Handover control method and user device

Abstract

A handover control method and a user equipment are provided. [Solution] A handover control method for performing a conditional handover of a user equipment from a source cell to which the user equipment is connected to a target cell includes the user equipment receiving a handover command from the source cell, the handover command including a list of candidate cells that are candidates for the target cell and handover conditions specified for each candidate cell in the list, and the user equipment performing a handover to the specific candidate cell for which the handover conditions are satisfied, when the handover conditions corresponding to the specific candidate cell in the list included in the received handover command are satisfied.

Description

[Technical field]

[0001] The present disclosure relates to a handover control method and a user equipment. [Background technology]

[0002] In a mobile communication system, a user device performs handover to change a cell to which the user device is connected from a source cell to a target cell as the user device moves. In recent years, there has been a demand for reducing communication interruption time during handover and improving the reliability of handover in an environment with unstable wireless conditions. Summary of the Invention

[0003] A handover control method according to a first aspect is a handover control method for performing a conditional handover of a user equipment from a source cell to which the user equipment is connected to a target cell, the handover control method including: receiving, by the user equipment, a handover command from the source cell, the handover command including a list of candidate cells that are candidates for the target cell and a handover condition specified for each candidate cell in the list; and, when a handover condition corresponding to a specific candidate cell in the list included in the received handover command is satisfied, performing a handover by the user equipment to the specific candidate cell for which the handover condition is satisfied.

[0004] A user equipment according to a second aspect includes a receiving unit that, when performing a conditional handover of the user equipment from a source cell to which the user equipment is connected to a target cell, receives from the source cell a handover command including a list of candidate cells that are candidates for the target cell and a handover condition specified for each candidate cell in the list, and a control unit that, when a handover condition corresponding to a specific candidate cell in the list included in the received handover command is satisfied, performs a handover to the specific candidate cell for which the handover condition is satisfied. [Brief description of the drawings]

[0005] [Figure 1] 1 is a diagram showing a configuration of a mobile communication system according to an embodiment; [Diagram 2] FIG. 2 is a diagram illustrating a configuration of a user device according to an embodiment. [Diagram 3] FIG. 2 is a diagram illustrating a configuration of a base station according to an embodiment. [Figure 4] FIG. 2 is a diagram illustrating a configuration of a protocol stack of a wireless interface according to an embodiment. [Diagram 5] FIG. 2 is a diagram illustrating a configuration of a radio frame according to an embodiment. [Figure 6] FIG. 2 is a diagram showing an operation sequence of a mobile communication system according to an embodiment. [Figure 7] FIG. 11 is a diagram showing information (information elements) included in a handover command according to one embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0006] A mobile communication system according to an embodiment will be described with reference to the drawings. In the description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

[0007] (Configuration of a mobile communication system) First, a configuration of a mobile communication system according to an embodiment will be described. Fig. 1 is a diagram showing a configuration of an LTE (Long Term Evolution) system, which is a mobile communication system according to an embodiment. The LTE system is a system based on the 3GPP (3rd Generation Partnership Project) standard.

[0008] In the following, the LTE system is exemplified as a mobile communication system, but the 3GPP standard fifth generation (5G) system may be at least partially applied to the mobile communication system.

[0009] As shown in FIG. 1, the LTE system includes a user equipment (UE) 100, a radio access network (E-UTRAN: Evolved-UMTS Terrestrial Radio Access Network) 10, and a core network (EPC: Evolved Packet Core) 20.

[0010] In the 5G system, the radio access network is called NG-RAN (Next Generation RAN) and the core network is called 5GC (5G Core Network).

[0011] The UE 100 is a mobile communication device. The UE 100 performs wireless communication with the eNB 200 that manages a cell (serving cell) in which the UE 100 is located. The UE 100 may be any device that is used by a user, and may be, for example, a mobile phone terminal (including a smartphone), a tablet terminal, a notebook PC, a sensor or a device provided in a sensor, a vehicle or a device provided in a vehicle (Vehicle UE), or an aircraft or a device provided in an aircraft (Aerial UE).

[0012] E-UTRAN 10 includes base stations (eNB: evolved Node-B) 200. eNBs 200 are connected to each other via an X2 interface, which is an interface between base stations. eNB 200 manages one or more cells. eNB 200 performs wireless communication with UE 100 that has established a connection with its own cell. eNB 200 has a radio resource management (RRM) function, a routing function for user data (hereinafter simply referred to as "data"), a measurement control function for mobility control and scheduling, and the like. "Cell" is used as a term indicating the smallest unit of a wireless communication area. "Cell" is also used as a term indicating a function or resource for performing wireless communication with UE 100. One cell belongs to one carrier frequency.

[0013] In the 5G system, the base station is called a gNB, and the base station interface is called an Xn interface. Note that the gNB can connect to the EPC, the eNB can connect to the 5GC, and the gNB and the eNB can be connected via an inter-base station interface (Xn interface, X2 interface).

[0014] The EPC 20 includes a mobility management entity (MME) and a serving gateway (S-GW) 300. The MME performs various mobility controls for the UE 100. The MME manages information on a tracking area (TA) in which the UE 100 is located by communicating with the UE 100 using NAS (Non-Access Stratum) signaling. A tracking area is an area consisting of multiple cells. The S-GW controls data forwarding. The MME and S-GW are connected to the eNB 200 via an S1 interface, which is an interface between a base station and a core network.

[0015] In a 5G system, the core network entity that performs various mobility controls for UE100 is called AMF (Access and Mobility Management Function), the core network entity that controls data transfer is called UPF (User Plane Function), and the interface between the base station and the core network is called the NG interface.

[0016] 2 is a diagram showing a configuration of the UE 100 (user equipment). The UE 100 includes a receiving unit 110, a transmitting unit 120, and a control unit .

[0017] The receiving unit 110 performs various receptions under the control of the control unit 130. The receiving unit 110 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 130.

[0018] The transmission unit 120 performs various transmissions under the control of the control unit 130. The transmission unit 120 includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) output by the control unit 130 into a radio signal and transmits it from the antenna.

[0019] The control unit 130 performs various controls in the UE 100. The control unit 130 includes at least one processor and at least one memory. The memory stores programs executed by the processor and information used in the processing by the processor. The processor may include a baseband processor and a CPU (Central Processing Unit). The baseband processor performs modulation / demodulation and encoding / decoding of a baseband signal. The CPU executes programs stored in the memory to perform various processing. The processor executes the processing described below.

[0020] 3 is a diagram showing a configuration of the eNB 200 (base station). The eNB 200 includes a transmitter 210, a receiver 220, a controller 230, and a backhaul communication unit 240.

[0021] The transmission unit 210 performs various transmissions under the control of the control unit 230. The transmission unit 210 includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) output by the control unit 230 into a radio signal and transmits it from the antenna.

[0022] The receiving unit 220 performs various receptions under the control of the control unit 230. The receiving unit 220 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 230.

[0023] The control unit 230 performs various controls in the eNB 200. The control unit 230 includes at least one processor and at least one memory. The memory stores programs executed by the processor and information used in the processing by the processor. The processor may include a baseband processor and a CPU. The baseband processor performs modulation / demodulation and encoding / decoding of a baseband signal. The CPU executes programs stored in the memory to perform various processing. The processor executes the processing described below.

[0024] The backhaul communication unit 240 is connected to a neighboring eNB or a neighboring gNB via an inter-base station interface (X2 interface or Xn interface). The backhaul communication unit 240 is connected to an MME / S-GW 300 or an AMF / UPF via a base station-core network interface (S1 interface or NG interface).

[0025] Fig. 4 is a diagram showing the configuration of a protocol stack of a radio interface in an LTE system. As shown in Fig. 4, the radio interface protocol is divided into the first to third layers of the OSI reference model. The first layer is a physical (PHY) layer. The second layer includes a medium access control (MAC) layer, a radio link control (RLC) layer, and a packet data convergence protocol (PDCP) layer. The third layer includes a radio resource control (RRC) layer. The PHY layer, the MAC layer, the RLC layer, the PDCP layer, and the RRC layer constitute an access stratum (AS) layer.

[0026] The PHY layer performs encoding / decoding, modulation / demodulation, antenna mapping / demapping, and resource mapping / demapping. Data and control information are transmitted between the PHY layer of the UE 100 and the PHY layer of the eNB 200 via a physical channel.

[0027] The MAC layer performs data priority control, retransmission processing using hybrid ARQ (HARQ), random access procedures, etc. Data and control information are transmitted between the MAC layer of the UE 100 and the MAC layer of the eNB 200 via a transport channel. The MAC layer of the eNB 200 includes a scheduler. The scheduler determines the uplink and downlink transport format (transport block size, modulation and coding scheme (MCS)) and the resource blocks to be assigned to the UE 100.

[0028] The RLC layer transmits data to the RLC layer on the receiving side by utilizing the functions of the MAC layer and the PHY layer. Data and control information are transmitted between the RLC layer of the UE 100 and the RLC layer of the eNB 200 via logical channels.

[0029] The PDCP layer performs header compression / decompression and encryption / decryption.

[0030] The RRC layer is defined only in the control plane that handles control information. RRC signaling for various settings is transmitted between the RRC layer of the UE 100 and the RRC layer of the eNB 200. The RRC layer controls logical channels, transport channels, and physical channels in response to establishment, re-establishment, and release of radio bearers. When there is a connection (RRC connection) between the RRC of the UE 100 and the RRC of the eNB 200, the UE 100 is in RRC connected mode. When there is no connection (RRC connection) between the RRC of the UE 100 and the RRC of the eNB 200, the UE 100 is in RRC idle mode.

[0031] The NAS layer, which is located above the RRC layer, performs session management, mobility management, etc. NAS signaling is transmitted between the NAS layer of the UE 100 and the NAS layer of the MME 300C. The UE 100 has a function of an application layer and the like in addition to a protocol of a radio interface.

[0032] In the 5G system, a Service Data Adaptation Protocol (SDAP) layer is provided above the PDCP layer in the user plane. The SDAP layer maps IP flows, which are the units for QoS control by the core network, to radio bearers, which are the units for QoS control by the AS (Access Stratum).

[0033] FIG. 5 is a diagram showing a configuration of a radio frame used in an LTE system. A radio frame is composed of 10 subframes on a time axis. Each subframe is composed of two slots on a time axis. The length of each subframe is 1 ms. The length of each slot is 0.5 ms. Each subframe includes multiple resource blocks (RBs) on a frequency axis. Each subframe includes multiple symbols on a time axis. Each resource block includes multiple subcarriers on a frequency axis. Specifically, one RB is composed of 12 subcarriers and one slot. One resource element (RE) is composed of one symbol and one subcarrier. Of the radio resources (time-frequency resources) allocated to UE 100, frequency resources can be specified by resource blocks, and time resources can be specified by subframes (or slots).

[0034] In the downlink, the section of the first few symbols of each subframe is an area used mainly as a physical downlink control channel (PDCCH) for transmitting downlink control information, and the remaining part of each subframe is an area used mainly as a physical downlink shared channel (PDSCH) for transmitting downlink data.

[0035] In the uplink, both ends of each subframe in the frequency direction are regions that are mainly used as a Physical Uplink Control Channel (PUCCH) for transmitting uplink control information, and the remaining part of each subframe is a region that is mainly used as a Physical Uplink Shared Channel (PUSCH) for transmitting uplink data.

[0036] (Mobile communication system operation) Next, an operation of a mobile communication system according to an embodiment will be described. The LTE system will be described as an example, but the 5G system may be applied. In that case, eNB may be replaced with gNB, and X2 interface may be replaced with Xn interface.

[0037] (1) Overview of Conditional Handover In a typical handover procedure, the eNB 200 determines handover of the UE 100. For example, the UE 100 transmits a measurement report on the radio condition to the source cell in response to deterioration of the radio condition between the UE 100 and the source cell and / or improvement of the radio condition between the UE 100 and the target cell.

[0038] Note that the handover of the UE 100 is an operation of switching the serving cell of the UE 100 from a source cell to a target cell. In the following, an example in which the source cell and the target cell belong to different eNBs will be described, but the source cell and the target cell may belong to the same eNB.

[0039] The source eNB 200, which manages the source cell, determines handover of the UE 100 based on the measurement report transmitted from the UE 100, and transmits a handover request including a UE context to a target eNB, which manages a target cell. Then, when the source eNB 200 receives a handover request confirmation response from the target eNB, it transmits a handover command to the UE 100. When the UE 100 receives the handover command, it starts handover to the target eNB, and transmits a random access signal to the target eNB.

[0040] On the other hand, in a conditional handover procedure, the UE 100 itself determines the handover of the UE 100. Specifically, the source eNB 200 transmits a handover request in advance to a candidate eNB that manages a candidate cell that is a candidate for the target cell. Here, the number of candidate eNBs is not limited to one, and may be multiple. Therefore, multiple candidate eNBs may receive the handover request.

[0041] The source eNB 200 transmits a handover command in advance to the UE 100. After receiving the handover command, the UE 100 suspends the handover until a handover condition is satisfied, and when the handover condition is satisfied, starts the handover and transmits a random access signal to one of the candidate eNBs.

[0042] In such a conditional handover, the source eNB 200 does not make a handover decision based on a measurement report, but the UE 100 itself makes a handover decision. Therefore, even if the radio condition between the UE 100 and the source eNB 200 is unstable, the handover is immediately performed according to the radio condition, thereby improving the reliability (robustness) of the handover.

[0043] In one embodiment, the UE 100 receives a handover command from a source cell (source eNB 200) including a list of candidate cells (hereinafter referred to as a cell list) that are candidates for a target cell and a handover condition specified for each candidate cell in the cell list. When a handover condition corresponding to a specific candidate cell in the cell list included in the received handover command is satisfied, the UE 100 performs a handover to the specific candidate cell for which the handover condition is satisfied.

[0044] In this way, the cell list is included in the handover command. Although a method in which the source eNB 200 predicts the moving direction of the UE 100 and selects only one cell at the moving destination as a candidate cell is also conceivable, such a method has a problem that conditional handover cannot be performed when the prediction is incorrect. In one embodiment, the cell list is included in the handover command, so that such a problem can be easily solved.

[0045] Furthermore, by specifying handover conditions for each candidate cell, handover conditions can be specified in detail according to the attributes of the candidate cells (eg, cell type and size).

[0046] In the following, the source eNB200 is referred to as "source eNB200S" and the candidate eNB is referred to as "candidate eNB200T."

[0047] (2) Example of operation sequence Fig. 6 is a diagram showing an example of an operation sequence of a mobile communication system according to an embodiment. Note that the signaling indicated by the dashed lines in Fig. 6 is not essential.

[0048] As shown in FIG. 6, in step S101, the UE 100 in the RRC connected mode establishes a wireless connection (RRC connection) with a cell (source cell) of the source eNB 200S.

[0049] The source eNB200S determines a candidate cell (candidate eNB200T) that is a candidate for a target cell when handing over the UE100. The candidate eNB200T may be a neighboring eNB adjacent to the source eNB200S. The source eNB200S may determine the candidate eNB200T when the UE100 connects to its own cell and transmit a handover request to the candidate eNB200T. The source eNB200S may transmit a handover request to a plurality of eNBs (a plurality of candidate eNBs). Here, it is assumed that the source eNB200S determines a cell of the candidate eNB200T1 and a cell of the candidate eNB200T2 as candidate cells. Hereinafter, the cell of the candidate eNB200T1 is referred to as candidate cell #1, and the cell of the candidate eNB200T2 is referred to as candidate cell #2.

[0050] In step S102, the source eNB200S transmits a handover request message for a conditional handover to the candidate eNB200T1 over the inter-base station interface (X2 interface).

[0051] Also, in step S103, the source eNB200S transmits a handover request message for a conditional handover to the candidate eNB200T2 over the inter-base station interface (X2 interface).

[0052] The handover request message includes the UE context, may be a new message dedicated to the conditional handover, or may be an existing handover request message that includes information (information element) indicating the conditional handover.

[0053] Each of candidate eNB200T1 and candidate eNB200T2 receives a handover request message from source eNB200S and retains the UE context included in the handover request message.

[0054] In step S104, the candidate eNB200T1 sends a handover request acknowledgement (Ack) message to the source eNB200S over the X2 interface.

[0055] The handover request ACK message transmitted by the candidate eNB200T1 includes a wireless communication setting determined by the candidate eNB200T1. The wireless communication setting is a setting to be applied to wireless communication with the candidate eNB200T1 when the UE100 is handed over to the candidate eNB200T1. The wireless communication setting may include a setting for each layer. For example, the wireless communication setting may include at least one of a PDCP setting, an RLC setting, a MAC setting, and a PHY setting.

[0056] The handover request ACK message transmitted by the candidate eNB200T1 may include information for the UE100 to omit a random access procedure for the candidate eNB200T1. Such information includes timing advance information and / or uplink radio resource information. The timing advance information is information indicating a timing advance value (TA value) to be applied when the UE100 performs uplink transmission to the candidate eNB200T1. The uplink radio resource information is information indicating uplink radio resources (time and frequency resources) to be used when the UE100 performs uplink transmission to the candidate eNB200T1. The uplink radio resource information may be referred to as an uplink grant (UL grant). The uplink radio resource information may further include information indicating a modulation and coding scheme (MCS) to be applied to the uplink transmission.

[0057] In step S105, the candidate eNB200T2 sends a handover request ACK message to the source eNB200S over the X2 interface.

[0058] The handover request ACK message transmitted by the candidate eNB200T2 may include information (information elements) similar to the handover request ACK message transmitted by the candidate eNB200T1. That is, the handover request ACK message transmitted by the candidate eNB200T2 includes a wireless communication configuration determined by the candidate eNB200T2. The handover request ACK message transmitted by the candidate eNB200T2 may include timing advance information and / or uplink radio resource information as information for the UE100 to omit a random access procedure for the candidate eNB200T2.

[0059] By receiving these handover request ACK messages, the source eNB200S acquires radio communication configurations from each candidate eNB200. In addition, the source eNB200S acquires timing advance information and / or uplink radio resource information.

[0060] In step S106, the source eNB 200S transmits to the UE 100 a handover command (for example, an RRC connection reconfiguration message) including information (information elements) related to the conditional handover.

[0061] FIG. 7 is a diagram showing an example of information (information elements) included in a handover command according to an embodiment.

[0062] 7, the handover command transmitted from the source eNB 200S to the UE 100 includes a list of candidate cells (cell list). The cell list is a list including identifiers (e.g., physical cell identifiers) of the candidate cells #1 and #2. Here, an example is shown in which there are two candidate cells in the cell list, but the number of candidate cells may be three or more.

[0063] The handover command transmitted by the source eNB200S to the UE100 includes a handover condition specified for each candidate cell in the cell list. The handover condition is, for example, a condition that a radio condition between the UE100 and the source eNB200 (source cell) has deteriorated and / or a radio condition between the UE100 and a candidate eNB (candidate cell) has improved. The handover condition may include at least one of a first threshold value to be compared with the radio condition of the source cell and a second threshold value to be compared with the radio condition of the candidate cell. The radio condition may be, for example, a received reference signal quality (RSRP, RSRQ, RS-SINR, etc.) or a packet retransmission count (RLC retransmission count, etc.).

[0064] In addition, the handover command that the source eNB 200S transmits to the UE 100 includes a wireless communication setting that is specified for each candidate cell in the cell list.

[0065] Furthermore, the handover command may further include timing advance information and / or uplink radio resource information specified for each candidate cell in the cell list. If such information is included, the handover command may further include information indicating that the random access procedure can be omitted for the corresponding candidate cell.

[0066] Furthermore, the handover command may further include information specifying the time (validity period) for which the handover command is valid.

[0067] In steps S107 and S108, before the UE 100 performs handover, the source eNB 200 starts forwarding a copy of the downlink data of the UE 100 to the candidate eNB 200T1 and the candidate eNB 200T2 that manage the candidate cells in the cell list. That is, the source eNB 200 starts a forwarding process of the copy of the downlink data (so-called data forwarding).

[0068] In step S109, when the UE 100 receives a handover command from the source eNB 200S, the UE 100 determines whether any handover condition included in the received handover command is satisfied. For example, the UE 100 measures radio conditions of the source eNB 200S (source cell) and each candidate eNB 200T, and compares the radio conditions with thresholds to determine whether the handover condition is satisfied.

[0069] In step S110, when a handover condition corresponding to a specific candidate cell in the cell list is satisfied, the UE 100 determines to perform a handover to the specific candidate cell for which the handover condition is satisfied. Here, the description will be given on the assumption that the handover condition corresponding to the candidate eNB 200T1 (candidate cell #1) is satisfied.

[0070] In step S111, the UE 100 may transmit a notification (handover notification) indicating execution of handover to the source eNB 200S. Based on the notification, the source eNB 200S knows that the UE 100 executes handover. The handover execution notification may include information indicating the cell selected by the UE 100 as the handover destination.

[0071] In step S112, the UE 100 starts a random access procedure for the candidate cell #1 (candidate eNB 200T1) selected as the handover destination, and transmits a random access signal to the cell.

[0072] In step S113, the candidate eNB200T1 calculates a timing advance based on the random signal, determines an uplink radio resource to be allocated to the UE100, and transmits a random access response to the UE100 including information on the calculated timing advance and the determined uplink radio resource.

[0073] Here, when the timing advance information and / or uplink radio resource information (UL grant) corresponding to the candidate cell #1 is included in the handover command, the UE 100 may omit the random access procedure for the candidate cell #1 (candidate eNB200T1), that is, the transmission of the random access signal (step S112) and the reception of the random access response (step S113). Here, the description will proceed on the assumption that the UE 100 omits the random access procedure.

[0074] In step S114, when performing handover to the candidate cell #1 (candidate eNB200T1), the UE 100 omits transmission of a random access signal to the candidate cell #1 and transmits an uplink message to the candidate cell #1 by using timing advance information and / or uplink radio resource information corresponding to the candidate cell #1. Here, the uplink message may be an RRC reconfiguration completion message, which is a type of RRC message. This completes the connection process between the UE 100 and the candidate cell #1 (candidate eNB200T1), and the handover is completed.

[0075] In step S115, when the handover is completed, the UE 100 transmits and receives data to and from the candidate cell #1 (candidate eNB200T1) using the wireless communication setting corresponding to the candidate cell #1 (candidate eNB200T1) included in the handover command. The candidate eNB200T1 may transmit downlink data transferred in advance from the UE 100 by data forwarding to the UE 100.

[0076] In step S116, the candidate eNB200T1 may transmit a handover notification to the source eNB200S over the X2 interface indicating that the UE 100 has been handed over (connected) to the candidate eNB200T1.

[0077] In step S117, in response to receiving the handover notification from the candidate eNB200T1, the source eNB200S may notify the candidate eNB200T2 of the handover cancellation. In response to receiving the handover cancellation, the candidate eNB200T2 may discard the UE context that was included in the handover request (step S103) and that was held by the candidate eNB200T2.

[0078] As described above, in the handover control method for performing a conditional handover of the UE 100 from a source cell to which the UE 100 is connected to a target cell, the UE 100 receives a handover command from the source cell, including a list of candidate cells (cell list) that are candidates for the target cell and a handover condition specified for each candidate cell in the cell list. When the handover condition corresponding to a specific candidate cell (candidate cell #1) in the cell list included in the received handover command is satisfied, the UE 100 performs a handover to the specific candidate cell (candidate cell #1) for which the handover condition is satisfied.

[0079] In this way, by specifying handover conditions for each candidate cell, it is possible to specify handover conditions in a detailed manner according to the attributes of the candidate cells (eg, cell type and size).

[0080] In one embodiment, the handover command further includes a wireless communication setting specified for each candidate cell in the cell list. After the UE 100 connects to the specific candidate cell (candidate cell #1), the UE 100 performs wireless communication with the specific candidate cell (candidate cell #1) using the wireless communication setting corresponding to the specific candidate cell (candidate cell #1).

[0081] In one embodiment, the source eNB 200S managing the source cell acquires wireless communication settings from the candidate eNB 200T managing the candidate cell in the cell list. The source eNB 200S transmits a handover command to the UE 100, the handover command including the wireless communication settings acquired from the candidate eNB 200T.

[0082] This enables proper wireless communication after the conditional handover.

[0083] In one embodiment, the handover command further includes timing advance information and / or uplink radio resource information designated for each candidate cell in the cell list. When performing handover to a specific candidate cell (candidate cell #1), the UE 100 omits transmission of a random access signal to the specific candidate cell (candidate cell #1) and transmits an uplink message to the specific candidate cell (candidate cell #1) by using the timing advance information and / or uplink radio resource information corresponding to the specific candidate cell (candidate cell #1).

[0084] In one embodiment, the source eNB 200S managing the source cell obtains timing advance information and / or radio resource information from the candidate eNB 200T managing the candidate cell in the cell list, and transmits a handover command to the UE 100 including the timing advance information and / or radio resource information obtained from the candidate eNB 200T.

[0085] This allows the random access procedure to be omitted during conditional handover.

[0086] In one embodiment, the source eNB 200S managing the source cell starts forwarding a copy of the user equipment's downlink data to the candidate eNB 200T managing the candidate cell in the cell list before the UE 100 performs handover.

[0087] This enables the candidate eNB200T to promptly start downlink data transmission after the conditional handover.

[0088] (Other embodiments) In the above-described embodiment, an example has been described in which a handover command including uplink radio resource information is transmitted from the source eNB200S to the UE 100. However, when each candidate eNB200T broadcasts the uplink radio resource information by a SIB (System Information Block), the UE 100 may receive the uplink radio resource information from each candidate eNB200T and, based on the received uplink radio resource information, omit the random access procedure (steps S112 and S113 in FIG. 6) and transmit an uplink message (step S114 in FIG. 6).

[0089] A program may be provided that causes a computer to execute each process performed by the UE 100 (or the eNB 200). The program may be recorded in a computer-readable medium. Using the computer-readable medium, it is possible to install the program in the computer. Here, the computer-readable medium on which the program is recorded may be a non-transient recording medium. The non-transient recording medium is not particularly limited, and may be, for example, a recording medium such as a CD-ROM or a DVD-ROM.

[0090] Furthermore, circuits for executing the processes performed by the UE 100 (or the eNB 200) may be integrated, and at least a part of the UE 100 (or the eNB 200) may be configured as a semiconductor integrated circuit (chip set, SoC).

[0091] Although one embodiment has been described in detail above with reference to the drawings, the specific configuration is not limited to that described above, and various design changes, etc. are possible without departing from the spirit and scope of the present invention.

[0092] This application claims priority to U.S. Provisional Application No. 62 / 752,524, filed October 30, 2018, the entire contents of which are incorporated herein by reference.

Claims

1. A source base station that manages a source cell, The source base station includes a control unit that transmits an RRC message to a user equipment connected to the source cell, the RRC message including a list of candidate cells that are candidates for the target cell and a handover condition specified for each candidate cell in the list, in order to perform a conditional handover of the user equipment from the source cell to a target cell; In response to transmitting the RRC message to the user equipment, the control unit starts forwarding a copy of downlink data of the user equipment to a candidate base station that manages a candidate cell in the list before the user equipment starts a handover to a specific candidate cell in the list. Source base station.

2. The control unit starts forwarding a copy of downlink data of the user equipment to a candidate base station that manages a candidate cell in the list before receiving a notification from the user equipment indicating execution of handover to the specific candidate cell. The source base station according to claim 1 .

3. A mobile communication system having a source base station that manages a source cell, The source base station transmits an RRC message to a user equipment connected to the source cell, the RRC message including a list of candidate cells that are candidates for the target cell and a handover condition specified for each candidate cell in the list, in order to perform a conditional handover of the user equipment from the source cell to a target cell; In response to the source base station transmitting the RRC message to the user equipment, before the user equipment initiates a handover to a particular candidate cell in the list, the source base station starts forwarding a copy of downlink data of the user equipment to a candidate base station that manages a candidate cell in the list. Mobile communication system.

4. A method performed in a source base station managing a source cell, comprising: transmitting an RRC message to a user equipment connected to the source cell, the RRC message including a list of candidate cells that are candidates for the target cell and a handover condition specified for each candidate cell in the list, in order to perform a conditional handover of the user equipment from the source cell to a target cell; In response to transmitting the RRC message to the user equipment, before the user equipment initiates a handover to a particular candidate cell in the list, initiating forwarding of a copy of downlink data of the user equipment to a candidate base station that manages a candidate cell in the list. method.

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