME. [Figure 6] 1 is a flowchart showing an outline of operations from cell search to standby operation performed by a mobile terminal (UE) in an LTE communication system. [Figure 7]FIG. 1 is a diagram illustrating the concept of a cell configuration when macro eNBs and small eNBs are mixed. [Figure 8] FIG. 2 is a diagram illustrating an example of coverage of an eNB in the communication system of the first embodiment. [Figure 9] FIG. 2 is a diagram illustrating an example of coverage of an eNB in the communication system of the first embodiment. [Figure 10] FIG. 2 is a diagram showing an example of a sequence of handover-related processing in the communication system of the first embodiment. [Figure 11] FIG. 11 is a diagram showing an example of a sequence of HO pre-processing in step ST908 of FIG. [Figure 12] FIG. 11 is a diagram showing an example of the sequence of the HO process in step ST928 of FIG. [Figure 13] FIG. 11 is a diagram showing an example of a sequence of post-HO processing in step ST949 of FIG. [Figure 14] FIG. 10 is a diagram showing an example of a sequence of handover-related processing in a communication system according to a first modification of the first embodiment. [Figure 15] FIG. 15 is a diagram showing an example of the sequence of HO pre-processing in step ST1009 of FIG. [Figure 16] FIG. 15 is a diagram showing an example of a sequence of post-HO processing in step ST1010 of FIG. [Figure 17] FIG. 10 is a diagram showing an example of a sequence of handover-related processing in a communication system according to a second embodiment. [Figure 18] FIG. 10 is a diagram showing an example of a sequence of handover-related processing in a communication system according to a second embodiment. [Figure 19] FIG. 10 is a diagram showing an example of a sequence of handover-related processing in a communication system according to a second embodiment. [Figure 20] FIG. 11 is a diagram showing an example of a sequence of handover-related processing in a communication system according to a third embodiment. [Figure 21] FIG. 11 is a diagram showing an example of a sequence of handover-related processing in a communication system according to a third embodiment. [Figure 22]FIG. 11 is a diagram showing an example of a sequence of handover-related processing in a communication system according to a third embodiment. [Figure 23] FIG. 11 is a diagram showing an example of a sequence of handover-related processing in a communication system according to a fourth embodiment. [Figure 24] FIG. 11 is a diagram showing an example of a sequence of handover-related processing in a communication system according to a fourth embodiment. [Figure 25] FIG. 11 is a diagram showing an example of a sequence of handover-related processing in a communication system according to a fourth embodiment. [Figure 26] FIG. 13 is a diagram showing an example of a sequence of handover-related processing in a communication system according to a fifth embodiment. [Figure 27] FIG. 13 is a diagram showing an example of a sequence of handover-related processing in a communication system according to a fifth embodiment. [Figure 28] FIG. 13 is a diagram showing an example of a sequence of handover-related processing in a communication system according to a fifth embodiment. [Figure 29] FIG. 29 is a diagram showing an example of the sequence of MeNB HO processing for EPS bearer #1 in step ST2025 of FIG. 28. [Figure 30] FIG. 29 is a diagram showing an example of the sequence of MeNB HO processing for EPS bearer #1 in step ST2025 of FIG. 28. [Figure 31] FIG. 10 is a diagram illustrating an example of the state of data transmission and reception between a UE. [Figure 32] FIG. 20 is a diagram showing an example of a sequence of MeNB HO processing for EPS bearer #1 in a communication system of embodiment 6. [Figure 33] FIG. 20 is a diagram showing an example of a sequence of MeNB HO processing for EPS bearer #1 in a communication system of embodiment 6. [Figure 34] FIG. 20 is a diagram showing an example of a sequence of MeNB HO processing for EPS bearer #1 in a communication system of embodiment 7. [Figure 35] FIG. 20 is a diagram showing an example of a sequence of MeNB HO processing for EPS bearer #1 in a communication system of embodiment 7. DETAILED DESCRIPTION OF THE INVENTION

[0058] Embodiment 1 Fig. 2 is a block diagram showing the overall configuration of an LTE communication system 700 being discussed in 3GPP. Referring to Fig. 2, the radio access network is called E-UTRAN (Evolved Universal Terrestrial Radio Access Network) 70. A mobile terminal device (hereinafter referred to as "mobile terminal (User Equipment: UE)") 71, which is a communication terminal device, is capable of wireless communication with a base station device (hereinafter referred to as "base station (E-UTRAN NodeB: eNB)") 72, and transmits and receives signals via wireless communication.

[0059] If the control protocols for the mobile terminal 71, such as RRC (Radio Resource Control), and the user plane, such as PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control), MAC (Medium Access Control), and PHY (Physical layer), terminate at the base station 72, the E-UTRAN is composed of one or more base stations 72.

[0060] The control protocol RRC (Radio Resource Control) between the mobile terminal 71 and the base station 72 performs broadcasting, paging, RRC connection management, etc. The states of the base station 72 and the mobile terminal 71 in RRC include RRC_IDLE and RRC_CONNECTED.

[0061] In RRC_IDLE, PLMN (Public Land Mobile Network) selection, system information (SI) broadcast, paging, cell reselection, mobility, etc. are performed. In RRC_CONNECTED, the mobile terminal has an RRC connection and can transmit and receive data with the network. In addition, in RRC_CONNECTED, handover (HO), measurement of neighbor cells, etc. are performed.

[0062] The base stations 72 are classified into eNBs 76 and Home-eNBs 75. The communication system 700 includes an eNB group 72-1 including a plurality of eNBs 76, and a Home-eNB group 72-2 including a plurality of Home-eNBs 75. A system configured from an EPC (Evolved Packet Core) core network and an E-UTRAN 70 radio access network is referred to as an EPS (Evolved Packet System). The EPC core network and the E-UTRAN 70 radio access network may be collectively referred to as a "network."

[0063] The eNB 76 is connected to a Mobility Management Entity (MME), or a Serving Gateway (S-GW), or an MME / S-GW unit (hereinafter sometimes referred to as an "MME unit") 73 including an MME and an S-GW via an S1 interface, and control information is communicated between the eNB 76 and the MME unit 73. Multiple MME units 73 may be connected to one eNB 76. The eNBs 76 are connected to each other via an X2 interface, and control information is communicated between the eNBs 76.

[0064] The Home-eNB 75 is connected to the MME unit 73 via an S1 interface, and control information is communicated between the Home-eNB 75 and the MME unit 73. A plurality of Home-eNBs 75 are connected to one MME unit 73. Alternatively, the Home-eNB 75 is connected to the MME unit 73 via a HeNBGW (Home-eNB GateWay) 74. The Home-eNB 75 and the HeNBGW 74 are connected via an S1 interface, and the HeNBGW 74 and the MME unit 73 are connected via the S1 interface.

[0065] One or more Home-eNBs 75 are connected to one HeNBGW 74, and information is communicated through the S1 interface. The HeNBGW 74 is connected to one or more MME units 73, and information is communicated through the S1 interface.

[0066] The MME unit 73 and HeNBGW 74 are upper devices, specifically upper nodes, and control connections between the eNB 76 and Home-eNB 75, which are base stations, and the mobile terminal (UE) 71. The MME unit 73 constitutes the EPC, which is a core network. The base station 72 and HeNBGW 74 constitute the E-UTRAN 70.

[0067] Furthermore, 3GPP is considering the following configuration: The X2 interface between Home-eNBs 75 is supported. That is, Home-eNBs 75 are connected via the X2 interface, and control information is communicated between the Home-eNBs 75. From the MME unit 73, HeNBGW 74 appears as the Home-eNB 75. From the Home-eNB 75, HeNBGW 74 appears as the MME unit 73.

[0068] In either case where the Home-eNB 75 is connected to the MME unit 73 via the HeNBGW 74 or where the Home-eNB 75 is connected directly to the MME unit 73, the interface between the Home-eNB 75 and the MME unit 73 is the same, the S1 interface.

[0069] The base station device 72 may configure one cell or multiple cells. Each cell has a predetermined range as its coverage, which is a range within which communication with the mobile terminal 71 is possible, and performs wireless communication with the mobile terminal 71 within the coverage. When one base station device 72 configures multiple cells, each cell is configured to be able to communicate with the mobile terminal 71.

[0070] FIG. 3 is a block diagram showing the configuration of the mobile terminal 71 shown in FIG. 2. The transmission process of the mobile terminal 71 shown in FIG. 3 will be described. First, control data from the protocol processing unit 801 and user data from the application unit 802 are stored in the transmission data buffer unit 803. The data stored in the transmission data buffer unit 803 is passed to the encoder unit 804, where it is subjected to encoding processes such as error correction. Some data may be output directly from the transmission data buffer unit 803 to the modulator unit 805 without being encoded. The data encoded by the encoder unit 804 is modulated by the modulator unit 805. The modulated data is converted into a baseband signal, and then output to the frequency converter unit 806, where it is converted into a radio transmission frequency. The transmission signal is then transmitted from the antenna 807 to the base station 72.

[0071] Furthermore, the receiving process of the mobile terminal 71 is performed as follows. A radio signal from the base station 72 is received by the antenna 807. The received signal is converted from a radio receiving frequency to a baseband signal by the frequency conversion unit 806, and demodulated by the demodulation unit 808. The demodulated data is passed to the decoder unit 809, where decoding processes such as error correction are performed. Of the decoded data, control data is passed to the protocol processing unit 801, and user data is passed to the application unit 802. A series of processes of the mobile terminal 71 is controlled by the control unit 810. Therefore, although the control unit 810 is omitted in FIG. 3, it is connected to each of the units 801 to 809.

[0072] Figure 4 is a block diagram showing the configuration of the base station 72 shown in Figure 2. The transmission process of the base station 72 shown in Figure 4 will be described. The EPC communication unit 901 transmits and receives data between the base station 72 and the EPC (MME unit 73, etc.), HeNBGW 74, etc. The other base station communication unit 902 transmits and receives data with other base stations. The EPC communication unit 901 and the other base station communication unit 902 each exchange information with a protocol processing unit 903. Control data from the protocol processing unit 903, and user data and control data from the EPC communication unit 901 and the other base station communication unit 902 are stored in a transmission data buffer unit 904.

[0073] The data stored in the transmission data buffer unit 904 is passed to an encoder unit 905, where it undergoes encoding processes such as error correction. Some data may be output directly from the transmission data buffer unit 904 to a modulator unit 906 without undergoing encoding processes. The encoded data is modulated by the modulator unit 906. The modulated data is converted into a baseband signal, and then output to a frequency converter unit 907, where it is converted into a radio transmission frequency. The transmission signal is then transmitted from an antenna 908 to one or more mobile terminals 71.

[0074] The reception process of the base station 72 is performed as follows: A radio signal from one or more mobile terminals 71 is received by an antenna 908. The received signal is converted from a radio reception frequency to a baseband signal by a frequency converter 907, and demodulated by a demodulator 909. The demodulated data is passed to a decoder 910, where decoding processes such as error correction are performed. Of the decoded data, control data is passed to the protocol processor 903 or the EPC communication unit 901 or other base station communication unit 902, and user data is passed to the EPC communication unit 901 and other base station communication unit 902. A series of processes of the base station 72 is controlled by a controller 911. Therefore, although the controller 911 is omitted in FIG. 4, it is connected to each of the units 901 to 910.

[0075] FIG. 5 is a block diagram showing the configuration of an MME. FIG. 5 shows the configuration of an MME 73a included in the MME unit 73 shown in FIG. 2 described above. A PDN GW communication unit 1001 transmits and receives data between the MME 73a and a PDN GW. A base station communication unit 1002 transmits and receives data via the S1 interface between the MME 73a and a base station 72. If the data received from the PDN GW is user data, the user data is passed from the PDN GW communication unit 1001 to the base station communication unit 1002 via a user plane communication unit 1003, and transmitted to one or more base stations 72. If the data received from the base station 72 is user data, the user data is passed from the base station communication unit 1002 to the PDN GW communication unit 1001 via the user plane communication unit 1003, and transmitted to the PDN GW.

[0076] If the data received from the PDN GW is control data, the control data is passed from the PDN GW communication unit 1001 to the control plane control unit 1005. If the data received from the base station 72 is control data, the control data is passed from the base station communication unit 1002 to the control plane control unit 1005.

[0077] The HeNBGW communication unit 1004 is provided when a HeNBGW 74 is present, and transmits and receives data via an interface (IF) between the MME 73a and the HeNBGW 74 depending on the information type. Control data received from the HeNBGW communication unit 1004 is passed from the HeNBGW communication unit 1004 to the control plane control unit 1005. The result of processing in the control plane control unit 1005 is transmitted to the PDN GW via the PDN GW communication unit 1001. In addition, the result of processing in the control plane control unit 1005 is transmitted to one or more base stations 72 via the base station communication unit 1002 by the S1 interface, and is also transmitted to one or more HeNBGWs 74 via the HeNBGW communication unit 1004.

[0078] The control plane control unit 1005 includes a NAS security unit 1005-1, an SAE bearer control unit 1005-2, an idle state mobility management unit 1005-3, and the like, and performs overall processing for the control plane. The NAS security unit 1005-1 performs security for NAS (Non-Access Stratum) messages, etc. The SAE bearer control unit 1005-2 performs management of SAE (System Architecture Evolution) bearers, etc. The idle state mobility management unit 1005-3 performs mobility management in the idle state (also called the LTE-IDLE state or simply idle), generation and control of paging signals in the idle state, addition, deletion, update, and search of tracking areas for one or more mobile terminals 71 under its control, and tracking area list management.

[0079] The MME 73a distributes paging signals to one or more base stations 72. The MME 73a also performs mobility control in an idle state. The MME 73a manages a tracking area list when the mobile terminal is in an idle state and an active state. The MME 73a initiates a paging protocol by transmitting a paging message to a cell belonging to a tracking area in which the UE is registered. The idle state mobility management unit 1005-3 may manage the CSG and CSG-ID of the Home-eNB 75 connected to the MME 73a, as well as the whitelist.

[0080] Next, an example of a cell search method in a communication system is shown. Fig. 6 is a flowchart showing an outline of the process from cell search to standby operation performed by a mobile terminal (UE) in an LTE communication system. When the mobile terminal starts a cell search, in step ST1, it synchronizes slot timing and frame timing using a primary synchronization signal (P-SS) and a secondary synchronization signal (S-SS) transmitted from surrounding base stations.

[0081] P-SS and S-SS are collectively called the synchronization signal (SS). The synchronization signal (SS) is assigned a synchronization code that corresponds one-to-one to the PCI assigned to each cell. 504 different PCIs are being considered. These 504 different PCIs are used to achieve synchronization and to detect (identify) the PCI of the synchronized cell.

[0082] Next, in step ST2, for the synchronized cell, a cell-specific reference signal (CRS), which is a reference signal (RS) transmitted from the base station for each cell, is detected and the RS received power (Reference Signal Received Power: RSRP) is measured. The RS uses a code that has a one-to-one correspondence with the PCI. By correlating with this code, it is possible to separate the cell from other cells. By deriving the code for the RS of the cell from the PCI identified in step ST1, it is possible to detect the RS and measure the RS received power.

[0083] Next, in step ST3, the cell with the best RS reception quality, for example, the cell with the highest RS reception power, that is, the best cell, is selected from one or more cells detected up to step ST2.

[0084] Next, in step ST4, the PBCH of the best cell is received to obtain the BCCH, which is broadcast information. A MIB (Master Information Block), which includes cell configuration information, is mapped to the BCCH on the PBCH. Therefore, the MIB can be obtained by receiving the PBCH and obtaining the BCCH. Examples of MIB information include the DL (downlink) system bandwidth (also called the transmission bandwidth configuration: dl-bandwidth), the number of transmitting antennas, and the SFN (System Frame Number).

[0085] Next, in step ST5, the DL-SCH of the cell is received based on the cell configuration information in the MIB, and SIB (System Information Block) 1 in the broadcast information BCCH is obtained. SIB 1 includes information on access to the cell, information on cell selection, and scheduling information for other SIBs (SIBk; k is an integer greater than or equal to 2). SIB 1 also includes a Tracking Area Code (TAC).

[0086] Next, in step ST6, the mobile terminal compares the TAC of the SIB1 received in step ST5 with the TAC part of the tracking area identity (TAI) in the tracking area list already held by the mobile terminal. The tracking area list is also called a TAI list. The TAI is identification information for identifying a tracking area, and is composed of an MCC (Mobile Country Code), an MNC (Mobile Network Code), and a TAC (Tracking Area Code). The MCC is a country code. The MNC is a network code. The TAC is a tracking area code number.

[0087] If the comparison result in step ST6 shows that the TAC received in step ST5 is the same as the TAC included in the tracking area list, the mobile terminal enters standby mode in the cell. If the comparison result shows that the TAC received in step ST5 is not included in the tracking area list, the mobile terminal requests a change of tracking area to perform a Tracking Area Update (TAU) to the core network (EPC) including the MME, etc., through the cell.

[0088] An apparatus constituting a core network (hereinafter sometimes referred to as a "core network side apparatus") updates the tracking area list based on the identification number (e.g., UE-ID) of the mobile terminal sent from the mobile terminal together with the TAU request signal. The core network side apparatus transmits the updated tracking area list to the mobile terminal. The mobile terminal rewrites (updates) the TAC list held by the mobile terminal based on the received tracking area list. Thereafter, the mobile terminal enters standby mode in the cell.

[0089] The widespread use of smartphones and tablet devices has led to an explosive increase in cellular wireless communication traffic, raising concerns about a shortage of wireless resources worldwide. In response to this, efforts are being made to develop small cells and promote spatial separation in order to improve frequency utilization efficiency.

[0090] In a conventional cell configuration, a cell configured by an eNB has a relatively wide coverage area. Conventionally, a cell is configured so that a certain area is covered by the relatively wide coverage areas of multiple cells configured by multiple eNBs.

[0091] In the case of small cell configuration, a cell configured by an eNB has a narrower coverage area than a cell configured by a conventional eNB. Therefore, as in the past, a larger number of small cell configuration eNBs are required to cover a certain area compared to conventional eNBs.

[0092] In the following description, a cell with a relatively large coverage, such as a cell configured by a conventional eNB, is referred to as a "macro cell," and an eNB that configures the macro cell is referred to as a "macro eNB." Also, a cell with a relatively small coverage, such as a cell configured as a small cell, is referred to as a "small cell," and an eNB that configures the small cell is referred to as a "small eNB."

[0093] The macro eNB may be, for example, a "Wide Area Base Station" as described in Non-Patent Document 8.

[0094] The small eNB may be, for example, a low-power node, a local area node, a hotspot, etc. Also, the small eNB may be a pico eNB constituting a pico cell, a femto eNB constituting a femto cell, a HeNB, a remote radio head (RRH), a remote radio unit (RRU), a remote radio equipment (RRE), or a relay node (RN). Also, the small eNB may be a "local area base station" or a "home base station" as described in Non-Patent Document 8.

[0095] 7 is a diagram showing the concept of cell configuration when macro eNBs and small eNBs are mixed. A macro cell configured by a macro eNB has a relatively wide coverage area 1301. A small cell configured by a small eNB has a coverage area 1302 that is smaller than the coverage area 1301 of the macro cell configured by the macro eNB.

[0096] When multiple eNBs are mixed, the coverage of a cell configured by one eNB may be included in the coverage of a cell configured by another eNB. In the cell configuration shown in Figure 7, as indicated by reference numerals "1304" and "1305," the coverage 1302 of a small cell configured by a small eNB may be included in the coverage 1301 of a macro cell configured by a macro eNB.

[0097] Also, as indicated by reference numeral "1305," the coverage of multiple, for example, two small cells 1302 may be included within the coverage of one macro cell 1301. A mobile terminal (UE) 1303 is included, for example, within the coverage 1302 of a small cell and communicates via the small cell.

[0098] Furthermore, in the cell configuration shown in FIG. 7, as indicated by reference numeral "1306," there may be cases where coverage 1301 of a macro cell configured by a macro eNB and coverage 1302 of a small cell configured by a small eNB overlap in a complex manner.

[0099] Also, as indicated by reference numeral "1307," there may be cases where the coverage 1301 of a macro cell configured by a macro eNB and the coverage 1302 of a small cell configured by a small eNB do not overlap.

[0100] Furthermore, as indicated by reference numeral "1308," there may be cases where the coverage 1302 of multiple small cells formed by multiple small eNBs is formed within the coverage 1301 of one macro cell formed by one macro eNB.

[0101] 8 and 9 are diagrams illustrating an example of coverage of an eNB in the communication system according to the first embodiment. 8 and 9 illustrate a case in which a UE 57 in dual connectivity performs HO between macro cells 51 and 53.

[0102] In the following description, the macro cell that performs dual connectivity is sometimes referred to as a "master cell," and the eNB that constitutes the master cell is sometimes referred to as a "master eNB (abbreviated as MeNB)." In addition, the MeNB that is the source of HO is sometimes referred to as a "source MeNB (abbreviated as S-MeNB)," and the MeNB that is the destination of HO is sometimes referred to as a "target MeNB (abbreviated as T-MeNB)."

[0103] Furthermore, a small cell that provides dual connectivity is sometimes referred to as a "secondary cell," and an eNB that constitutes a secondary cell is sometimes referred to as a "secondary eNB (abbreviated as SeNB)."

[0104] 8 and 9, the S-MeNB is indicated by reference symbol "51," and the coverage of the S-MeNB 51 is indicated by reference symbol "52." The T-MeNB is indicated by reference symbol "53," and the coverage of the T-MeNB 53 is indicated by reference symbol "54." The SeNB is indicated by reference symbol "55," and the coverage of the SeNB 55 is indicated by reference symbol "56."

[0105] Fig. 9 shows a case where another SeNB 58 is present in addition to the SeNB 55 shown in Fig. 8. In Fig. 9, the SeNB 55 shown in Fig. 8 is assumed to be a source SeNB (hereinafter may be referred to as "source SeNB") 55, and the other SeNB is assumed to be a destination SeNB (hereinafter may be referred to as "destination SeNB") 58. Also, in Fig. 9, reference symbol "56" indicates the coverage of the source SeNB 55, and reference symbol "59" indicates the coverage of the destination SeNB 58.

[0106] In this embodiment, a case will be described in which, due to the movement of UE 57 shown in Figures 8 and 9, measurements of cells surrounding UE 57 show that the reception quality of S-MeNB 51 has deteriorated, and a measurement report is made showing that the reception quality of T-MeNB 53 has improved.

[0107] Fig. 10 is a diagram showing an example of a sequence of handover-related processing in the communication system of embodiment 1. Fig. 11 is a diagram showing an example of a sequence of HO pre-processing in step ST908 of Fig. 10. Fig. 12 is a diagram showing an example of a sequence of HO processing in step ST928 of Fig. 10. Fig. 13 is a diagram showing an example of a sequence of HO post-processing in step ST949 of Fig. 10. Here, handover-related processing refers to processing related to handover (HO), and includes HO pre-processing, HO processing, and HO post-processing.

[0108] This embodiment discloses a method for a UE in dual connectivity to perform HO between macro cells in the case of Alternative 1A of the dual connectivity user plane architecture described in Non-Patent Document 11 (see 8.1.1.1 of Non-Patent Document 11).

[0109] In option 1A of the user plane architecture, communication is performed via a path that performs communication from the S-GW via the MeNB (hereinafter may be referred to as an "MeNB via path") and a path that performs communication from the S-GW via an SeNB, which is a small cell (hereinafter may be referred to as an "SeNB via path"). The MeNB via path is a path that uses bearer 1 and is used for packet data communication, for example, in steps ST902 and ST903. The SeNB via path is a path that uses bearer 2 and is used for packet data communication, for example, in steps ST904 and ST905.

[0110] The method by which the MeNB notifies a UE in dual connectivity of a measurement control message will be disclosed below.

[0111] In the example shown in FIG. 10, in step ST901, the S-MeNB notifies the UE of a measurement control message. Measurements of neighboring SeNBs may be configured in the measurement control message. Alternatively, measurements of frequencies for the SeNB may be configured. Furthermore, as measurement configuration, events or event criteria for the SeNB or frequencies for the SeNB may be configured separately from the MeNB.

[0112] The setting parameters include an SeNB identifier, a frequency, an event number for reporting, a reception quality threshold, a measurement period, etc. The reception quality includes RSRP (Reference Signal Received Power), RSRQ (Reference Signal Received Quality), etc.

[0113] The UE that has received the measurement control message in Step ST901 measures neighboring cells (MeNB and SeNB).

[0114] In Step ST906, the UE notifies the S-MeNB of a measurement report message. In Step ST907, the S-MeNB that has received the measurement report message uses the result of the measurement report to decide whether or not to perform handover (HO) of the UE to the T-MeNB. In the example shown in FIG. 10, the S-MeNB decides in Step ST907 to perform HO of the UE to the T-MeNB.

[0115] When the S-MeNB decides in Step ST907 to perform HO of the UE to the T-MeNB, it proceeds to Step ST908, and in Step ST908, it performs SeNB release processing, which is HO pre-processing shown in FIG.

[0116] Specifically, when the S-MeNB decides to HO the UE to the T-MeNB in step ST907 of FIG. 10, the S-MeNB proceeds to step ST909 of FIG.

[0117] In Step ST909, the S-MeNB notifies the SeNB of an SeNB Release Request message. In Step ST910, the SeNB that has received the SeNB Release Request message notifies the S-MeNB of an SeNB Release Response message.

[0118] In Step ST911, the S-MeNB notifies the UE of an RRC connection reconfiguration message as information related to RRC.

[0119] In Step ST912, the SeNB performs SN Status Transfer to transfer the status of the PDCP sequence number (SN) to the S-MeNB. Specifically, the SeNB notifies the S-MeNB of PDCP SN information. In addition, in Step ST913, the SeNB may perform data forwarding to the S-MeNB to transfer data whose transmission has not been completed, thereby establishing lossless communication.

[0120] Furthermore, whether to perform transfer or SN reordering may be determined depending on the bearer type, Quality of Service (QoS), the delay time occurring in the backhaul of the destination cell, and the number of transfers (retransmission of transferred data). For example, for services that do not have a problem even if data loss occurs, such as voice data such as VoIP (Voice over Internet Protocol), transfer or reordering is not performed. Also, for example, transfer is not performed when transferred data is retransmitted during handover. Also, for services that require real-time performance, transfer is not performed when the delay time occurring in the backhaul of the destination cell is large. By doing so, resources can be released quickly and stable operation can be provided.

[0121] In Step ST914, the S-MeNB stores the data transferred from the SeNB in a buffer.

[0122] In Step ST915, the UE notifies the S-MeNB of an RRC connection reconfiguration complete message. When RRC setting and radio synchronization between the S-MeNB and the UE are thereby completed, communication between the UE and the S-MeNB starts in Step ST916, and communication between the UE and the S-GW starts in Step ST917.

[0123] In Step ST918, the S-MeNB notifies the MME of a Path Switch Request message requesting a path switch. In Step ST919, the MME that has been notified of the Path Switch Request message notifies the S-GW of a Modify Bearer Request message requesting a change of the bearer.

[0124] In step ST918 or step ST919, information indicating whether or not the SeNB has been changed may be added. Alternatively, information indicating that the request is not a path switch request or a bearer change request due to HO of the UE may be added. Alternatively, information identifying the UE that is the HO target may not be added, thereby indicating that the request is not a path switch request or a bearer change request due to HO of the UE.

[0125] This allows the MME, S-GW, or a separately provided location registration management function unit to omit the process of updating the UE's location information and the process of managing radio resources associated with updating the location information, unlike normal HO processing.

[0126] In Step ST920, the S-GW that has been notified of the bearer change request message changes the destination of bearer 2 that has been used for communication from the SeNB to the S-MeNB. At this time, the S-GW may change the bearer for a path via the S-MeNB, taking into consideration the user accommodation status of the S-MeNB.

[0127] In Step ST924, the S-GW notifies the MME of a Modify Bearer Response message indicating that the bearer modification request has been accepted. In Step ST925, the MME that has been notified of the Modify Bearer Response message notifies the S-MeNB of a Path Switch Request Ack message indicating that the path switching request has been accepted.

[0128] In this way, the S-MeNB transmits and receives two bearers, bearer 1 and bearer 2, to and from the same UE in steps ST902 and ST903 in FIG. 10 and steps ST916 and ST922 in FIG.

[0129] At this time, the S-GW may notify the end of the forwarding process by adding an end marker to the PDCP to be transmitted to the SeNB in Step ST921. This allows the SeNB to recognize the end of the forwarding data, and therefore makes it possible to release the forwarding buffer at an efficient timing.

[0130] Furthermore, the SeNB may add an end marker and transfer the data to the S-MeNB in Step ST923, thereby enabling the S-MeNB to release the transfer acceptance buffer at an efficient timing.

[0131] In Step ST926, the S-MeNB notifies the SeNB of a UE context release message instructing it to release the UE context. When the S-MeNB has been notified of the end marker in Step ST923, in addition to notifying the SeNB of the UE context release message, the S-MeNB notifies the SeNB of the receipt of the end marker in Step ST926. By notifying the SeNB of the receipt of the UE context release message or the end marker in this way, the SeNB can release the UE management information in Step ST927.

[0132] The presence or absence of an end marker may be determined according to the type of bearer or QoS. For services that do not suffer from data loss, such as voice data such as VoIP, resources can be released without attaching an end marker, thereby achieving the effect of enabling prompt resource management and providing stable operation.

[0133] As shown in Fig. 8, when UE 57 moves and approaches the boundary of coverage 52 of S-MeNB 51, in step ST907 of Fig. 10 described above, S-MeNB 51 determines to perform HO to T-MeNB 53 based on the measurement report of UE 57. At this time, the measurement value of SeNB 55 in communication also becomes good.

[0134] When the measurement value of SeNB 55 is good, S-MeNB 51 determines that UE 57 is still present within coverage 56 of SeNB 55 after HO. Then, in step ST909 of FIG. 11 , S-MeNB 51 may notify SeNB 55 by adding resource reservation instruction information or an ID that can reserve resources to the release request message.

[0135] This allows SeNB55 to not release the RRC-related settings or information or radio synchronization-related settings or information that it has set, and can reduce the resource allocation process and resynchronization process when reconfiguring dual connectivity from T-MeNB53 to SeNB55 after HO to T-MeNB53.

[0136] After the SeNB releases the UE management information in Step ST927, a handover process from the S-MeNB to the T-MeNB is performed in Step ST928 of Fig. 10. As a result, both of the two bearers are switched from the S-MeNB to the T-MeNB.

[0137] Specifically, the handover process in Step ST928 is executed as shown in FIG. 12. In Step ST929, the S-MeNB notifies the T-MeNB, which is the HO target, of a handover request message. The HO request message includes E-RAB (E-UTRAN Radio Access Bearer) information for performing HO.

[0138] In Step ST930, the T-MeNB performs admission control to check its capacity. If the T-MeNB determines that it can accept HO based on the result of the admission control, it notifies the S-MeNB of an HO request acceptance response (Handover Request Ack) message in Step ST931.

[0139] Upon receiving the HO request response possible message in Step ST931, the S-MeNB notifies the UE of an RRC connection reconfiguration message including mobility control information in Step ST932.

[0140] In Step ST933, the S-MeNB performs SN status transfer to the T-MeNB. Specifically, the S-MeNB notifies the T-MeNB of PDCP SN information.

[0141] In Step ST934, the S-MeNB may perform data forwarding to forward data that has not been transmitted to the T-MeNB. In Step ST935, the T-MeNB stores the data forwarded from the S-MeNB in a buffer.

[0142] In Step ST936, the UE completes the radio configuration and notifies the T-MeNB of an RRC connection reconfiguration complete message. When the radio configuration is completed in this manner, in Step ST937, communication is started between the UE and the T-MeNB, and in Step ST938, communication is started between the UE and the S-GW.

[0143] In Step ST939, the T-MeNB notifies the MME of a Path Switch Request message. In Step ST940, the MME that has been notified of the Path Switch Request message notifies the S-GW of a Modify Bearer Request message.

[0144] In step ST939 or step ST940, information indicating whether or not the SeNB has been changed may be added. Alternatively, information indicating that the request is a path switch request or a bearer change request by the HO of the UE may be added. Alternatively, information identifying the UE that is the HO target may be added to indicate that the request is a path switch request or a bearer change request by the HO of the UE. Alternatively, a conventional path switch request or a bearer change request may be added to indicate that the request is a conventional HO of the UE.

[0145] This allows the MME, S-GW, or a separately provided location registration management function unit to perform the UE location information update process and the radio resource management process associated with the location information update as normal HO processing.

[0146] In Step ST941, the S-GW that has been notified of the bearer change request message changes the destination of bearer 2 that has been used for communication to the T-MeNB. At this time, the S-GW may change the bearer for a path via the T-MeNB, taking into consideration the user accommodation status of the T-MeNB.

[0147] As a result of the above, Bearer 1 and Bearer 2 are set up via the T-MeNB, and in step ST937, communication is performed between the UE and the T-MeNB, and in step ST943, communication is performed between the S-GW and the T-MeNB.

[0148] In Step ST945, the S-GW notifies the MME of a Modify Bearer Response message. In Step ST946, the MME that has been notified of the Modify Bearer Response message notifies the T-MeNB of a Path Switch Request Ack message.

[0149] In order to perform lossless transmission, the S-GW may add an end marker to the final data to the S-MeNB in Step ST942, which enables the S-MeNB to release the transfer buffer at an efficient timing.

[0150] In Step ST944, the S-MeNB may add an end marker to the final transfer data, thereby enabling the T-MeNB to release the transfer acceptance buffer at an efficient timing.

[0151] In Step ST947, the T-MeNB notifies the S-MeNB of a UE context release message. When the S-MeNB has been notified of the end marker in Step ST944, in Step ST947, the S-MeNB notifies the S-MeNB of the UE context release message and also notifies the S-MeNB of the receipt of the end marker. By notifying the S-MeNB of the UE context release message or the receipt of the end marker in this way, the S-MeNB can release the management information of the UE in Step ST948.

[0152] After the S-MeNB releases the UE management information in Step ST948, an SeNB addition process, which is a post-HO process, is performed in Step ST949 in Fig. 10. Specifically, the SeNB addition process in Step ST949 is performed as shown in Fig. 13.

[0153] After switching to the T-MeNB by the HO procedure in Step ST928 in FIG. 10, the T-MeNB notifies the SeNB of an SeNB addition request message in Step ST950 in FIG.

[0154] In Step ST951, the SeNB notifies the T-MeNB of an SeNB addition response message.

[0155] In Step ST952, the T-MeNB notifies the UE of an RRC connection reconfiguration message as information related to RRC.

[0156] In addition, in step ST951, the SeNB notifies the SeNB of an SeNB addition response message indicating that the SeNB addition request has been accepted, and in response thereto, the T-MeNB notifies the SeNB of PDCP SN information as an SN status transfer in step ST953.

[0157] In Step ST954, the T-MeNB may perform data forwarding to the SeNB to transfer data that has not been completely transmitted, thereby establishing lossless communication.

[0158] The T-MeNB may determine whether to perform data transfer or SN reordering depending on the type of bearer or QoS. For bearers that do not suffer from data loss, such as voice data such as VoIP, omitting data transfer or SN reordering can provide the effect of quickly releasing resources and providing stable operation.

[0159] In Step ST955, the SeNB stores the data transferred from the T-MeNB in a buffer.

[0160] In Step ST956, the UE completes radio synchronization and notifies the T-MeNB of an RRC Connection Reconfiguration Complete message indicating that the reconfiguration of the RRC connection between the SeNB and the UE has been completed. When the radio synchronization is completed in this manner, in Step ST957, communication is started between the UE and the SeNB, and in Step ST958, communication is started between the UE and the S-GW.

[0161] Furthermore, in Step ST959, the SeNB notifies the T-MeNB of an SeNB addition completion message indicating completion of the radio configuration, specifically, completion of the addition of the SeNB. The T-MeNB that has been notified of the SeNB addition completion message notifies the MME of a path switch request message in Step ST960. The MME that has been notified of the path switch request message notifies the S-GW of a modify bearer request message in Step ST961.

[0162] In step ST960 or step ST961, information indicating whether or not the SeNB has been changed (added) may be added. Alternatively, information indicating that the request is not a path switch request or a bearer change request due to HO of the UE may be added. Alternatively, information identifying the UE that is the HO target may not be added, thereby indicating that the request is not a path switch request or a bearer change request due to HO of the UE.

[0163] This allows the MME, S-GW, or a separately provided location registration management function unit to omit the process of updating the UE's location information and the process of managing radio resources associated with updating the location information, unlike normal HO processing.

[0164] In Step ST962, the S-GW that has been notified of the bearer change request message changes the destination of bearer 2 that has been used for communication to the S-MeNB. At this time, the S-GW may change the bearer for a path via the SeNB, taking into consideration the user accommodation status of the SeNB.

[0165] In Step ST966, the S-GW notifies the MME of a Modify Bearer Response message. In Step ST967, the MME that has been notified of the Modify Bearer Response message notifies the T-MeNB of a Path Switch Request Ack message indicating completion of path switching. As a result, Bearer 2 is set via the SeNB, and in Step ST957, communication is performed between the UE and the SeNB, and in Step ST964, communication is performed between the S-GW and the SeNB. In Step ST968, the T-MeNB may release management information for Bearer 2 of the UE.

[0166] At this time, in Step ST963, the S-GW may add an end marker to the PDCP to be transmitted to the T-MeNB to notify the end of the transfer process. This allows the T-MeNB to recognize the end of the transfer data, making it possible to release the transfer buffer at an efficient timing.

[0167] Furthermore, in step ST965, the T-MeNB may add an end marker and transfer the data to the SeNB, thereby enabling the SeNB to release the transfer buffer at an efficient timing.

[0168] The presence or absence of an end marker may be determined according to the type of bearer or QoS. For services that do not suffer from data loss, such as voice data such as VoIP, resources can be released without attaching an end marker, thereby achieving the effect of enabling prompt resource management and providing stable operation.

[0169] As shown in Fig. 9 above, when UE 57 moves and approaches the boundary of coverage 52 of S-MeNB 51, it releases source SeNB 55, performs HO from S-MeNB 51 to T-MeNB 53, and then adds SeNB 58. At this time, T-MeNB 53 may determine SeNB 58 to be added based on, in addition to the measurement report notified in step ST906 of Fig. 10, information on the moving direction of UE 57, information on the moving speed of UE 57, location information of surrounding SeNBs (GPS information of each SeNB, or information indicating which MeNB each SeNB is close to), the cell size of the destination SeNB (which may be the maximum transmission power value of the SeNB equivalent to the cell size), information on whether the UE is included in a CSG (Closed Subscriber Group) of the SeNB, or a combination of these. Although a UE is described above, it may also be a mobile router that aggregates multiple UEs. As a result of these determinations, SeNB designation information that designates the SeNB to be added after HO may be added to the HO request message notified in step ST929 of FIG.

[0170] For example, when a UE is on a train, even if the measurement information of the UE indicates that the measurement value for the SeNB with which the UE is communicating is better than that of the destination SeNB, if the destination SeNB has a quality that allows communication and the location information indicates that the destination SeNB is within the coverage of the T-MeNB, adding the destination SeNB can reduce the dual connectivity processing. In other words, if the UE moves at high speed after adding the source SeNB, the source SeNB can be immediately released, preventing an increase in processing due to adding the destination SeNB.

[0171] Furthermore, for example, when a source SeNB is communicating via CSG, even if the destination SeNB is temporarily in good condition due to shadowing caused by a human body or the like, the source SeNB is added because the source SeNB is communicating via CSG or the moving speed is slow. This allows for continuous stable communication.

[0172] As described above, in the present embodiment, when a UE is connected to one macro cell and one small cell and performs dual connectivity, as the UE moves, HO pre-processing is performed before HO processing in which the macro cell to which the UE is connected is switched from the source macro cell, S-MeNB, to the destination macro cell, T-MeNB, and HO post-processing is performed after HO processing. In HO pre-processing, the small cell, SeNB, is released and the connection with the SeNB is terminated. In HO post-processing, an SeNB is added and the connection with the SeNB is re-established.

[0173] This allows a UE in dual connectivity to achieve HO between macro cells in the case of option 1A of the dual connectivity user plane architecture described in Non-Patent Document 11.

[0174] First embodiment, variant 1 Fig. 14 is a diagram showing an example of a sequence of handover-related processing in a communication system according to a first modification of the first embodiment. Fig. 15 is a diagram showing an example of a sequence of pre-HO processing in step ST1009 of Fig. 14. Fig. 16 is a diagram showing an example of a sequence of post-HO processing in step ST1010 of Fig. 14. The handover-related processing of this modification is similar to the handover-related processing of the first embodiment shown in Figs. 10 to 13 described above, and therefore the same steps are given the same step numbers and their explanations will be omitted.

[0175] This embodiment discloses a method for a UE in dual connectivity to perform HO between macro cells in the case of Alternative 3C of the dual connectivity user plane architecture described in Non-Patent Document 11 (see 8.1.1.8 of Non-Patent Document 11).

[0176] In user plane architecture option 3C, the MeNB performs bearer splitting, dividing the bearer (bearer 2) between the MeNB and the UE into a path for direct communication (bearer 1) and a path for communication via a small cell.

[0177] In the case of bearer splitting, an E-RAB corresponding to one EPS bearer is split into two paths at the S-MeNB, and data is communicated between the S-GW and the UE. One path is where data is directly communicated between the S-MeNB and the UE in Step ST1001. The other path is where data is communicated between the S-MeNB and the UE via the SeNB in Steps ST1003 and ST1004.

[0178] In step ST1002, data is communicated between the S-MeNB and the S-GW via one path. The communication between the SeNB and the MeNB is performed between different eNBs. If the communication quality of the communication path between the different eNBs is poor, data loss may occur. In order to solve this problem, it is advisable to introduce a delivery confirmation process in the communication between the SeNB and the MeNB. For example, it is advisable to introduce a retransmission process. In this way, it is possible to reduce data loss in the communication between the SeNB and the MeNB.

[0179] A case where a UE in dual connectivity performs inter-MeNB HO by bearer splitting will be described.

[0180] A case will be described in which the reception quality of the S-MeNB measured by measurement deteriorates due to movement of the UE, the reception quality of the T-MeNB improves, and measurement reporting is performed according to the event criteria.

[0181] In this embodiment, the SeNB release process is performed as follows: In step ST907 of FIG. 15, the S-MeNB, which has received the measurement report from the UE in step ST906 of FIG. 14, decides to perform HO of the UE to the T-MeNB using the result of the measurement report.

[0182] When the S-MeNB decides to HO the UE to the T-MeNB, in Step ST909, it transmits an SeNB Release Request message to the SeNB, requesting the release of the SeNB. In addition, in Step ST911, the S-MeNB transmits an RRC connection re-establishment request message to the UE as information related to RRC. As a result, the S-MeNB stops bearer splitting of Bearer 2, and in Steps ST916 and ST917, transmits and receives Bearer 2 directly to and from the UE.

[0183] After the release of the SeNB, handover (HO) processing is performed from the S-MeNB to the T-MeNB in step ST928 of Fig. 14. The HO processing in step ST928 is executed in the same manner as the HO processing shown in Fig. 12 described above. By this HO processing, both of the two bearers are switched from the S-MeNB to the T-MeNB.

[0184] After switching to the T-MeNB, the post-HO process of step ST1010 in Fig. 14 is executed. Specifically, in step ST950 in Fig. 16, the T-MeNB notifies the SeNB of an addition request message, and in step ST952, notifies the UE of information related to RRC for performing communication. As a result, bearer 2 is bearer split.

[0185] One bearer of the bearer split bearer 2 is used when data is communicated directly between the T-MeNB and the UE in Step ST1005, and the other bearer is used when data is communicated between the T-MeNB and the UE via the SeNB in Steps ST1007 and ST1008. Data is communicated between the T-MeNB and the S-GW via one path in Step ST1006. In this way, bearer splitting can be processed without sending a path switch request to the MME and the S-GW.

[0186] As described above, according to this modification, even in the case of option 3C of the dual connectivity user plane architecture described in Non-Patent Document 11, that is, in the case where communication is performed between a macro cell and a UE, and between a small cell and a UE, using a bearer-splitted bearer, a UE in dual connectivity can achieve HO between macro cells.

[0187] Embodiment 2 17 to 19 are diagrams showing an example of a sequence of handover-related processing in a communication system according to the second embodiment. Fig. 17 and Fig. 18 are connected by a boundary line BL1. Fig. 18 and Fig. 19 are connected by a boundary line BL2. The handover-related processing according to this embodiment is similar to the handover-related processing according to the first embodiment shown in Figs. 10 to 13 described above, and therefore the same steps are given the same step numbers and their explanations are omitted.

[0188] This embodiment discloses a method for performing HO without releasing the SeNB in the case of Alternative 1A of the dual connectivity user plane architecture described in Non-Patent Document 11 (see 8.1.1.1 of Non-Patent Document 11).

[0189] In user plane architecture option 1A, communication is performed via two paths: in steps ST902 and ST903, communication is performed from the S-GW via the MeNB using bearer 1; and in steps ST904 and ST905, communication is performed from the S-GW via the small cell using bearer 2.

[0190] In this embodiment, when a UE in dual connectivity performs inter-MeNB HO, the connection with the SeNB is not released. That is, inter-MeNB HO is performed while maintaining the dual connectivity of the UE.

[0191] The method by which the MeNB notifies a UE in dual connectivity of a measurement control message will be disclosed below.

[0192] 17 to 19, in step ST901, the S-MeNB notifies the UE of a measurement control message. Measurements of neighboring SeNBs may be configured in the measurement control message. Alternatively, measurements of frequencies for the SeNB may be configured. Furthermore, as measurement configuration, events or event criteria for the SeNB or frequencies for the SeNB may be configured separately from the MeNB.

[0193] The setting parameters include an SeNB identifier, a frequency, an event number for reporting, a threshold value for reception quality, a measurement period, etc. The reception quality includes RSRP, RSRQ, etc.

[0194] The UE that has received the measurement control message in Step ST901 measures the MeNB and SeNB.

[0195] Here, a case will be described in which the reception quality of the S-MeNB measured by measurement deteriorates due to movement of the UE, the reception quality of the T-MeNB improves, and measurement reporting is performed according to the event criteria.

[0196] In Step ST906, the UE performs a measurement report to the S-MeNB. The measurement report from the UE may include an identifier of the SeNB used for dual connectivity and reception quality.

[0197] In Step ST907, the S-MeNB, which has received the measurement report from the UE in Step ST906, decides to perform HO of the UE to the T-MeNB using the result of the measurement report.

[0198] In step ST1100, the S-MeNB notifies the T-MeNB of an HO request message. This HO request message may include information about the SeNB (hereinafter, may be referred to as "SeNB information"). The SeNB information may include, for example, an SeNB identifier, the reception quality of the SeNB at the UE, and so on.

[0199] Furthermore, information related to the bearer of the SeNB (hereinafter sometimes referred to as "bearer information") includes an E-RAB identifier. When a plurality of bearers are set in the SeNB, the MeNB may provide an identifier for each bearer. The MeNB may notify the identifier of each bearer in association with information related to the bearer configuration. The SeNB may also notify the identifier of the bearer in association with the identifier of the bearer. Information related to the bearer configuration set for each bearer includes, for example, QoS parameters, specifically, a QoS Class Identifier (abbreviated as QCI), Allocation and Retention Priority (abbreviated as ARP), Guaranteed Bit Rate QoS Information, etc.

[0200] This enables the setting for each bearer, and the S-MeNB can notify the T-MeNB of the bearer setting used by the SeNB. The T-MeNB can acquire the bearer setting for each bearer of the SeNB, and can determine whether or not a change for each bearer is necessary. The bearer setting may be notified in association with information related to the bearer information of the SeNB.

[0201] An example of setting information related to RRC is an RRC context. The RRC context includes, for example, an access stratum (AS) configuration indicating a radio resource configuration, and a radio resource management (RRM) configuration indicating radio resource management (RRM) information.

[0202] The information about RRC includes information about RRC for the MeNB and information about RRC for the SeNB. It is preferable to be able to distinguish the information about each RRC. For example, an RRC context for the MeNB and an RRC context for the SeNB are created separately. Alternatively, information for the MeNB and information for the SeNB may be created separately within one RRC context.

[0203] This makes it possible to set information relating to the RRC for the SeNB and information relating to the RRC for the MeNB separately, and enables the S-MeNB to notify the T-MeNB of information relating to the RRC for each of the MeNB and the SeNB. The T-MeNB can obtain information relating to the RRC for each of the S-MeNB and the SeNB, and the T-MeNB can determine whether or not it is necessary to change the settings relating to the RRC for the SeNB.

[0204] In Step ST1101, the T-MeNB determines whether to change the SeNB using information received from the S-MeNB in the HO request message. This process determines whether the T-eNB can take over the SeNB bearer set up at the S-eNB. For example, there is a limit to the number of S-eNBs that can be accommodated at the T-eNB, and this limit may make it impossible to accept the request. In addition, in the case of option 3C of the user plane architecture in dual connectivity, whether the S-MeNB can accommodate a bearer that has been bearer split is also used as information for the determination.

[0205] When it is determined that the SeNB is to be changed, the process proceeds to Step ST1102. Moreover, not only when it is determined that the SeNB is to be changed, but also when the T-MeNB decides to release the SeNB in the HO process, the process may proceed to Step ST1102.

[0206] In Step ST1102, the T-MeNB executes the SeNB release procedure of Step ST908 shown in FIG. 10 described above to release the SeNB, and then executes the HO procedure of Step ST908. Specifically, the T-MeNB notifies the S-MeNB of information indicating a request to execute the SeNB release procedure (hereinafter, may be referred to as "SeNB release procedure execution request information"). The S-MeNB that has been notified of the SeNB release procedure execution request information from the T-MeNB may apply the method disclosed in Embodiment 1, and description thereof will be omitted here.

[0207] In this case, the HO request from the S-MeNB to the T-MeNB in Step ST1100 may be omitted. The T-MeNB may perform admission control in Step ST930 after notifying the S-MeNB of SeNB release procedure execution request information.

[0208] The T-MeNB may notify the S-MeNB of the SeNB release procedure execution request information using an HO request acceptance response message. For example, this may be done using the HO request acceptance response message in step ST1104 after admission control. The T-MeNB that has decided to release the SeNB in the HO procedure determines that HO will be performed only between MeNBs, performs admission control accordingly, and notifies the S-MeNB of an HO request acceptance response message including the SeNB release procedure execution request information. If the system specifies that the SeNB is released during MeNB HO, the HO request acceptance response message may indicate a request to execute the SeNB release procedure. The S-MeNB that is performing dual connectivity and has received the HO request acceptance response message from the T-MeNB executes the SeNB release procedure.

[0209] The notification of the SeNB release procedure execution request information from the T-MeNB to the UE may be notified using an RRC Connection Reconfiguration message including MCI (Mobility Control Information). For example, the RRC connection reconfiguration message including MCI in Step ST1106 may be used. If it is specified as a system that the SeNB is released in the event of MeNB HO, the RRC connection reconfiguration message including MCI may indicate a request to execute the SeNB release procedure. The UE constituting the SeNB that receives the RRC connection reconfiguration message including MCI from the S-MeNB executes the SeNB release procedure.

[0210] If the T-MeNB determines not to change the SeNB, it decides in Step ST1103 to perform HO processing while maintaining the use of the SeNB. In other words, it changes only the MeNB. If it determines not to change the SeNB, it does not perform processing to change the bearer 2 using the SeNB.

[0211] In Step ST930, the T-MeNB performs admission control for the HO request. This admission control may be performed before or together with the above-mentioned determination of whether or not to change the SeNB.

[0212] Since the SeNB is not changed, admission control is performed by changing the MeNB. In admission control, the T-MeNB performs RRC-related settings for the T-MeNB. The RRC-related settings include an AS configuration indicating the configuration of radio resources, an RRM configuration indicating RRM information, and the like. Information included in an RRC connection reconfiguration message may also be configured.

[0213] The UE-specific RACH preamble configuration used in the T-MeNB may be configured separately from the UE-specific RACH preamble configuration used in the SeNB. Also, the C-RNTI (Cell-Radio Network Temporary Identifier) used in the T-MeNB may be configured separately from the C-RNTI used in the SeNB. When the SeNB supports other UEs not served by the T-MeNB, the C-RNTI can be configured separately for each UE, enabling flexible control.

[0214] Alternatively, the UE-specific RACH preamble configuration used in the T-MeNB may be set to the same as the UE-specific RACH preamble configuration used in the SeNB. Also, the C-RNTI used in the T-MeNB may be set to the same as the C-RNTI used in the SeNB. In cases such as when the SeNB does not support other UEs not served by the T-MeNB, there is no need to set each UE individually, and control can be performed with a single value, simplifying control.

[0215] The RRC configuration for the T-MeNB requires information that enables the UE to reconfigure the RRC connection with the T-MeNB.

[0216] When the T-MeNB decides to accept the HO request, in Step ST1104, the T-MeNB notifies the S-MeNB of an HO request acceptance response message including an HO instruction message. This HO instruction message may include the above-mentioned setting information related to RRC for the T-MeNB.

[0217] Furthermore, when maintaining bearer 2 using the SeNB, the T-MeNB may notify the S-MeNB that HO is possible while maintaining the configuration of bearer 2 using the SeNB. Furthermore, when maintaining the configuration of bearer 2 using the SeNB, the T-MeNB may notify the S-MeNB that HO is possible while maintaining the RRC-related settings for the SeNB. These notifications may be made using an HO request acceptance response message. They may be notified together with an HO request acceptance response message for a bearer that does not use a SeNB. They may be notified separately from an HO request acceptance response message for a bearer that does not use a SeNB.

[0218] The HO request acceptance response message or the HO instruction message may include information indicating whether or not there is a change in the RRC settings of the SeNB. The S-MeNB may acquire the information and, if it determines that there is a change, may further acquire the RRC settings of the SeNB. This simplifies the processing when there is no change.

[0219] If there is no change in the RRC configuration of the SeNB, the configuration information does not need to be included in the HO command message, which makes it possible to reduce the amount of information to be signaled.

[0220] Alternatively, the HO command message may include configuration information related to the RRC for the SeNB, which enables the S-MeNB to compare the configuration information related to the RRC for the SeNB acquired from the T-MeNB with the configuration information related to the RRC for the SeNB held by its own cell, thereby enabling reliable verification that there are no changes.

[0221] The S-MeNB, which has received the HO request acceptance response message in Step ST1104, can recognize that the HO is performed while maintaining the SeNB for the UE in dual connectivity with the T-MeNB.

[0222] The S-MeNB, which recognizes that HO will be performed while maintaining the SeNB, does not request the SeNB to change the configuration of bearer 2. It does not request a change in settings related to RRC for the SeNB.

[0223] In Step ST1105, a process of notifying the SeNB of information indicating that HO will be activated is performed between the T-MeNB, S-MeNB, and SeNB. For example, the T-MeNB notifies the SeNB of information indicating that HO will be activated. This notification may be performed when the T-MeNB transmits an HO request acceptance response message to the S-MeNB. Alternatively, the S-MeNB that receives the HO request acceptance response message from the T-MeNB may notify the SeNB of information indicating that HO will be activated.

[0224] In step ST1108 of FIG. 18, the S-MeNB transfers the PDCP SN information and data that has not yet been transmitted to the T-MeNB, and establishes lossless communication.

[0225] A UE in dual connectivity may be configured not to change the Bearer 2 configuration by SeNB activation during inter-MeNB HO. If the bearer configuration is changed by SeNB activation during HO, the process will be performed when the MeNB changes from S-MeNB to T-MeNB, which complicates control and increases the possibility of malfunction. By not changing the Bearer 2 configuration by SeNB activation during HO, it is possible to reduce malfunction.

[0226] A method for preventing SeNB-initiated changes to the configuration of Bearer 2 during HO will be disclosed below.

[0227] The T-MeNB notifies the SeNB of information indicating that HO will be activated. This notification may be made when the T-MeNB transmits an HO request acceptance response message to the S-MeNB. Alternatively, the S-MeNB that receives the HO request acceptance response message from the T-MeNB may notify the SeNB of information indicating that HO will be activated.

[0228] Upon receiving the information indicating that HO has been initiated, the SeNB does not initiate a request to change the configuration of bearer 2. It is preferable that the request is not initiated until the HO is completed.

[0229] Alternatively, during the HO, the S-MeNB or T-MeNB may notify the SeNB of Nack (or rejection) in response to the Bearer 2 configuration change request from the SeNB. Reason information may be included, and the reason may be set to indicate that the HO is in progress.

[0230] The method by which the SeNB recognizes the completion of HO is disclosed below. The T-MeNB may notify the SeNB of information indicating that HO has been completed. This notification may be made when the T-MeNB receives a path switch request acceptance response message from the MME during the HO process. Alternatively, the completion of the MeNB change process, which will be described later, may be regarded as the completion of HO.

[0231] By doing so, it is possible to prevent the configuration of Bearer 2 for a UE activated by the SeNB from being changed during HO processing for the UE in dual connectivity, thereby reducing malfunctions in the system.

[0232] Another method will be disclosed. It has been disclosed that when HO is performed while the SeNB is maintained, the S-MeNB does not request the SeNB to change the configuration of bearer 2 or the settings related to RRC for the SeNB. However, as another method, when HO is performed while the SeNB is maintained, the S-MeNB may notify the SeNB that HO is being performed while the SeNB is maintained. Alternatively, the S-MeNB may notify the SeNB that a request to change the configuration of bearer 2 or a request to change the settings related to RRC for the SeNB will not be made. This notification may be made together with information indicating that the above-mentioned HO is activated.

[0233] This allows the SeNB to recognize whether or not the HO is to be performed while maintaining the SeNB during the HO. In the case of the HO while maintaining the SeNB, the SeNB can prevent the configuration of Bearer 2 for the UE activated by the SeNB from being changed during the HO. This makes it possible to obtain the same effects as described above.

[0234] In Step ST1106, the S-MeNB that has received the HO request acceptance response message notifies the UE of an RRC Connection Reconfiguration message including Mobility Control Information (MCI) in order to cause the UE to perform HO to the T-MeNB. Furthermore, the S-MeNB that has received the HO request acceptance response message may notify the UE that there is no change in the bearer using the SeNB.

[0235] The S-MeNB notifies the UE that there is no change in the SeNB for E-RAB carrying bearer 2 during the HO. It may also notify that there is no change in the RRC settings for the SeNB. This notification may be included in an RRC connection reconfiguration message including the MCI.

[0236] If necessary, the S-MeNB may notify the UE of the configuration information related to RRC for the SeNB if the S-MeNB is aware of the configuration information. The UE can compare the received configuration information related to RRC for the SeNB to which the UE is connected with the received configuration information, and can verify that there are no changes.

[0237] In Step ST1106, the UE that has received the RRC connection reconfiguration message including the MCI performs HO from the S-MeNB to the T-MeNB while maintaining synchronization and connection with the SeNB. The UE performs HO while maintaining radio resources with the SeNB. In this state, the UE is able to communicate with the SeNB.

[0238] A method for handling uplink data from a UE to an SeNB is disclosed below. When performing HO while maintaining connection with the SeNB, the UE can continue transmitting uplink data of bearer 2. Alternatively, the UE can stop transmitting uplink data during HO and store the data in a buffer until HO from the S-MeNB to the T-MeNB is completed. This makes it possible to wait for establishment of a control plane, and reduce buffering processing of control information between the SeNB and S-MeNB, or between the SeNB and T-MeNB, when a control plane path has not yet been established, thereby simplifying the processing in the SeNB.

[0239] In Step ST1106, the UE receives the MCI instructing HO to the T-MeNB from the S-MeNB, and in Step ST1107 stops transmitting uplink data of bearer 1 and stores the data in a buffer.

[0240] In Step ST1109, the UE performs a process to change connection from the S-MeNB to the T-MeNB in accordance with the MCI. After receiving the MCI, the UE disconnects from the S-MeNB and establishes a connection with the T-MeNB. The UE reconfigures the RRC connection for the T-MeNB using the content received in the RRC connection reconfiguration message in Step ST1106, and establishes a connection with the T-MeNB.

[0241] In Step ST1110, the UE performs RA processing with the T-MeNB, and in Step ST1111, notifies the T-MeNB of an RRC connection reconfiguration complete message.

[0242] After the UE receives the RRC connection reconfiguration completion message from the T-MeNB, the UE is able to perform data communication directly with the T-MeNB.

[0243] After notifying the T-MeNB of the completion of the RRC connection reconfiguration message, the UE transmits uplink data of the bearer 1.

[0244] A method for handling downlink data notified from the SeNB to the UE is disclosed below. When the SeNB performs HO while maintaining connection with the UE, the SeNB can maintain transmission of downlink data of Bearer 2.

[0245] Alternatively, the SeNB can stop transmitting downlink data during HO and store the data in a buffer until HO from the S-MeNB to the T-MeNB is completed. This makes it possible to wait for the establishment of the control plane, and reduce buffering processing of control information between the SeNB and the S-MeNB or between the SeNB and the T-MeNB when the control plane path is not yet established, thereby simplifying the processing of the SeNB.

[0246] The SeNB is notified by the MeNB of the configuration of a bearer for dual connectivity for a UE, and the MeNB notifies the SeNB of configuration information related to RRC for the UE.

[0247] Therefore, the SeNB needs to recognize which MeNB has notified it of the bearer configuration, which MeNB it should notify of the setting information related to the RRC configured by the SeNB, etc. In other words, it needs to recognize the MeNB.

[0248] In addition, the SeNB performs data communication for UEs that have dual connectivity between MeNBs. The SeNB needs to recognize which MeNB to transmit data from to the UE and which MeNB to transmit data from the UE to. In other words, the SeNB needs to recognize the MeNB.

[0249] Although the former MeNB and the latter MeNB may be configured separately, the case where they are the same will be described here. In the following description, this MeNB will be referred to as a controlling MeNB of the SeNB.

[0250] In the HO process of a UE during dual connectivity, when the SeNB is not changed but the MeNB is changed from an S-MeNB to a T-MeNB, a method is disclosed in which the controlling MeNB of the SeNB recognizes which MeNB has been changed to.

[0251] The following two methods (1) and (2) are disclosed as specific examples of a method for a controlling MeNB of an SeNB to recognize which MeNB has been changed to. (1) The S-MeNB notifies the SeNB of a change in the controlling MeNB. (2) The T-MeNB notifies the SeNB of the change of the controlling MeNB.

[0252] A specific example of the specific example (1) will be disclosed below. When the T-MeNB receives an RRC connection reconfiguration complete message from the UE, the T-MeNB notifies the S-MeNB of a controlling MeNB change request message from the SeNB to request the SeNB to change the controlling MeNB. The notification may be performed using X2 signaling. The notification may include an identifier of the SeNB that will change the controlling MeNB. The notification may also include information on the reason for the change. The notification may include information indicating that the MeNB has been changed due to HO.

[0253] Upon receiving the notification, the S-MeNB notifies the SeNB that will change the controlling MeNB of a controlling MeNB change request message requesting the change of the controlling MeNB. The notification may be performed using X2 signaling or signaling on an interface provided between the MeNB and the SeNB.

[0254] The following five items (1) to (5) are disclosed as specific examples of information to be included in the signaling. (1) Information indicating a change in the controlling MeNB. (2) Identifier of the controlling MeNB after the change. Here, the identifier of the T-MeNB. (3) A bearer identifier of a path using the SeNB, which may be an E-RAB identifier (which may be an E-RAB ID, etc.) or an EPS bearer identifier. (4) Identifier of the UE that performs dual connectivity using the bearer. (5) A combination of (1) to (4) above.

[0255] The SeNB that has received the controlling MeNB change request message identifies the bearer for which the MeNB is to be changed from the information included in the controlling MeNB change request message. The SMeNB changes the controlling MeNB of the bearer to the changed MeNB. The management of the bearer in the SeNB should be performed in association with the identifier of the controlling MeNB. In other words, it is preferable to associate the identifier of the bearer with the identifier of the controlling MeNB. In this way, the SeNB can change the controlling MeNB of the bearer for the UE that performs dual connectivity.

[0256] After the controlling MeNB is changed, the SeNB shall accept a request to modify or release the bearer only from the changed controlling MeNB, and shall perform data communication with the changed MeNB.

[0257] By doing this, the SeNB can recognize which MeNB the controlling MeNB has been changed to, and can perform communication for control purposes such as modifying the bearer for the UE and requesting its release, as well as data communication, with the changed controlling MeNB.

[0258] The SeNB that has changed the controlling MeNB may notify the S-MeNB of a controlling MeNB change response message. The message may include information indicating that the controlling MeNB has been changed.

[0259] The S-MeNB that receives the controlling MeNB change response message from the SeNB may notify the changed controlling MeNB, here the T-MeNB, of a message indicating that the controlling MeNB change of the SeNB has been completed.

[0260] By doing this, the changed MeNB, in this case the T-MeNB, can recognize that the controlling MeNB of the SeNB has been changed, and can start communication with the SeNB for control purposes such as modifying the bearer for the UE, requesting release, and data communication.

[0261] After the change, the MeNB may notify the SeNB of a message confirming that the controlling MeNB for the bearer for the UE has changed, and the SeNB may respond to the message.

[0262] A specific example of the specific example (2) is disclosed below. When the T-MeNB receives an RRC connection reconfiguration complete message from the UE, it notifies the SeNB of a controlling MeNB change request message. The SeNB identifier may be the SeNB information included in the HO request message received from the S-MeNB. The notification may be performed using X2 signaling. Alternatively, the notification may be performed using signaling on an interface between the MeNB and the SeNB. The information included in the signaling may be the same as that disclosed in the method of the specific example (1).

[0263] The processing in the SeNB that receives the controlling MeNB change request message from the T-MeNB is the same as in the specific example (1) above, and therefore the description thereof will be omitted.

[0264] By doing this, the SeNB can recognize which MeNB the controlling MeNB has been changed to, and it becomes possible to perform communication for control purposes such as modifying the bearer for the UE, requesting release, and data communication with the changed controlling MeNB.

[0265] The SeNB that has changed the controlling MeNB may notify the T-MeNB of a controlling MeNB change response message. The message may include information indicating that the controlling MeNB has been changed.

[0266] The T-MeNB that receives the controlling MeNB change response message from the SeNB may notify the previous controlling MeNB, here the S-MeNB, of a message indicating that the controlling MeNB change of the SeNB has been completed.

[0267] By doing this, the MeNB before the change, in this case the S-MeNB, can recognize that the controlling MeNB of the SeNB has been changed, and can terminate control communications, such as modifying the bearer for the UE and requesting its release, as well as data communications, with the SeNB.

[0268] In addition, when HO of a UE in dual connectivity is performed using an SeNB, the SeNB can perform communication for control of bearer configuration and data communication with the changed MeNB (T-MeNB).

[0269] Therefore, data communication can be performed between the UE and the T-MeNB via the SeNB.

[0270] A specific example of the MeNB change process of the SeNB will be described. In Step ST1111, the T-MeNB that has received the RRC connection reconfiguration complete message from the UE notifies the S-MeNB of a controlling MeNB change request message requesting a change of the controlling MeNB of the SeNB in Step ST1112. The controlling MeNB change request message of the SeNB includes an identifier of the SeNB that will change the controlling MeNB.

[0271] In Step ST1113, the S-MeNB that has received this message notifies the SeNB that will change the controlling MeNB of a controlling MeNB change request message. The controlling MeNB change request message includes information indicating the change of the controlling MeNB, an identifier of the T-MeNB, a bearer identifier of the path using the SeNB, an identifier of the bearer in the SeNB for changing the controlling MeNB, an identifier of the S-MeNB, and an identifier of the UE that is the target of HO for performing dual connectivity using the SeNB. In this way, the SeNB can recognize for which bearer established by which MeNB the controlling MeNB should be changed.

[0272] In Step ST1114, the SeNB that has received the controlling MeNB change request message changes the controlling MeNB using the received information. As a result, the SeNB performs communication for control of bearers and data communication with the T-MeNB, which is the controlling MeNB after the change. The SeNB that has changed the controlling MeNB ends communication for control and data communication with the controlling MeNB before the change.

[0273] In Step ST1115, the SeNB that has changed the controlling MeNB in Step ST1114 notifies the S-MeNB of a controlling MeNB change response message to notify that the controlling MeNB has been changed. In Step ST1116, the S-MeNB that has received the controlling MeNB change response message notifies the T-MeNB of the SeNB of a controlling MeNB change response message to notify that the change of the controlling MeNB of the SeNB has been completed. This allows the T-MeNB to recognize that the controlling MeNB of the SeNB has been changed to the T-MeNB. Therefore, the T-MeNB can perform communication for control of bearers and data communication with the SeNB.

[0274] As described above, in this embodiment, notification of a controlling MeNB change request message to the SeNB is executed via the S-MeNB, but the controlling MeNB change request message and the controlling MeNB change response message may be transmitted and received directly between the T-MeNB and the SeNB without going through the S-MeNB. In this case, information about the S-MeNB may be included in the controlling MeNB change request message and used to determine whether the controlling MeNB change request is an invalid controlling MeNB change request in the controlling MeNB change process of the SeNB.

[0275] Step ST1112 to step ST1116 shown in Fig. 18 are designated as step ST1117. Step ST1117 shows a controlling MeNB change process of the SeNB performed among the T-MeNB, the S-MeNB, and the SeNB.

[0276] The subsequent path switching process in the S-GW for changing the MeNB is the same as that in Fig. 12, and therefore description thereof will be omitted. The path switching process enables data communication via the T-MeNB between the S-GW and the UE that was the target of HO.

[0277] Regarding data communication between the S-GW and the UE, one is direct data communication between the S-MeNB and the UE, and the other is direct data communication between the SeNB and the UE. By using the method disclosed in this embodiment, it becomes possible for a UE in dual connectivity by a bearer to perform inter-MeNB HO.

[0278] Another method for the controlling MeNB change process of the SeNB is disclosed. In the method disclosed above, the controlling MeNB change process of the SeNB is performed after the T-MeNB receives an RRC connection reconfiguration complete message from the UE.

[0279] As another method, the controlling MeNB change process of the SeNB may be performed after the T-MeNB notifies the S-MeNB of the HO request acceptance response message, or after the S-MeNB receives the HO request acceptance response message from the T-MeNB, or after the S-MeNB transmits an RRC connection reconfiguration message including the MCI to the UE. The controlling MeNB change process of the SeNB may be performed by applying the above-mentioned method.

[0280] By doing so, it becomes possible to change the controlling MeNB to a T-MeNB in the SeNB early, and therefore it becomes possible to perform control data communication between the SeNB and the T-MeNB early.

[0281] Another method for the control MeNB change process of the SeNB will be disclosed. The control MeNB change process of the SeNB may be performed after the T-MeNB completes the path switch process for the MME and S-GW. When the T-MeNB receives a path switch request acceptance response message from the MME, it notifies the SeNB of the change of the MeNB.

[0282] Alternatively, when the S-MeNB receives a UE context release message from the T-MeNB, the S-MeNB may notify the SeNB of the change of the MeNB. The above-mentioned method may be applied to the control MeNB change process of the SeNB.

[0283] In this way, after the HO process, including the path switching from the MME and S-GW to the T-MeNB, is completed, data communication can be performed via a bearer using the SeNB. By reliably performing the process after the HO process is completed, it is possible to avoid complicating the control of data communication.

[0284] As described above, according to the present embodiment, when the HO process is initiated, the SeNB, which is a small cell, is notified that the macro cell that controls the small cell will be changed. This allows the HO process to be executed while maintaining the path for communication from the S-GW via the small cell, so that user-plane data communication can be continued even during the HO process.

[0285] Furthermore, since the control sequence is simpler compared to when HO is performed after releasing the SeNB as in embodiment 1, it is possible to reduce the amount of information to be signaled in the HO sequence during dual connectivity.

[0286] Embodiment 3 20 to 22 are diagrams showing an example of a sequence of handover-related processing in a communication system according to the third embodiment. Fig. 20 and Fig. 21 are connected by a boundary line BL3. Fig. 21 and Fig. 22 are connected by a boundary line BL4. The handover-related processing of this embodiment is similar to the handover-related processing of the first embodiment shown in Figs. 10 to 13 and the second embodiment shown in Figs. 17 to 19 described above, and therefore the same steps are given the same step numbers and their explanations are omitted.

[0287] This embodiment discloses a method for performing HO without releasing the SeNB in the case of Alternative 3C of the dual connectivity user plane architecture described in Non-Patent Document 11 (see 8.1.1.8 of Non-Patent Document 11).

[0288] In user plane architecture option 3C, the MeNB performs bearer splitting, dividing the bearer between the MeNB and the UE into a path for direct communication and a path for communication via a small cell.

[0289] When a UE in dual connectivity using bearer split performs inter-MeNB HO, the connection with the SeNB is not released. That is, the UE performs inter-MeNB HO while maintaining dual connectivity.

[0290] In the case of bearer splitting, as described in the first modification of the first embodiment, an E-RAB corresponding to one EPS bearer is split into two paths at the S-MeNB, and data is communicated between the S-GW and the UE. One is that in step ST1001, data is communicated directly between the S-MeNB and the UE. The other is that in steps ST1003 and ST1004, data is communicated between the S-MeNB and the UE via the SeNB. In step ST1002, data is communicated between the S-MeNB and the S-GW via one path.

[0291] A case where a UE in dual connectivity performs inter-MeNB HO by bearer splitting will be described.

[0292] The method by which the MeNB notifies the UE in dual connectivity using bearer split of the measurement control message can be the same as that disclosed in the first embodiment, and a description thereof will be omitted here.

[0293] Here, a case will be described in which the reception quality of the S-MeNB deteriorates as a result of measurements, the reception quality of the T-MeNB improves, and measurement reporting is performed according to the event criteria.

[0294] In Step ST906, the UE performs a measurement report to the S-MeNB. The measurement report from the UE may include an identifier of the SeNB used for dual connectivity and reception quality.

[0295] The S-MeNB, which has received the measurement report from the UE, uses the result of the report to decide to HO the UE to the T-MeNB.

[0296] In Step ST1201, the S-MeNB notifies the T-MeNB of an HO request message. This message may include SeNB information. Alternatively, the S-MeNB may notify bearer information related to a bearer performing bearer splitting using the SeNB. Alternatively, the S-MeNB may notify information related to bearer splitting. Alternatively, the S-MeNB may notify information related to RRC in bearer splitting.

[0297] The SeNB information includes, for example, an identifier of the SeNB, and the reception quality of the SeNB at the UE.

[0298] Bearer information relating to a bearer performing bearer splitting includes an E-RAB identifier.

[0299] The information regarding bearer splitting includes information regarding the configuration (hereinafter also referred to as "split bearer configuration") of a bearer split into a direct path between the MeNB and the UE and a path between the MeNB and the UE via the SeNB (hereinafter also referred to as "split bearer configuration"). The MeNB may set up a split bearer for each split path. An identifier may be provided for each path. The identifier of each path may be notified in association with information regarding the split bearer configuration set for each path. In addition, in the case of a path using an SeNB, the identifier of the SeNB may be notified in association with the identifier of the path. The information regarding the split bearer configuration set for each path includes, for example, QoS parameters, specifically, QCI, ARP, and bandwidth guaranteed service quality information.

[0300] This enables a split bearer to be set for each path, and the S-MeNB can notify the T-MeNB of the split bearer setting for each path. The T-MeNB can obtain the split bearer setting for each path and determine whether or not it is necessary to change the split bearer setting for each path.

[0301] It is preferable to notify bearer information relating to bearers performing bearer splitting and information relating to the split bearer configuration in association with each other, thereby making it possible to distinguish between bearers performing bearer splitting and bearers not performing bearer splitting.

[0302] An example of configuration information related to RRC in bearer splitting is an RRC context. The RRC context includes, for example, an AS configuration indicating the configuration of radio resources, an RRM configuration indicating RRM information, and the like.

[0303] The information about RRC includes information about RRC for the MeNB and information about RRC for the SeNB. It is preferable to be able to distinguish the information about each RRC. For example, an RRC context for the MeNB and an RRC context for the SeNB are created separately. Alternatively, information for the MeNB and information for the SeNB may be created separately within one RRC context.

[0304] This makes it possible to set information about the RRC for the SeNB and information about the RRC for the MeNB separately, and the S-MeNB can notify the T-MeNB of information about the RRC for each of the MeNB and the SeNB. The T-MeNB can obtain information about the RRC for each of the S-MeNB and the SeNB, and the T-MeNB can determine whether or not it is necessary to change the settings related to the RRC for the SeNB.

[0305] In Step ST1101, the T-MeNB uses the information received in the HO request message from the S-MeNB to determine whether or not to change the SeNB. The process of Step ST1101 is the same as the process of Step ST1101 in FIG. 17, and therefore description thereof will be omitted.

[0306] If it is determined that the SeNB should be changed, the mobile station proceeds to Step ST 1102. The process of Step ST 1102 can be performed by applying the method disclosed in the second embodiment, and therefore, a description thereof will be omitted here.

[0307] If it is determined that the SeNB will not be changed, the T-MeNB decides in Step ST1103 to perform HO processing while maintaining the use of the SeNB. In other words, the T-MeNB decides to change only the MeNB. If it is determined that the SeNB will not be changed, the split bearer configuration used by the SeNB is maintained. Processing to change the split bearer configuration to the SeNB is not performed.

[0308] In Step ST1202, the T-MeNB performs admission control for the HO request. This admission control may be performed before or together with the above-mentioned determination of whether or not to change the SeNB.

[0309] Since the SeNB is not changed, admission control is performed by changing the MeNB. In admission control, the T-MeNB performs RRC-related settings for the T-MeNB. The RRC-related settings include an AS configuration indicating the configuration of radio resources, an RRM configuration indicating RRM information, and the like. Information included in an RRC connection reconfiguration message may also be configured.

[0310] The UE-specific RACH preamble configuration used in the T-MeNB may be set separately from the UE-specific RACH preamble configuration used in the SeNB. Also, the C-RNTI used in the T-MeNB may be set separately from the C-RNTI used in the SeNB. When the SeNB supports other UEs not served by the T-MeNB, the C-RNTI can be set individually for each UE, enabling flexible control.

[0311] Alternatively, the UE-specific RACH preamble configuration used in the T-MeNB may be set to the same as the UE-specific RACH preamble configuration used in the SeNB. Also, the C-RNTI used in the T-MeNB may be set to the same as the C-RNTI used in the SeNB. In cases such as when the SeNB does not support other UEs not served by the T-MeNB, there is no need to set each UE individually, and control can be performed with a single value, simplifying control.

[0312] The RRC configuration for the T-MeNB requires information that enables the UE to reconfigure the RRC connection with the T-MeNB.

[0313] When the T-MeNB decides to accept the HO request, in Step ST1203, the T-MeNB notifies the S-MeNB by including an HO instruction message in an HO request acceptance response message. This HO instruction message may include the above-mentioned setting information related to RRC for the T-MeNB.

[0314] Furthermore, when maintaining the configuration of a split bearer using an SeNB, the T-MeNB notifies the S-MeNB of the SeNB identifier, the E-RAB identifier, information on bearer splitting, information on RRC in bearer splitting, etc. Alternatively, the T-MeNB may notify the S-MeNB that HO is possible while maintaining the configuration of a split bearer using an SeNB. Furthermore, when maintaining the configuration of a split bearer using an SeNB, the T-MeNB may notify the S-MeNB that HO is possible while maintaining the settings related to RRC for the SeNB.

[0315] This reduces the amount of information to be notified. These notifications may be made using an HO request acceptance response message. They may be notified together with the HO request acceptance response message for a bearer that does not use an SeNB. They may also be notified separately from the HO request acceptance response message for a bearer that does not use an SeNB.

[0316] The HO request acceptance response message or the HO instruction message may include information indicating whether or not there is a change in the RRC settings of the SeNB. The S-MeNB may acquire the information and, if it determines that there is a change, may further acquire the RRC settings of the SeNB. This simplifies the processing when there is no change.

[0317] If there is no change in the RRC settings of the SeNB, the setting information does not need to be included in the HO command message, which makes it possible to reduce the amount of information to be signaled.

[0318] Alternatively, the HO command message may include configuration information related to the RRC for the SeNB, which enables the S-MeNB to compare the configuration information related to the RRC for the SeNB acquired from the T-MeNB with the configuration information related to the RRC for the SeNB held by its own cell, thereby enabling reliable verification that there are no changes.

[0319] The S-MeNB, which has received the HO request acceptance response message in Step ST1203, can recognize that the HO is performed while maintaining the SeNB for the UE in dual connectivity with the T-MeNB.

[0320] The S-MeNB, which recognizes that HO will be performed while maintaining the SeNB, does not request the SeNB to change the split bearer configuration, nor does it request a change in settings related to RRC for the SeNB.

[0321] A UE in dual connectivity may be configured not to change the bearer split configuration due to SeNB activation during inter-MeNB HO. If a change in the bearer split configuration due to SeNB activation occurs during HO, the process will be performed when the MeNB changes from S-MeNB to T-MeNB, which complicates control and increases the possibility of malfunction. By not changing the bearer split configuration due to SeNB activation during HO, it is possible to reduce malfunction.

[0322] A method for preventing SeNB-initiated bearer splitting configuration changes during HO is disclosed below.

[0323] The T-MeNB notifies the SeNB of information indicating that HO will be activated. This notification may be made when the T-MeNB transmits an HO request acceptance response message to the S-MeNB. Alternatively, the S-MeNB that receives the HO request acceptance response message from the T-MeNB may notify the SeNB of information indicating that HO will be activated.

[0324] Upon receiving the information indicating that HO has been initiated, the SeNB does not initiate a request to change the bearer splitting configuration until the HO is completed.

[0325] Alternatively, during the HO, the S-MeNB or T-MeNB may notify the SeNB of a NACK (or rejection) in response to the bearer splitting configuration change request from the SeNB. Reason information may be included, and the reason may be set to indicate that the HO is in progress.

[0326] In the processes of FIGS. 20 to 22, in step ST1204, these processes are performed among the T-MeNB, the S-MeNB, and the SeNB.

[0327] The method by which the SeNB recognizes the completion of HO is disclosed below. The T-MeNB may notify the SeNB of information indicating that HO has been completed. This notification may be made when the T-MeNB receives a path switch request acceptance response message from the MME during the HO process. Alternatively, the completion of the MeNB change process, which will be described later, may be regarded as the completion of HO.

[0328] This prevents the SeNB from changing the bearer splitting configuration for a UE during HO processing for the UE in dual connectivity, thereby reducing malfunctions in the system.

[0329] Another method will be disclosed. It has been disclosed that when HO is performed while the SeNB is maintained, the S-MeNB does not request the SeNB to change the split bearer configuration or the setting related to RRC for the SeNB. However, as another method, when HO is performed while the SeNB is maintained, the S-MeNB may notify the SeNB that HO is performed while the SeNB is maintained. Alternatively, the S-MeNB may notify the SeNB that a request to change the split bearer configuration or a request to change the setting related to RRC for the SeNB will not be made. This may be notified together with information indicating that the above-mentioned HO is activated.

[0330] This allows the SeNB to recognize whether or not the HO is to be performed while maintaining the SeNB during the HO. In the case of the HO while maintaining the SeNB, the SeNB can prevent the SeNB from changing the bearer splitting configuration for the UE activated by the SeNB during the HO. This makes it possible to obtain the same effects as described above.

[0331] In Step ST1205, the S-MeNB that has received the HO request acceptance response message notifies the UE of an RRC Connection Reconfiguration message including MCI (Mobility Control Information) in order to cause the T-MeNB to perform HO. Furthermore, the S-MeNB that has received the HO request acceptance response message may notify the UE that there is no change in bearer splitting using the SeNB.

[0332] The S-MeNB notifies the UE that there is no change in the SeNB for E-RAB that performs the split bearer during the HO. It may also notify that there is no change in the RRC configuration for the SeNB. This notification may be included in the RRC connection reconfiguration message that includes the MCI.

[0333] If necessary, the S-MeNB may notify the UE of the configuration information related to RRC for the SeNB if the S-MeNB is aware of the configuration information. The UE can compare the received configuration information related to RRC for the SeNB to which the UE is connected with the received configuration information, and can verify that there are no changes.

[0334] The UE that has received the RRC connection reconfiguration message including the MCI in Step ST1205 performs HO from the S-MeNB to the T-MeNB while maintaining synchronization and connection with the SeNB. The UE performs HO while maintaining radio resources with the SeNB. For example, the UE does not reset the MAC setting for the SeNB, or does not reconfigure the RLC setting for the SeNB. In this state, the UE is able to communicate with the SeNB.

[0335] The configuration for the MeNB follows the configuration information related to RRC for the MeNB in the RRC connection reconfiguration message including the MCI. Since PDCP is located in the MeNB, the PDCP configuration may follow the configuration for the MeNB. For example, the configuration for the S-MeNB is reconfigured to the configuration for the T-MeNB. In the RRC connection reconfiguration message including the MCI, the configuration information related to RRC for the MeNB and the configuration information related to RRC for the SeNB may be separately provided so that they can be configured separately. This allows the UE to reconfigure, maintain, or transition to default configurations for the SeNB and the MeNB separately. In this example, the UE can reconfigure the configuration for the MeNB while maintaining the configuration for the SeNB, so that it is possible to change from an S-MeNB to a T-MeNB while maintaining connection to the SeNB.

[0336] A method for handling uplink data from a UE to an SeNB is disclosed below. Even when performing HO while maintaining connection with the SeNB, the UE does not recognize whether the SeNB is connected to an S-MeNB or a T-MeNB. During HO processing, if the UE transmits uplink data to the SeNB when the SeNB is not yet connected to the T-MeNB, the SeNB cannot transmit the uplink data to the T-MeNB, resulting in loss of the uplink data. Here, a method for solving such a problem is disclosed.

[0337] After receiving MCI from the S-MeNB, the UE stops transmitting uplink data and stores it in a buffer. Furthermore, the UE stops transmitting uplink data and stores it in a buffer until RRC connection reconfiguration with the T-MeNB is completed. The UE transmits uplink data after being notified of an RRC connection reconfiguration complete message with the T-MeNB. The UE may stop transmitting new uplink data after receiving MCI from the S-MeNB and store it in a buffer. The uplink data may be all uplink data transmitted from the UE. The UE may stop transmitting uplink data before sorting the uplink data to be transmitted via a direct path to the T-MeNB and the uplink data to be transmitted via a path to the T-MeNB via the SeNB, and store it in a buffer. Alternatively, even if the uplink data to be transmitted via a direct path to the T-MeNB and the uplink data to be transmitted via a path to the T-MeNB via the SeNB are sorted within the UE, both may be stopped and stored in a buffer. The UE may perform uplink data processing by numbering the PDCP SN, which can solve the problem of uplink data loss.

[0338] In Step ST1205, the UE receives the MCI instructing HO to the T-MeNB from the S-MeNB, and stops transmitting uplink data and stores it in a buffer in Step ST1206. In Step ST1207, the UE performs a process of changing connection from the S-MeNB to the T-MeNB in accordance with the MCI.

[0339] In Step ST1207, after receiving the MCI, the UE disconnects from the S-MeNB and connects to the T-MeNB. The UE reconfigures the RRC connection for the T-MeNB by using the content received in the RRC connection reconfiguration message in Step ST1205, and connects to the T-MeNB.

[0340] In Step ST1212, the UE performs RA processing with the T-MeNB, and in Step ST1213, notifies the T-MeNB of an RRC connection reconfiguration complete message.

[0341] After the UE receives the RRC connection reconfiguration completion message from the T-MeNB, the UE is able to perform data communication directly with the T-MeNB.

[0342] After receiving an RRC connection reconfiguration completion message with the T-MeNB, the UE transmits uplink data. The uplink data is divided in the UE into a path for direct transmission to the T-MeNB and a path for transmission to the T-MeNB via the SeNB, and the UE transmits the uplink data to both the T-MeNB and the SeNB.

[0343] In Step ST1219, the UE transmits uplink data via a path directly to the T-MeNB. In Step ST1220, the UE transmits the uplink data to the SeNB. The uplink data may be transmitted in accordance with the PDCP SN.

[0344] The UE or SeNB may perform synchronization processing between the UE and the SeNB as necessary while the UE is undergoing HO from the S-MeNB to the T-MeNB. Alternatively, the UE may perform the synchronization processing before starting uplink data transmission to the SeNB. RA processing may also be used.

[0345] The UE may continue transmission processing of uplink data to the S-MeNB that was being transmitted and received with the SeNB before receiving the MCI, i.e., before the transmission of uplink data was stopped. The UE may also continue transmission processing of uplink data that was being transmitted directly with the S-MeNB until the transmission is unsuccessful. This is done using the overhead compression setting notified by the S-MeNB.

[0346] The UE may start transmitting to the T-MeNB, starting with data whose transmission failed among the uplink data that was transmitted and received between the S-MeNB directly or via the SeNB before receiving the MCI, i.e., before the transmission of the uplink data was stopped. The UE may start transmitting to the T-MeNB, starting with the data whose transmission failed and has the smallest PDCP SN. The UE may transmit this data to the T-MeNB directly or via the SeNB.

[0347] This method minimizes data loss. Furthermore, since this method requires less buffer space for uplink data in the SeNB, it is possible to reduce the buffer capacity of the SeNB and simplify its configuration. This makes it possible to reduce the cost of the SeNB.

[0348] Transmission to the T-MeNB may use the header compression setting set by the T-MeNB, both in the case of transmission to the T-MeNB directly and in the case of transmission to the T-MeNB via an SeNB.

[0349] In a path for transmitting uplink data via the SeNB to the T-MeNB, the SeNB may not yet be connected to the T-MeNB. In such a case, the SeNB may store the uplink data from the UE in a buffer until the MeNB configuration change with the T-MeNB is completed. After the MeNB configuration change with the T-MeNB is completed, the SeNB transmits the uplink data from the UE to the T-MeNB. The SeNB may reorder the data using the PDCP SN and transmit it.

[0350] This solves the problem of data loss occurring in the SeNB even when a MeNB setting change between the SeNB and the T-MeNB is delayed for some reason and uplink data transmission from the UE to the SeNB occurs.

[0351] Another method for solving the problem of uplink data loss is disclosed. The UE transmits uplink data to the SeNB without waiting for connection with the T-MeNB to be established. The SeNB stores the uplink data from the UE in a buffer until the MeNB configuration change with the T-MeNB is completed. After the MeNB configuration change with the T-MeNB is completed, the SeNB transmits the uplink data from the UE to the T-MeNB. The SeNB may reorder the data using the PDCP SN of the data before transmitting it. This solves the problem of data loss in the SeNB.

[0352] In this case, the amount of buffering required in the SeNB is larger than in the above-mentioned method, but when the MeNB configuration is changed between the SeNB and the T-MeNB, it becomes possible to immediately transmit the buffered uplink data from the SeNB to the T-MeNB, so that the uplink data from the UE can be delivered to the T-MeNB early.

[0353] A specific example in which the UE transmits uplink data to the SeNB before notifying the T-MeNB of connection completion will be described.

[0354] After the UE receives the MCI from the S-MeNB, it transmits new uplink data to the T-MeNB via the SeNB. The T-MeNB notifies the UE of the header compression setting via the S-MeNB, and the UE uses the header compression setting for new uplink communication with the T-MeNB via the SeNB.

[0355] In this case, the UE may stop transmitting uplink data to the S-MeNB that was being performed with the SeNB before receiving the MCI. Data whose uplink data transmission to the S-MeNB has failed may also be transmitted to the SeNB with header compression set by the T-MeNB. The UE may start transmitting data with the smallest PDCP SN from among the data whose transmission has failed to the T-MeNB via the SeNB.

[0356] Alternatively, the UE may transmit uplink data to both the S-MeNB and the T-MeNB between the UE and the SeNB. The SeNB may determine which header compression is used to determine which direction the uplink data is destined for. Alternatively, as another method, information indicating which direction the uplink data is destined for may be included in or added to the uplink data from the UE to the SeNB and transmitted. For example, the information may be included as one bit. As a method for setting one bit, for example, when the SeNB transmits and receives only user plane data, the D / C bit of the PDCP format may be set as the bit for the information.

[0357] The SeNB buffers uplink data from the UE until the MeNB configuration change with the T-MeNB is completed, and after the MeNB configuration change with the T-MeNB is completed, the SeNB transmits the uplink data from the UE to the T-MeNB. The SeNB may reorder and transmit the data using the PDCP SN of the data.

[0358] The following describes uplink data received by the S-MeNB from the UE. The uplink data received by the S-MeNB from the UE, including both uplink data received directly from the UE and uplink data received via the SeNB, may be managed by the PDCP SN. The S-MeNB reorders the data according to the PDCP SN and transmits it to the S-GW.

[0359] When the S-MeNB transmits MCI to the UE, uplink data being communicated directly with the UE when the MCI is transmitted may be transmitted from the S-MeNB to the S-GW after successful delivery of the data to the S-MeNB. Similarly, uplink data being communicated with the UE via the SeNB when the MCI is transmitted may be transmitted to the S-GW after successful delivery of the data to the S-MeNB. The S-GW may perform order management of the data received from the S-MeNB and the data received from the T-MeNB.

[0360] When the UE receives the MCI, if the data that the UE judged to have been transmitted has not been successfully delivered to the S-MeNB for some reason, the UE does not transmit the data to the T-MeNB, which can prevent data loss. Also, instead of transmitting from the S-MeNB to the S-GW, the data may be forwarded to the T-MeNB. The T-MeNB may reorder the data according to the PDCP SN and transmit it to the S-GW. This provides the same effect as the method of transmitting to the S-GW.

[0361] In addition, it may be possible to configure whether the S-MeNB transmits the data to the S-GW or forwards it to the T-MeNB. The subject of configuration may be a node on the RAN side or the core network side. The configuration information may be notified to the S-MeNB in advance. For example, the MME or the S-GW may notify the S-MeNB of the configuration information in advance.

[0362] Furthermore, when the S-MeNB transmits MCI to the UE, the data that was successfully delivered after the immediately preceding data that failed to be delivered may be transmitted to the S-GW or forwarded to the T-MeNB (forwarding). In the UE, uplink transmission is performed from the data that failed to be delivered to the T-MeNB. If the UE fails to deliver the data following that data to the T-MeNB after completing HO, the forwarded data can be used.

[0363] The method for handling downlink data notified from the MeNB to the UE via the SeNB will be disclosed below.

[0364] The S-MeNB that has received the HO request acceptance response message in step ST1203 stops transmission of new downlink data and stores the data in a buffer. The downlink data may be all downlink data to be transmitted to the UE for which HO is performed. The S-MeNB may stop transmission of the downlink data to be transmitted via a direct path to the UE and downlink data to be transmitted via a path to the UE via the SeNB before sorting the data and store the data in a buffer. Alternatively, even if the downlink data to be transmitted via a direct path to the UE and the downlink data to be transmitted via a path to the UE via the SeNB are sorted within the S-MeNB, the S-MeNB may stop transmission of both types of data and store the data in a buffer. PDCP SN numbering may be performed as part of the downlink data processing in the S-MeNB.

[0365] The S-MeNB forwards (forwards) to the T-MeNB the downlink data that was transmitted and received with the UE before receiving the HO request acceptance response message, i.e., before data transmission was stopped, including the downlink data that was not delivered.

[0366] Of the downlink data that has not been delivered, it is advisable to forward it to the T-MeNB in order of the data with the smallest PDCP SN. It is advisable to forward it to the T-MeNB in order of the data with the smallest PDCP SN, including the downlink data transmitted via a direct path from the S-MeNB to the UE and the downlink data transmitted via a path via the SeNB.

[0367] In Step ST1208, the S-MeNB starts transfer (forwarding) to the T-MeNB, performs SN status transfer in Step ST1209, and performs data forwarding in Step ST1210. The transfer is managed by the PDCP SN, and data is transferred up to the end marker from the S-GW notified in Step ST942 and Step ST944.

[0368] In Step ST1211, the T-MeNB stores the data forwarded from the S-MeNB in a buffer.

[0369] By doing so, even if the UE has dual connectivity using the SeNB, it is possible to prevent downlink data loss during HO processing.

[0370] The T-MeNB stores downlink data in a buffer until the direct RRC connection reconfiguration with the UE is completed and until the MeNB change procedure in the SeNB is completed. The T-MeNB may buffer downlink data via a direct path with the UE and downlink data via a path via the SeNB before or after separating them using bearer splitting. Either can be managed by the numbered PDCP SN. Reordering can also be performed in the UE using the PDCP SN.

[0371] In the case of the method of storing data in a buffer before separation, when the bearer splitting configuration using the SeNB is changed, after the new bearer splitting configuration is set for both paths, scheduling for data separation suited to the new bearer splitting configuration is possible, thereby enabling efficient scheduling for dual connectivity UEs.

[0372] On the other hand, in the case of the method of storing the data in a buffer after separation, when both paths become available, it is possible to immediately transmit the downlink data to the UE using both paths, which means that an increase in delay can be prevented.

[0373] When a direct connection with the UE is completed and when the MeNB change process in the SeNB is completed, the T-MeNB schedules the downlink data separately for each path and transmits the downlink data to the UE using each path.

[0374] Another method is disclosed. The T-MeNB starts downlink data transmission to the UE upon completion of direct connection with the UE or completion of MeNB change notification with the SeNB, whichever occurs first.

[0375] If the T-MeNB buffers the data before splitting it, it will send downstream data over the earlier path until both paths are established, and then start bearer splitting when both paths are established.

[0376] When the T-MeNB stores the data in a buffer after separating the data, it is preferable to transmit the downlink data in accordance with the establishment of each path.

[0377] In this way, downlink data can be transmitted to the UE as soon as possible.

[0378] The SeNB is notified by the MeNB of the bearer splitting configuration for dual connectivity for the UE, and the MeNB notifies the SeNB of the RRC-related configuration information for the UE.

[0379] Therefore, the SeNB needs to recognize from which MeNB the bearer splitting configuration has been notified, to which MeNB the setting information related to the RRC set by the SeNB should be notified, etc. In other words, the SeNB needs to recognize the MeNB.

[0380] In addition, the SeNB performs data communication for UEs that have dual connectivity between MeNBs. The SeNB needs to recognize which MeNB to transmit data from to the UE and which MeNB to transmit data from the UE to. In other words, the SeNB needs to recognize the MeNB.

[0381] Although the former MeNB and the latter MeNB may be configured separately, the case where they are the same will be described here. This MeNB is referred to as a controlling MeNB of the SeNB.

[0382] In the HO process of a UE during dual connectivity, when the SeNB is not changed but the MeNB is changed from an S-MeNB to a T-MeNB, a method of recognizing which MeNB has been changed to as the controlling MeNB of the SeNB will be disclosed below. As specific examples, the following two methods (1) and (2) will be disclosed. (1) The S-MeNB notifies the SeNB of a change in the controlling MeNB. (2) The T-MeNB notifies the SeNB of the change of the controlling MeNB.

[0383] A specific example of the specific example (1) will be disclosed. When the T-MeNB receives an RRC connection reconfiguration complete message from the UE, the T-MeNB notifies the S-MeNB of a controlling MeNB change request message from the SeNB. The notification may be performed using X2 signaling. The notification may include an identifier of the SeNB that changes the controlling MeNB. The notification may also include information on the reason for the change. The notification may include information indicating that the MeNB has been changed due to HO.

[0384] Upon receiving the notification, the S-MeNB notifies the SeNB that will change the Controlling MeNB of the change. The notification may be performed using X2 signaling or signaling on an interface between the MeNB and the SeNB.

[0385] As specific examples of information to be included in the signaling, the following seven items (1) to (7) are disclosed. (1) Information indicating a change in the controlling MeNB. (2) Identifier of the controlling MeNB after the change. Here, the identifier of the T-MeNB. (3) A bearer identifier of a path using the SeNB, which may be an E-RAB identifier (which may be an E-RAB ID, etc.) or an EPS bearer identifier. (4) Identifier of the split bearer that changes the controlling MeNB in the SeNB. (5) Identifier of the MeNB that requested the bearer split configuration. Here, the identifier of the S-MeNB. (6) Identifier of the UE that performs dual connectivity using bearer splitting. (7) A combination of (1) to (6) above.

[0386] Upon receiving the controlling MeNB change notification message, the SeNB identifies the split bearer for which the MeNB is to be changed from the information included in the change notification message. The SeNB changes the controlling MeNB of the split bearer to the changed MeNB. The management of the split bearer in the SeNB should be performed in association with the identifier of the controlling MeNB. In other words, the identifier of the split bearer should be associated with the identifier of the controlling MeNB. In this way, the SeNB can change the controlling MeNB of the split bearer for the UE that performs dual connectivity.

[0387] After the controlling MeNB is changed, the SeNB shall accept a request to modify or release the split bearer only from the changed controlling MeNB, and shall perform data communication with the changed MeNB.

[0388] By doing this, the SeNB can recognize which MeNB the controlling MeNB has been changed to, and can perform communication for control purposes such as modifying the split bearer for the UE and requesting its release, as well as data communication, with the changed controlling MeNB.

[0389] The SeNB that has changed the controlling MeNB may notify the S-MeNB of a controlling MeNB change response message. The message may include information indicating that the controlling MeNB has been changed.

[0390] The S-MeNB that receives the controlling MeNB change response message from the SeNB may notify the changed controlling MeNB, here the T-MeNB, of a message indicating that the controlling MeNB change of the SeNB has been completed.

[0391] By doing this, the changed MeNB, in this case the T-MeNB, can recognize that the controlling MeNB of the SeNB has been changed, and can start communication with the SeNB for control purposes such as modifying the split bearer for the UE, requesting release, and data communication.

[0392] After the change, the MeNB may notify the SeNB of a message confirming that the controlling MeNB of the split bearer for the UE has changed, and the SeNB may respond to the message.

[0393] A specific example of the specific example (2) will be disclosed. When the T-MeNB receives an RRC connection reconfiguration complete message from the UE, it notifies the SeNB of a controlling MeNB change request message. The SeNB identifier may be the SeNB information included in the HO request message received from the S-MeNB. The notification may be performed using X2 signaling. Alternatively, the notification may be performed using signaling on an interface between the MeNB and the SeNB. The information included in the signaling may be the same as that disclosed in the method of the specific example (1).

[0394] The processing in the SeNB that receives the controlling MeNB change request message from the T-MeNB is the same as in the specific example (1) above, and therefore the description thereof will be omitted.

[0395] By doing this, the SeNB can recognize which MeNB the controlling MeNB has been changed to, and can perform communication for control purposes such as modifying the split bearer for the UE and requesting its release, as well as data communication, with the changed controlling MeNB.

[0396] The SeNB that has changed the controlling MeNB may notify the T-MeNB of a controlling MeNB change response message. The message may include information indicating that the controlling MeNB has been changed.

[0397] The T-MeNB that receives the controlling MeNB change response message from the SeNB may notify the previous controlling MeNB, here the S-MeNB, of a message indicating that the controlling MeNB change of the SeNB has been completed.

[0398] By doing this, the MeNB before the change, in this case the S-MeNB, can recognize that the controlling MeNB of the SeNB has been changed, and can terminate communication with the SeNB for control purposes such as modifying the split bearer for the UE, requesting release, and data communication.

[0399] In addition, when HO of a UE in dual connectivity is performed using an SeNB, the SeNB can perform communication with the changed MeNB (T-MeNB) for control of the split bearer configuration and data communication.

[0400] Therefore, data communication can be performed between the UE and the T-MeNB via the SeNB.

[0401] A specific example of the MeNB change process of the SeNB will be described. In Step ST1214, the T-MeNB that has received the RRC connection reconfiguration complete message from the UE in Step ST1213 of Fig. 21 notifies the S-MeNB of a change request message for the SeNB to change the controlling MeNB. The message includes an identifier of the SeNB that will change the controlling MeNB.

[0402] In Step ST1215, the S-MeNB that has received this message notifies the SeNB that will change the controlling MeNB of a message requesting change of the controlling MeNB. The message includes information indicating the change of the controlling MeNB, an identifier of the T-MeNB, a bearer identifier of the path using the SeNB, an identifier of the split bearer in the SeNB for changing the controlling MeNB, an identifier of the S-MeNB, and an identifier of the UE that is the target of HO for performing dual connectivity using the SeNB. In this way, the SeNB can recognize for which split bearer established by which MeNB the controlling MeNB should be changed.

[0403] In Step ST1216, the SeNB that has received the message changes the controlling MeNB by using the information received in Step ST1215. As a result, the SeNB thereafter performs control communication and data communication regarding the split bearer with the T-MeNB that is the MeNB after the change. The SeNB that has changed the controlling MeNB ends control communication and data communication with the controlling MeNB before the change.

[0404] In Step ST1217, the SeNB that has changed the controlling MeNB notifies the S-MeNB of a controlling MeNB change response message to notify that the controlling MeNB has been changed. In Step ST1218, the S-MeNB that has received the response message notifies the T-MeNB of the SeNB of a controlling MeNB change response message to notify that the change of the controlling MeNB of the SeNB has been completed. This allows the T-MeNB to recognize that the MeNB of the SeNB has been changed to the T-MeNB. Therefore, the T-MeNB becomes able to perform communication for control of the split bearer and data communication with the SeNB.

[0405] As a result, in steps ST1220 and ST1221, both downlink data from the T-MeNB to the UE and uplink data from the UE to the T-MeNB can be communicated via the SeNB.

[0406] Step ST1214 to step ST1218 shown in Fig. 12 are designated as step ST1222. Step ST1222 shows the MeNB change process of the SeNB performed in the T-MeNB, S-MeNB, and SeNB.

[0407] The subsequent path switching process in the S-GW for changing the MeNB from step ST939 to step ST947 is the same as that in FIG. 12, and therefore description thereof will be omitted.

[0408] This enables data communication between the S-GW and the UE that was the target of HO via the T-MeNB, or via the T-MeNB and the SeNB.

[0409] Regarding data communication between the S-GW and the UE, one is that data is directly communicated between the S-MeNB and the UE in Step ST1005, and the other is that data is communicated between the S-MeNB and the UE via the SeNB in Steps ST1007 and ST1008. In Step ST1006, data is communicated between the S-MeNB and the S-GW via one path.

[0410] By using the method disclosed in this embodiment, a UE in dual connectivity using bearer splitting can perform inter-MeNB HO.

[0411] Another method will be disclosed for step ST1222 of the MeNB change process to an SeNB. In the method disclosed above, the MeNB change process to an SeNB is performed after the T-MeNB receives an RRC connection reconfiguration complete message from the UE.

[0412] As another method, the MeNB change process to the SeNB may be performed after the T-MeNB notifies the S-MeNB of the HO request acceptance response message, or after the S-MeNB receives the HO request acceptance response message from the T-MeNB, or after the S-MeNB transmits an RRC connection reconfiguration message including the MCI to the UE. The MeNB change process to the SeNB may be performed by applying the above-mentioned method.

[0413] By doing so, it becomes possible to change the controlling MeNB to the T-MeNB in the SeNB early, and therefore it becomes possible to perform data communication between the SeNB and the T-MeNB early.

[0414] Therefore, the UE does not need to stop transmitting or buffer uplink data until the RRC connection reconfiguration with the T-MeNB is completed. Alternatively, the UE does not need to stop transmitting or buffer uplink data at the SeNB. The UE can transmit uplink data to the T-MeNB via the SeNB early by using a split bearer with the SeNB.

[0415] In addition, the T-MeNB does not need to stop transmitting or buffer downlink data until it receives the RRC connection reconfiguration completion from the UE. The T-MeNB can quickly transmit uplink data to the UE via the SeNB by using a split bearer with the SeNB.

[0416] This means that even if data communication via a direct path between the UE and the T-MeNB is not yet performed, communication between the UE and the T-MeNB is possible using a path via the SeNB. This enables data communication processing during HO to be performed with low latency, and data loss during HO can be reduced.

[0417] Another method will be disclosed for step ST1222 of the MeNB change processing to the SeNB. The T-MeNB may perform the MeNB change processing to the SeNB after completing the path switch processing to be performed on the MME and S-GW. When the T-MeNB receives a path switch request acceptance response message from the MME, the T-MeNB notifies the SeNB of the change of the MeNB.

[0418] Alternatively, the S-MeNB may notify the SeNB of the change of MeNB when it receives a UE context release message from the T-MeNB. The MeNB change process to the SeNB may be performed using the above-mentioned method.

[0419] In this way, it becomes possible to perform data communication by a split bearer using the SeNB after the HO process, including the path switching from the MME and S-GW to the T-MeNB, is completed. By performing the data communication after the HO process is completed reliably, it becomes possible to avoid complicating the control of data communication.

[0420] Embodiment 4 23 to 25 are diagrams showing an example of a sequence of handover-related processing in a communication system according to the fourth embodiment. Fig. 23 and Fig. 24 are connected by a boundary line BL5. Fig. 24 and Fig. 25 are connected by a boundary line BL6. The handover-related processing of this embodiment is similar to the handover-related processing of the first embodiment shown in Figs. 10 to 13, the second embodiment shown in Figs. 17 to 19, and the third embodiment shown in Figs. 20 to 22 described above. Therefore, the same steps are given the same step numbers, and descriptions thereof will be omitted.

[0421] In this embodiment, the method of handling downlink data is different from that in the above-described embodiment 3. In this embodiment, the method of handling downlink data notified from the MeNB to the UE via the SeNB will be disclosed below.

[0422] In this embodiment, the S-MeNB that has received the HO request acceptance response message in step ST1203 stops transmitting new downlink data and stores the data in a buffer. The downlink data may be all downlink data to be transmitted to the UE for which HO is to be performed. The S-MeNB stops transmitting the downlink data after sorting it into downlink data to be transmitted via a direct path to the UE and downlink data to be transmitted via a path to the UE via the SeNB, and stores the data in a buffer, and does not stop transmitting the downlink data to be transmitted via a path to the UE via the SeNB. PDCP SN numbering may be performed as part of the downlink data processing in the S-MeNB.

[0423] The S-MeNB forwards (forwards) to the T-MeNB downlink data that was transmitted and received with the UE before receiving the HO request acceptance response message, i.e., before data transmission was stopped, including the downlink data that was unsuccessfully delivered. Of the downlink data that was unsuccessfully delivered, the S-MeNB should forward (forward) data with the smallest PDCP SN to the T-MeNB. Of the downlink data transmitted via a direct path from the S-MeNB to the UE, the S-MeNB should forward (forward) data with the smallest PDCP SN to the T-MeNB.

[0424] In Step ST1301, the S-MeNB starts transfer (forwarding) to the T-MeNB, performs SN status transfer in Step ST1209, and performs data forwarding in Step ST1210. The transfer is managed by the PDCP SN, and data is transferred up to the end marker from the S-GW notified in Step ST942 and Step ST944.

[0425] The subsequent steps are executed in the same manner as in Embodiment 3. Specifically, in Step ST1211, the T-MeNB stores the data transferred (forwarded) from the S-MeNB in a buffer.

[0426] By doing so, even if the UE has dual connectivity using the SeNB, it is possible to prevent downlink data loss during HO processing.

[0427] In this embodiment, the T-MeNB stores downlink data in a buffer until the direct RRC connection reconfiguration with the UE is completed and until the MeNB change process in the SeNB is completed. The T-MeNB may separate downlink data via a direct path with the UE and downlink data via a path via the SeNB by bearer splitting and then store the separated data in a buffer. The data can be managed using numbered PDCP SNs. Reordering can also be performed in the UE using the PDCP SNs.

[0428] In the case of the method of storing data in a buffer before separation, when the bearer splitting configuration using the SeNB is changed, after the new bearer splitting configuration is set for both paths, scheduling for data separation suited to the new bearer splitting configuration is possible, thereby enabling efficient scheduling for dual connectivity UEs.

[0429] On the other hand, in the case of the method of storing the data in a buffer after separation, when both paths become available, the downlink data can be immediately transmitted to the UE using both paths, which means that an increase in delay can be prevented.

[0430] The subsequent steps are executed in the same manner as in embodiment 3. When a direct connection with the UE is completed and when the MeNB change process in the SeNB is completed, the T-MeNB schedules downlink data separately for each path and transmits the downlink data to the UE using each path.

[0431] Another method is disclosed. The T-MeNB starts downlink data transmission to the UE upon completion of direct connection with the UE or completion of MeNB change notification with the SeNB, whichever occurs first.

[0432] In this way, downlink data can be transmitted to the UE as soon as possible.

[0433] Embodiment 5. 26 to 28 are diagrams showing an example of a sequence of handover-related processes in a communication system according to Embodiment 5. Fig. 26 and Fig. 27 are connected by a boundary line BL7. Fig. 27 and Fig. 28 are connected by a boundary line BL8.

[0434] In this embodiment, before handover from an S-MeNB to a T-MeNB is performed, both the SeNB and the S-MeNB transmit and receive data to and from the UE using bearer 2 (hereinafter may be referred to as "EPS bearer #2") as a bearer corresponding to EPS (hereinafter may be referred to as "EPS bearer"). During handover processing, only the SeNB transmits and receives data to and from the UE. After handover processing, both the SeNB and the T-MeNB transmit and receive data to and from the UE.

[0435] As an example, a UE that transmits and receives data between both an SeNB and an S-MeNB using EPS bearer #2 will be described. When this UE performs handover from an S-MeNB to a T-MeNB, immediately before the handover, EPS bearer #2 is entirely migrated so as to pass only through the RLC / MAC / PHY of the SeNB, without passing through the RLC / MAC of the S-MeNB. Alternatively, EPS bearer #2, including GTPu / PDCP, is entirely migrated so that all of its lower layers below the S-GW pass only through the RLC / MAC / PHY of the SeNB, without passing through the S-MeNB. Alternatively, EPS bearer #2 is entirely migrated so as to pass all of the GTPu / PDCP / RLC / MAC / PHY of the SeNB, instead of being entirely migrated so as to pass only through the RLC / MAC / PHY of the SeNB. In the following description, this complete migration of bearers to pass through the SeNB may be referred to as "complete migration to the SeNB."

[0436] In step ST2001 of FIG. 26, the S-MeNB, the T-MeNB, the MME, and the S-GW provide an area restriction (Area Restriction Provided).

[0437] Steps ST2002 to ST2011 for determining HO are performed in the same manner as in the fourth embodiment. Specifically, in step ST2002, the S-MeNB notifies the UE of a measurement control message. Measurements of neighboring SeNBs may be configured in the measurement control message. Alternatively, measurements of frequencies for the SeNB may be configured. Furthermore, as measurement configuration, events or event criteria for the SeNB or frequencies for the SeNB may be configured separately from the MeNB.

[0438] The setting parameters include an SeNB identifier, a frequency, an event number for reporting, a reception quality threshold, a measurement period, etc. The reception quality includes RSRP (Reference Signal Received Power), RSRQ (Reference Signal Received Quality), etc.

[0439] The UE that has received the measurement control message in Step ST2002 measures neighboring cells, that is, MeNBs and SeNBs.

[0440] In Step ST2003, packet data is communicated between the UE and the S-MeNB, and in Step ST2004, packet data is communicated between the S-MeNB and the S-GW.

[0441] If EPS bearer #2 is bearer split, in Step ST2005, packet data is communicated directly between the UE and the S-MeNB, and in Step ST2006, packet data is communicated directly between the S-MeNB and the S-GW.

[0442] Furthermore, in steps ST2007 and ST2008, packet data is communicated between the UE and the S-MeNB via the SeNB. Specifically, in step ST2007, packet data is communicated between the UE and the SeNB, and in step ST2008, packet data is communicated between the SeNB and the S-MeNB.

[0443] In Step ST2009, the S-MeNB notifies the UE of uplink allocation information. In Step ST2010, the UE notifies the S-MeNB of a measurement report message.

[0444] In Step ST2011, the S-MeNB that has received the measurement report message uses the result of the measurement report to decide whether or not to perform handover (HO) of the UE to the T-MeNB. In the example shown in FIG. 26, the S-MeNB decides in Step ST2011 to perform HO of the UE to the T-MeNB.

[0445] When the S-MeNB decides to HO the UE to the T-MeNB in Step ST2011, it proceeds to Step ST2012. In Step ST2012, the S-MeNB decides whether to modify the EPS bearer #2 for full handover to the SeNB (hereinafter, may be referred to as "modification for full handover of EPS bearer #2 to the SeNB"). In the example shown in FIG. 26, the S-MeNB decides to modify the EPS bearer #2 for full handover to the SeNB.

[0446] In step ST2013, the S-MeNB performs a process to confirm whether EPS bearer #2 full migration is possible with respect to the SeNB. Here, the process to confirm whether EPS bearer #2 full migration is possible with respect to the SeNB refers to a process of confirming with the SeNB whether EPS bearer #2 full migration is possible with respect to the SeNB, that is, whether EPS bearer #2 can be fully migrated to the SeNB. Specifically, as the process to confirm whether EPS bearer #2 full migration is possible with respect to the SeNB, the S-MeNB notifies the SeNB of a full migration confirmation signal to confirm whether EPS bearer #2 can be fully migrated to the SeNB. The SeNB that has received the full migration confirmation signal notifies the S-MeNB of whether EPS bearer #2 can be fully migrated to the SeNB.

[0447] In Step ST2014, the S-MeNB determines whether or not total transition to the SeNB is possible, based on the possibility of total transition notified by the SeNB. If it is determined that total transition to the SeNB is possible, the S-MeNB proceeds to Step ST2015, and if it is determined that total transition to the SeNB is not possible, the S-MeNB proceeds to Step ST2016.

[0448] In Step ST2015, the UE, SeNB, and S-MeNB perform a process of modifying the EPS bearer #2 to the S-MeNB. Specifically, in Step ST2015, a process of performing a complete handover of the EPS bearer #2 to the SeNB is performed.

[0449] In Step ST2016, the S-MeNB performs the HO procedure after releasing the SeNB, similar to Step ST1102 in Fig. 17. In another embodiment, if the S-MeNB has proceeded to Step ST2016 after determining that full handover to the SeNB is not possible, the S-MeNB may perform the HO procedure without releasing the SeNB.

[0450] In Step ST2017, packet data is communicated between the S-MeNB and the S-GW. In Step ST2018, packet data is communicated between the UE and the SeNB. In Step ST2019, packet data is communicated between the SeNB and the S-MeNB.

[0451] In Step ST2020, the S-MeNB notifies the T-MeNB, which is the HO target, of a handover request message. The HO request message includes SeNB information and information related to EPS bearer #2 (hereinafter, may be referred to as "EPS bearer #2 information"). The SeNB information and EPS bearer #2 information may be notified using a message separate from the HO request message.

[0452] In Step ST2021, the T-MeNB determines whether or not a change of SeNB is necessary. If it is determined that a change of SeNB is necessary, the T-MeNB proceeds to Step ST2022, and if it is determined that a change of SeNB is not necessary, the T-MeNB proceeds to Step ST2023.

[0453] In Step ST2022, the T-MeNB releases the SeNB and then performs handover (HO) processing in the same manner as in Step ST1102 of FIG.

[0454] In Step ST2023, the T-MeNB determines whether or not it is necessary to change the bearer configuration. If it is determined that it is necessary to change the bearer configuration, it proceeds to Step ST2025 in FIG. 28, and if it is determined that it is not necessary to change the bearer configuration, it proceeds to Step ST2024.

[0455] In Step ST2024, the SeNB and the T-MeNB perform an MeNB change confirmation process for the SeNB. Here, the MeNB change confirmation process refers to a process for confirming whether or not to change the MeNB from an S-MeNB to a T-MeNB. Specifically, as the MeNB change confirmation process, the T-MeNB notifies the SeNB of an MeNB change confirmation signal for confirming whether or not to change the MeNB from an S-MeNB to a T-MeNB. The SeNB that has received the MeNB change confirmation signal notifies the T-MeNB of whether or not to change the MeNB. The S-MeNB is not involved in the process of Step ST2024.

[0456] In Step ST2025 of FIG. 28, the UE, SeNB, S-MeNB, T-MeNB, MME, and S-GW perform the MeNB HO process for EPS bearer #1. Even when there is NAS signaling via the SeNB, the MeNB HO process for EPS bearer #1 in Step ST2025 is followed. Details of the MeNB HO process for EPS bearer #1 will be described later.

[0457] In Step ST2026, the SeNB, S-MeNB, and T-MeNB perform an MeNB change procedure for the SeNB. This is a procedure for notifying that the control plane (C-plane) of the SeNB, for example, the macro eNB (MeNB) that performs signaling, has been changed after the handover of the MeNB is completed. This procedure is necessary to notify that the MeNB of EPS bearer #2 has been changed to a T-MeNB and to enable data transmission and reception between the T-MeNB and the SeNB. Specifically, in Step ST2026, the S-MeNB notifies the SeNB of a signal indicating that the MeNB has been changed to a T-MeNB.

[0458] In Step ST2027, packet data is communicated between the UE and the SeNB. In Step ST2028, packet data may be communicated between the SeNB and the T-MeNB.

[0459] In Step ST2029, the T-MeNB determines whether to modify the EPS bearer #2 for the full handover of the EPS bearer #2 to the SeNB. In the example shown in FIG. 28, the T-MeNB determines to modify the EPS bearer #2 for the full handover of the EPS bearer #2 to the SeNB.

[0460] As a result, EPS bearer #2 is changed so that it is transmitted and received only from SeNBs that are not involved in handover during the handover process of the MeNB, and is therefore not affected by the handover. This simplifies the handover process, thereby reducing handover failures and data loss.

[0461] In Step ST2030, the UE, SeNB, and T-MeNB perform a process to confirm whether or not the full transfer of the EPS bearer #2 is possible with respect to the SeNB, and a process to correct the full transfer of the EPS bearer #2 to the SeNB. The S-MeNB is not involved in the process of Step ST2030.

[0462] In Step ST2031, packet data is communicated between the UE and the T-MeNB. In Step ST2032, packet data is communicated between the T-MeNB and the S-GW.

[0463] In Step ST2033, packet data is communicated between the UE and the SeNB. In Step ST2034, packet data is communicated between the SeNB and the T-MeNB.

[0464] 29 and 30 are diagrams showing an example of the sequence of the MeNB HO process for the EPS bearer #1 in step ST2025 of FIG.

[0465] In Step ST2041, the T-MeNB performs admission control to check its accommodation capacity in the same manner as in Step ST930 shown in Fig. 12. If the T-MeNB determines that it can accept HO based on the result of the admission control, in Step ST2042, it notifies the S-MeNB of an HO request acceptance response (Handover Request Ack) message in the same manner as in Step ST931.

[0466] In response to the HO request acceptance response message in Step ST2042, in Step ST2043, the SeNB, S-MeNB, and T-MeNB perform MeNB change processing for the SeNB. Here, the MeNB change processing is processing for notifying that the control plane (C-plane) of the SeNB, for example, the macro eNB (MeNB) that performs signaling, has been changed in the data flow from the T-MeNB to the S-MeNB and further from the S-MeNB to the SeNB, or in the data flow from the T-MeNB to the S-MeNB and from the T-MeNB to the SeNB. Specifically, the S-MeNB notifies the SeNB of a signal indicating that the MeNB has been changed to the T-MeNB. The fact that the MeNB has been changed may be notified using an HO request acceptance response (Handover Request Ack) message in Step ST2042.

[0467] In Step ST2044, packet data is communicated between the UE and the S-MeNB. In Step ST2045, packet data is communicated between the S-MeNB and the T-MeNB.

[0468] In Step ST2046, the S-MeNB notifies the UE of downlink allocation information. In Step ST2047, the S-MeNB notifies the UE of an RRC connection reconfiguration message including mobility control information. In Step ST2047, the S-MeNB may notify the UE that there is no change to the SeNB and EPS bearer #2.

[0469] The processes from Step ST2048 to Step ST2055 are the same as those in 3GPP TS36.300. Specifically, in Step ST2048, the UE detaches from the S-MeNB, which is the old cell, and starts synchronization with the T-MeNB, which is the new cell.

[0470] In Step ST2049, the S-MeNB transmits the packets stored in the buffer and the packets being transmitted to the T-MeNB, which is the target eNB.

[0471] In Step ST2050, the S-MeNB performs SN status transfer to transfer the status of the PDCP sequence number (SN) to the T-MeNB, in the same manner as in Step ST912 of FIG. 11. Furthermore, in Step ST2051, the S-MeNB may perform data forwarding to transfer data whose transmission has not been completed to the T-MeNB.

[0472] In Step ST2052, the T-MeNB stores the packet delivered from the S-MeNB in a buffer.

[0473] In Step ST2053, the UE synchronizes with the T-MeNB. In Step ST2054, the T-MeNB notifies the UE of uplink allocation information and a tracking area (TA) for the UE. In Step ST2055, the UE notifies the T-MeNB of an RRC connection reconfiguration complete message.

[0474] In step ST2056 of Figure 30, the SeNB, S-MeNB, and T-MeNB perform a process of changing the MeNB for the SeNB, in the same way as in step ST2026 of Figure 28. This is a process of notifying that the control plane (C-plane) of the SeNB, for example, the macro eNB (MeNB) that performs signaling, has been changed.

[0475] In Step ST2057, packet data may be communicated between the UE and the T-MeNB. In Step ST2058, packet data may be communicated between the UE and the SeNB. In Step ST2059, packet data may be communicated between the SeNB and the T-MeNB. In Step ST2060, the T-MeNB may transmit the packet data to the S-GW.

[0476] In Step ST2671, the T-MeNB, MME, and S-GW make a path switch request to request that the paths of both EPS bearer #1 and EPS bearer #2 be changed from the S-MeNB to the T-MeNB.

[0477] Specifically, in Step ST2061, the T-MeNB notifies the MME of a path switch request message. In Step ST2062, the MME that has been notified of the path switch request message notifies the S-GW of a modify bearer request message.

[0478] In Step ST2063, the S-GW that has been notified of the bearer modification request message changes the downlink path. In Step ST2064, the S-GW may add an end marker to the PDCP to be transmitted to the S-MeNB to notify the end of the forwarding process. In Step ST2066, the S-MeNB may add an end marker and forward the PDCP to the T-MeNB. In Step ST2065, packet data may be communicated between the T-MeNB and the S-GW.

[0479] In Step ST2067, the S-GW notifies the MME of a Modify Bearer Response message. In Step ST2068, the MME that has been notified of the Modify Bearer Response message notifies the T-MeNB of a Path Switch Request Ack message indicating completion of path switching. In this way, the processing of Step ST2671 ends.

[0480] In Step ST2069, the T-MeNB notifies the S-MeNB of a UE context release message. The S-MeNB that has been notified of the UE context release message releases the resources that it has allocated to the UE in Step ST2670. After the resource release process in Step ST2670, the MeNB change process for the SeNB in Step ST2026 in FIG. 28 described above is performed.

[0481] As described above, at the time of handover, the SeNB is configured to transmit and receive data using EPS bearer #2 only to the UE through the process of Step ST2029 in Fig. 28. How the process of Step ST2029 is performed will be described with reference to Fig. 31.

[0482] 31 is a diagram showing an example of the state of data transmission and reception with a UE. The S-GW 601 includes a PDCP processing eNB switching unit 602. The S-MeNB 603 includes a first PDCP processing unit 604, an RLC processing unit 605, a MAC processing unit 606, a PHY processing unit 607, and a second PDCP processing unit 608. The SeNB 609 includes a PDCP processing unit 610, a PDCP path switching unit 611, an RLC processing unit 612, a MAC processing unit 613, and a PHY processing unit 614.

[0483] Before handover switching, UE 615 transmits and receives data using EPS bearer #2 with both S-MeNB 603 and SeNB 609. For example, in the case of downlink, data is provided from S-GW 601 to first PDCP processing unit 604 and second PDCP processing unit 608 of S-MeNB 603. First and second PDCP processing units (hereinafter sometimes collectively referred to as "PDCP processing units") 604, 608 perform PDCP processing in LTE or LTE-A.

[0484] The data provided to second PDCP processing section 608 is provided to PDCP path switching section 611 of SeNB 609. PDCP path switching section 611 switches the PDCP path. Because handover is not in progress, PDCP path switching section 611 determines that the PDCP from second PDCP processing section 608 should be provided to RLC processing section 612, and provides the data from second PDCP processing section 608 to RLC processing section 612. RLC processing section 612 performs RLC processing in LTE or LTE-A.

[0485] The data provided to the RLC processing unit 612 is then provided to a MAC processing unit 613 and a PHY processing unit 614 in that order, and then provided to a UE 615 via wireless transmission. The MAC processing unit 613 performs MAC processing in LTE or LTE-A. The PHY processing unit 614 performs PHY processing in LTE or LTE-A.

[0486] As in this embodiment, when data transmission and reception at SeNB609 remains unchanged and only MeNB handover is performed, data transmission and reception using EPS bearer #2 is not performed between UE615 and S-MeNB603 during handover.

[0487] When a full transition including PDCP is made to the SeNB 609, the S-GW 601 switches the eNB that performs PDCP processing using the PDCP processing eNB switching unit 602. The PDCP processing eNB switching unit 602 transmits data to the PDCP processing unit 610 of the SeNB 609 using the EPS bearer #2, instead of the second PDCP processing unit 608 of the S-MeNB 603. The PDCP processing unit 610 performs PDCP processing in LTE or LTE-A.

[0488] Upon receiving data from S-GW 601, SeNB 609 performs PDCP processing on the received data using PDCP processing section 610 and provides the processed data to PDCP path switching section 611. SeNB 609 selects the PDCP data from its own PDCP processing section 610 instead of the PDCP data from second PDCP processing section 608 of S-MeNB 603 using PDCP path switching section 611, and provides this to RLC processing section 612.

[0489] The PDCP data given to the RLC processing unit 612 is then given to the MAC processing unit 613 and the PHY processing unit 614 in that order for processing, and finally transmitted to the UE 615 via wireless transmission.

[0490] By providing the PDCP processing eNB switching unit 602 and the PDCP path switching unit 611, it is possible to realize processing in which data is transmitted only through the SeNB that is not involved in handover during handover of the MeNB.

[0491] In the case of the uplink, the processing flow is the same as in the case of the downlink. In the case of the uplink, as in the case of the downlink, by providing the PDCP processing eNB switching unit 602 and the PDCP path switching unit 611, it is possible to realize processing in which data continuity is performed only by the SeNB that is not performing handover during handover of the MeNB.

[0492] Furthermore, after the handover process in step ST2025 in FIG. 28 described above, the process in the case where data is transmitted and received using EPS bearer #2 between both the SeNB and the T-MeNB is executed as follows. At the time of handover, the SeNB changes the connection from the S-MeNB to the T-MeNB. Thereafter, the T-MeNB changes the data transmission and reception path of only the SeNB to the data transmission and reception path between the SeNB and the T-MeNB. Alternatively, at the time of handover, a method may be considered in which connection is established using the data transmission and reception paths of both the SeNB and the T-MeNB at one time.

[0493] In this case, there are two possible ways to change the configuration of radio resources: (A) by setting up a split bearer configuration from the T-MeNB, or (B) by changing the switching of the data transmission and reception path.

[0494] As specific examples of the case where the split bearer configuration is set up from the T-MeNB in (A) above, the following four examples (A-1) to (A-4) are disclosed.

[0495] (A-1) The information that needs to be notified from the S-MeNB to the T-MeNB is SeNB identification information, specifically information regarding the destination address of the SeNB. This is because PDCP-processed data will be transmitted to the SeNB, and the SeNB identification information is required for this. Without the SeNB identification information, the T-MeNB does not know to which SeNB to transmit data or from which SeNB to receive data. In addition, the T-MeNB needs to know parameter information regarding which RRC connection parameters the SeNB is operating with, and so this information is also notified.

[0496] (A-2) Information that needs to be notified from the T-MeNB to the S-MeNB is information on whether the MeNB handover was successful or failed. In addition, when data is accumulated in the buffer of the S-MeNB, information indicating an instruction to perform data forwarding is also information that needs to be notified from the T-MeNB to the S-MeNB.

[0497] (A-3) The information that needs to be notified to the UE is a notification indicating that the MeNB has been switched by handover from the S-MeNB to the T-MeNB. In this case, the UE will transmit and receive data to and from the T-MeNB, not the S-MeNB.

[0498] (A-4) In the case of a split bearer configuration, the SeNB transmits and receives data to and from the S-GW via the PDCP processing unit of either the S-MeNB or the T-MeNB. Therefore, the information that needs to be notified to the SeNB is information about which PDCP processing unit of the MeNB the SeNB will transmit data to and receive data from.

[0499] As specific examples of the case where the above-mentioned (B) is addressed by changing the switching of the data transmission / reception path, the following four examples (B-1) to (B-4) are disclosed.

[0500] (B-1) The information that needs to be notified from the S-MeNB to the T-MeNB is the identification information of the SeNB. This is because if the S-MeNB does not know which SeNBs it was simultaneously communicating with before the handover, it will not know which SeNBs it may simultaneously communicate with after the handover. In addition, the T-MeNB needs to know parameter information about which RRC connection parameters the SeNB is operating with, so this information is also notified. In this specific example (B-1), it is assumed that the SeNB only transmits and receives data, and the T-MeNB is responsible for control information (signaling).

[0501] (B-2) The information that needs to be notified from the T-MeNB to the S-MeNB is, as in the specific example (A-2) above, information on whether the MeNB handover was successful or unsuccessful, and a notification instructing the S-MeNB to perform data forwarding if data is accumulated in the buffer.

[0502] (B-3) The information that needs to be notified to the UE is, as in the specific example (A-3), a notification indicating that the MeNB has been switched by handover from an S-MeNB to a T-MeNB. In this case, the UE will transmit and receive data to and from the T-MeNB, not the S-MeNB.

[0503] (B-4) The information that needs to be notified to the SeNB is information indicating that the MeNB has switched from an S-MeNB to a T-MeNB due to the completion (success) of handover. Since the SeNB is subordinate to the MeNB, it needs to know which MeNB among the subordinate MeNBs is transmitting and receiving a control signal, such as signaling, to the device itself, and follow the control from that MeNB.

[0504] In the sequences shown in Figures 26 to 30, the configuration of radio resources is changed at three times: (1) step ST2026, (2) step ST2056, and (3) step ST2043. Each of the times (1) to (3) will be specifically described below.

[0505] (1) In the timing of the MeNB change process for the SeNB in Step ST2026, the configuration of radio resources is changed after the timing of the downlink path change in Step ST2063. Therefore, since the MeNB is switched to a T-MeNB and the resources of the S-MeNB are released, it is possible to prevent the process from being erroneously performed using the resources before the handover. Furthermore, the process can be realized by simple processing and by a circuit with a relatively small scale.

[0506] (2) In the timing of the MeNB change process for the SeNB in Step ST2056, the configuration of radio resources is changed before the timing of the downlink path change in Step ST2063. Therefore, a contradiction occurs in that before the upper device is switched from an S-MeNB to a T-MeNB due to handover, radio communication with the UE is operating as it should after handover.

[0507] This makes the processing more complex than when it is performed at the timing of (1) above, but because the switching timing is earlier, it avoids the risks that arise when data is sent and received only by the SeNB, such as congestion caused by multiple UEs accessing the SeNB at the same time, and thereby enables stable operation.

[0508] (3) The timing of the MeNB change process for the SeNB in Step ST2043 is immediately after the T-MeNB notifies the SeNB of an Ack response to the handover request in Step ST2042. At this timing, the radio resource configuration is changed at an even earlier timing than the timing in (2) above.

[0509] This makes the processing in the event of a handover failure more complicated than when it is performed at the timing of (2) above, but it makes it possible to more reliably avoid the risks mentioned above, thereby achieving more stable operation.

[0510] At the timings (2) and (3) above, only uplink data can be transmitted and received depending on the radio resource configuration. Downlink data can be transmitted and received after the T-MeNB receives data from the S-GW.

[0511] Embodiment 6 32 and 33 are diagrams showing an example of a sequence of an MeNB HO process for an EPS bearer #1 in a communication system according to Embodiment 6. Figures 32 and 33 are connected by a boundary line BL10.

[0512] In the above-described fifth embodiment, the S-MeNB performs the reconfiguration, but in the present embodiment, the T-MeNB performs the reconfiguration. Specifically, in the sequence of the fifth embodiment shown in the above-described Figures 26 to 30, the same processing as in the fifth embodiment is performed except that the MeNB HO processing for the EPS bearer #1 in step ST2680 shown in Figures 32 and 33 is performed instead of step ST2025 shown in Figures 29 and 30.

[0513] The processing of step ST2680 shown in Figures 32 and 33 is similar to the MeNB HO processing for EPS bearer #1 of step ST2025 shown in Figures 29 and 30, so the same steps are assigned the same step numbers and common explanations will be omitted.

[0514] The processing of step ST2680 is the same as the processing of step ST2025 of embodiment 5 shown in Figures 29 and 30, except that in step ST2025 of embodiment 5, processing of steps ST2681 and ST2682 is performed instead of steps ST2046 and ST2047 in Figure 29.

[0515] In this embodiment, in Step ST2681, the T-MeNB notifies the UE of downlink allocation information. Furthermore, in Step ST2682, the T-MeNB notifies the UE of an RRC connection reconfiguration message. In this embodiment, the RRC connection reconfiguration message does not include mobility control information.

[0516] As described above, in the above-described fifth embodiment, the S-MeNB notifies the UE of the RRC connection reconfiguration message, but in the present embodiment, the T-MeNB notifies the UE of the RRC connection reconfiguration message.

[0517] This allows the optimal RRC connection to be set up by the T-MeNB, which is the destination of the handover and will establish communication connection, rather than the S-MeNB, which is the source of the handover, thereby stabilizing communication between the UE and the MeNB.

[0518] Embodiment 7 34 and 35 are diagrams showing an example of a sequence of an MeNB HO process for an EPS bearer #1 in a communication system according to Embodiment 7. Figures 34 and 35 are connected by a boundary line BL11.

[0519] In this embodiment, in the sequence of embodiment 5 shown in Figures 26 to 30 described above, the same processing as in embodiment 5 is performed except that instead of step ST2025 shown in Figures 29 and 30, MeNB HO processing for EPS bearer #1 is performed in step ST2690 shown in Figures 34 and 35.

[0520] The processing of step ST2690 shown in Figures 34 and 35 is similar to the MeNB HO processing for EPS bearer #1 of step ST2025 shown in Figures 29 and 30, so the same step numbers are assigned to the same steps and common explanations are omitted.

[0521] The process of step ST2690 is the same as the process of step ST2025 in the fifth embodiment shown in FIGS. 29 and 30, except that the process of step ST2691 is performed in addition to step ST2025 in the fifth embodiment shown in FIGS.

[0522] In the present embodiment, in Step ST2691, before Step ST2646 and Step ST2647, the T-MeNB notifies the S-MeNB of an RRC connection reconfiguration message and downlink allocation information.

[0523] In this manner, in the present embodiment, the T-MeNB notifies the S-MeNB, and then the S-MeNB notifies the UE of the content of the reconfiguration message. As a result, similar to the sixth embodiment described above, instead of notifying the UE of the reconfiguration message from the S-MeNB, the content of the reconfiguration message notified from the T-MeNB is notified to the UE via the S-MeNB.

[0524] Therefore, as in embodiment 6, the optimal RRC connection setting is performed not by the S-MeNB, which is the source of the handover, but by the T-MeNB, which is the destination of the handover to establish communication connection, thereby stabilizing communication between the UE and the MeNB.

[0525] The above-described embodiments and their modifications can be applied to a case where an SeNB configures a plurality of serving cells. Similarly, they can be applied to a case where an MeNB configures a plurality of serving cells. A set of serving cells configured by an SeNB is sometimes referred to as an SCG (secondary cell group), and a set of serving cells configured by an MeNB is sometimes referred to as an MCG (master cell group).

[0526] The above-described embodiments and their modifications are merely examples, and the embodiments and their modifications can be freely combined. Furthermore, any of the components of the embodiments and their modifications can be modified or omitted as appropriate.

[0527] Although the present disclosure has been described in detail, the above description is illustrative in all respects and is not restrictive. It is understood that countless variations not illustrated can be envisioned. [Explanation of symbols]

[0528] 51 S-MeNB, 52 S-MeNB coverage, 53 T-MeNB, 54 T-MeNB coverage, 55,58 SeNB, 56,59 SeNB coverage, 57 UE.

Claims

1. In a mobile communication system in which a master base station and a secondary base station concurrently perform wireless communication with a mobile terminal, A mobile communication system in which the measurement result regarding the secondary base station is transmitted from the mobile terminal to the master base station.

2. 2. The mobile communication system according to claim 1, A mobile communication system, wherein a measurement configuration for the secondary base station is transmitted from the master base station to the mobile terminal.

3. A master base station in a mobile communication system in which a master base station and a secondary base station concurrently perform wireless communication with a mobile terminal, A master base station that receives measurement results regarding the secondary base stations from the mobile terminal.

4. A secondary base station in a mobile communication system in which a master base station and a secondary base station concurrently perform wireless communication with a mobile terminal, A secondary base station, wherein measurement results relating to the secondary base station are transmitted from the mobile terminal to the master base station.

5. A mobile terminal in a mobile communication system in which a master base station and a secondary base station concurrently perform wireless communication with the mobile terminal, The mobile terminal transmits measurement results regarding the secondary base station to the master base station.

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