CN114009137A

CN114009137A - Terminal device

Info

Publication number: CN114009137A
Application number: CN201980097802.XA
Authority: CN
Inventors: 内野徹; 闵天杨; 高桥秀明
Original assignee: NTT Docomo Inc
Current assignee: NTT Docomo Inc
Priority date: 2019-06-26
Filing date: 2019-06-26
Publication date: 2022-02-01
Also published as: WO2020261460A1

Abstract

A terminal (UE) migrates to a target radio base station (gNB) without using a reconnection procedure. When a terminal migrates to a target radio base station in association with a radio link failure, the terminal starts a random access procedure with the target radio base station at a predetermined timing.

Description

Terminal device

Technical Field

The present invention relates to a terminal that performs wireless communication, and more particularly, to a terminal that migrates to a target wireless base station without using a reconnection procedure.

Background

In the third Generation Partnership Project (3rd Generation Partnership Project: 3GPP), Long Term Evolution (LTE) is standardized, LTE-Advanced (hereinafter, referred to as LTE including LTE-Advanced) is standardized for the purpose of further speeding up LTE, and fifth Generation mobile communication systems (5G, New Radio (NR), or Next Generation (NG)) are also standardized.

For example, in a conventional Handover (HO) procedure, a network determines a target radio base station (also referred to as a target cell) based on quality information such as a Measurement Report (Measurement Report) transmitted from a terminal (User Equipment, UE), and transmits a handover command (handover command) to the terminal after preparation for handover.

However, when the terminal passes through a point (point) of appropriate handover in preparation for handover on the network side, there is a problem that instantaneous interruption of a radio link occurs because the terminal does not receive a handover command from a source radio base station (also referred to as a source cell) and directly migrates to a target radio base station.

In order to solve such a problem, a procedure called Conditional HO (Conditional handover) has been studied (non-patent document 1). In the Conditional HO, a candidate destination cell for handover and a transition condition for the candidate destination cell are set in advance for the terminal.

In the Conditional HO, the terminal agrees to transmit an RRC reconfiguration complete to the target radio base station (non-patent document 2).

Thereby, the terminal can migrate to the target radio base station without waiting for a handover command from the network. That is, according to the Conditional HO, the terminal can migrate to the target radio base station without using a reconnection procedure in a radio resource control layer (RRC) with the target radio base station.

In addition, a procedure of early recovery from Radio Link Failure (RLF) using a cell migration procedure by the Conditional HO has also been studied (non-patent document 3).

Documents of the prior art

Non-patent document

Non-patent document 1: NR mobility enhancements for New WID, RP-190489,3GPP TSG RAN Meeting #83,3GPP, 3.2019, 3.3.months

Non-patent document 2 "Running CR for the introduction of NR mobility enhancement", R2-1906284,3GPP TSG-RAN WG2 Meeting #106,3GPP, 5.2019

Non-patent document 3 "Summary of mobility accessories from LTE mobility", R2-1908417,3GPP TSG RAN WG2 Meeting #106,3GPP, 5 months in 2019

Disclosure of Invention

However, if the cell migration procedure by Conditional HO is directly applied to recovery from RLF, it is considered that there is a problem as described below.

Specifically, as described above, in the Conditional HO, although the terminal transmits the RRC reconfiguration complete to the target Radio base station, in a specific situation such as when there is already data transmitted via the signaling Radio Bearer 1 (signaling Radio Bearer 1: SRB1), since the Buffer Status Report (BSR) is not triggered, there is a problem in that the random access procedure for transmitting the Scheduling Request (SR) is not started with the target Radio base station.

Accordingly, the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a terminal capable of reliably starting a random access procedure with a target radio base station even when a handover is made to the target radio base station without using a reconnection procedure as in a conventional HO.

According to one aspect of the present disclosure, there is provided a terminal including a control unit (control unit 250) configured to transition to a target radio base station without using a reconnection procedure, wherein the control unit starts a random access procedure with the target radio base station at a predetermined timing when the control unit transitions to the target radio base station in association with a radio link failure.

According to one aspect of the present disclosure, there is provided a terminal including a control unit (control unit 250) configured to migrate to a target radio base station without using a reconnection procedure, wherein the control unit determines to discard data transmitted via a signaling radio bearer when migrating to the target radio base station in association with a radio link failure.

Drawings

Fig. 1 is a schematic configuration diagram of the entire wireless communication system 10.

Fig. 2 is a functional block diagram of the UE 200.

Fig. 3 is an explanatory diagram of a conventional handover (handover) procedure.

Fig. 4 is an explanatory diagram of a handover procedure based on Conditional HO.

Fig. 5 is an explanatory diagram of a procedure of recovering from a handover failure (HOF) using a reconnection procedure in the RRC layer.

Fig. 6 is an explanatory diagram of a procedure of recovery from a handover failure (HOF) using a Conditional HO.

Fig. 7 is a diagram showing a recovery timing from a handover failure (HOF) using a reconnection procedure in the RRC layer.

Fig. 8 is a diagram showing a recovery sequence from a handover failure (HOF) using a Conditional HO.

Fig. 9 is a diagram showing images of interaction between layers in the terminal at the time of switching.

Fig. 10 is a diagram showing an image of interaction between layers within the terminal at the time of reconnection to a target wireless base station.

Fig. 11 is a diagram showing a relationship between data transmittable/non-transmittable periods of SRBs 0, SRBs 1, and SRBs 2/DRBs and transmission/reception timings of messages in RRC.

Fig. 12 is a diagram showing an example in which an RA procedure is not started in a case where a Conditional HO is applied to a Radio Link Failure (RLF).

Fig. 13 is a diagram showing an example in which an RA procedure is initiated in a case where the problem shown in fig. 12 is eliminated and a Conditional HO is applied to a Radio Link Failure (RLF).

Fig. 14 is a diagram showing an example of the hardware configuration of the UE 200.

Detailed Description

Hereinafter, embodiments will be described based on the drawings. The same or similar reference numerals are given to the same functions and structures, and the description thereof is appropriately omitted.

(1) General overall structure of wireless communication system

Fig. 1 is a schematic configuration diagram of the entire radio communication system 10 according to the embodiment. The Radio communication system 10 is a Radio communication system according to 5G New Radio (New air interface), and includes a Next Generation Radio Access Network (NG-RAN), not shown, and a User Equipment 200 (hereinafter, referred to as UE 200).

The NG-RAN includes radio base stations 100A (hereinafter referred to as a gNB100A) to 100C (hereinafter referred to as a gNB 100C). In addition, the specific configuration of the wireless communication system 10 including the number of gnbs and UEs is not limited to the example shown in fig. 1.

Further, the NG-RAN actually includes a plurality of NG-RAN nodes (NG-RAN nodes), specifically, a gNB (or NG-eNB), and is connected to a core network (5GC, not shown) according to 5G. In addition, NG-RANs 20 and 5GC may also be simply expressed as "networks".

The gnbs 100A-100C are radio base stations under 5G, and perform radio communication under 5G with the UE 200. The gnbs 100A to 100C and the UE200 support Massive MIMO that generates beams with higher directivity by controlling radio signals transmitted from a plurality of antenna elements, Carrier Aggregation (CA) using a plurality of Component Carriers (CCs), Dual Connectivity (DC) that simultaneously transmits CCs between a plurality of NG-RAN nodes and terminals, and the like.

gNB 100A-gNB 100C each form one or more cells. UE200 is able to migrate between cells (which may also be referred to as radio base stations) formed by gnbs 100A-gNB 100C. The "migration" typically refers to handover (handover) between cells (radio base stations), but may include behavior (behavior) of the UE200 in which a cell (radio base station) to which a connection is made is changed, such as cell reselection.

The cell (radio base station) of the migration destination to which the UE200 migrates is referred to as a target cell or a target radio base station. In addition, a cell (radio base station) of a migration source is referred to as a source cell or a source radio base station.

In the wireless communication system 10, a Conditional HO, i.e., a procedure in which the UE200 transitions to a target radio base station without using a reconnection procedure in a radio resource control layer (RRC), is used. In addition, in the radio communication system 10, a procedure of early recovery from Radio Link Failure (RLF) using a cell migration procedure by the Conditional HO is also adopted.

Details of the Conditional HO and the procedure for early recovery from RLF using the cell migration procedure by the Conditional HO will be described later.

(2) Functional block structure of wireless communication system

Next, a functional block configuration of the radio communication system 10 will be described. Specifically, the functional block structure of the UE200 will be described.

Fig. 2 is a functional block diagram of the UE 200. As shown in fig. 2, the UE200 includes a radio transmission unit 210, a radio reception unit 220, an RA procedure execution unit 230, a data discarding unit 240, and a control unit 250.

The radio transmission unit 210 transmits an uplink signal (UL signal) according to the NR. The radio receiving unit 220 receives a downlink signal (DL signal) according to the NR.

Specifically, the radio transmitter 210 and the radio receiver 220 perform radio communication via a control channel or a data channel.

The Control Channel includes a PDCCH (Physical Downlink Control Channel), a PUCCH (Physical Uplink Control Channel), a RACH (Random Access Channel), a DCI (Downlink Control Information) including an RA-RNTI (Random Access Radio Network Temporary Identifier), and a PBCH (Physical Broadcast Channel).

The data Channel includes a PDSCH (Physical Downlink Shared Channel), a PUSCH (Physical Uplink Shared Channel), and the like.

RA procedure execution unit 230 executes a Random Access (RA) procedure with the radio base station (specifically, any one of gNB100A to gNB 100C). Specifically, the RA procedure execution unit 230 transmits and receives a message according to the RA procedure under the control of the control unit 250.

In addition, the RA procedure may include contention-based (contention) random access (CBRA) and non-contention-based random access (CFRA) RA procedures.

In the case of CBRA, the RA procedure execution unit 230 transmits a Random Access Preamble (msg.1) to the gNB of the connection request destination, and receives a Random Access Response (msg.2) as a Response to the Random Access Preamble (Random Access Preamble) from the gNB. Thereafter, the RA procedure executing part 230 transmits a Scheduled Transmission (msg.3) to the gNB, and receives a Contention Resolution (Contention Resolution) (msg.4) from the gNB.

The RA procedure is executed, for example, in the initial access from the RRC _ IDLE state, the RRC connection re-establishment procedure, and the like, but in the present embodiment, as described below, the RA procedure is also executed when the Radio Link Failure (RLF) is accompanied by a transition to the target radio base station.

The data discarding part 240 manages UL data and discards the UL data according to control performed by the control part 250. Specifically, the data discarding unit 240 discards UL data that is not transmitted to a buffer (not shown) and is held, according to control performed by the control unit 250.

Specifically, the data discarding part 240 discards UL data transmitted via a Radio Bearer (RB), that is, discards unsent UL data held in the buffer, according to control by the control part 250.

In addition, the radio bearers include a Signaling Radio Bearer (SRB) and a Data Radio Bearer (DRB). SRB is for control plane data and DRB is for user plane data. The SRBs may be set to SRBs 0, SRBs 1, SRBs 2, and SRBs 3 according to the application.

The SRB0 is used for RRC messages using CCCH (Common Control Channel) logical channels. Specifically, the SRB0 is used for transmission and reception of a specific RRC message (RRC Setup Request, etc.).

The SRB1 uses a DCCH (Dedicated Control Channel) logical Channel for RRC messages (which may include piggybacked Non-Access Stratum (NAS) messages) and NAS messages prior to establishment of the SRB 2.

SRB2 is used for NAS messages, using DCCH logical channels. SRB2 has a lower priority than SRB1 and may be set by the network after AS security is activated.

The SRB3 is used for specific RRC messages when the UE200 is in E-UTRA-NR Dual Connectivity (EN-DC) using DCCH logical channels.

In addition, DRB is used for user data.

The control unit 250 controls each functional block constituting the UE 200. In particular, in the present embodiment, the migration (including handover) of the UE200 between the gnbs is controlled.

Specifically, the control unit 250 migrates to the target radio base station without using a reconnection procedure to the network. More specifically, the control unit 250 performs a cell migration procedure by the Conditional HO, and migrates to the target radio base station without using a reconnection procedure in the RRC layer.

When the Radio Link Failure (RLF) occurs and the radio base station moves to the target radio base station, the control unit 250 starts a random access procedure (RA procedure) with the target radio base station at a predetermined timing.

Specifically, the control unit 250 can control the RA procedure execution unit 230 to start the RA procedure according to a predetermined timing, for example, an instruction from the RRC layer or generation and/or transmission of a predetermined message in the RRC layer. The details of this timing will be described later.

Further, the control unit 250 can determine to discard at least data transmitted via the Signaling Radio Bearer (SRB) when the target radio base station is migrated with the RLF. Specifically, the control unit 250 controls the data discarding unit 240 to cause the data discarding unit 240 to discard data transmitted via the SRBs 0, SRBs 1, and SRBs 2 (or SRBs 3, if SRBs 3 are set).

Further, after discarding the data, the control unit 250 starts an RA procedure with the target radio base station based on the Buffer Status Report (BSR). Specifically, the control unit 250 discards the data and treats the data generated thereafter as high-priority data, thereby causing the RA procedure execution unit 230 to execute the RA procedure based on the normal BSR.

(3) Operation of a wireless communication system

Next, an operation of the radio communication system 10 will be described. Specifically, the operation of the Conditional HO and the operation of the RLF recovery from the use of the Conditional HO will be described, and the operation capable of solving the problem of the RLF recovery from the use of the Conditional HO will be described.

(3.1)Conditional HO

Fig. 3 is an explanatory diagram of a conventional handover procedure, and fig. 4 is an explanatory diagram of a handover procedure based on a Conditional HO (Conditional HO).

As shown in fig. 3, in a conventional handover procedure, a network (gNB) determines a target radio base station (T-gNB) based on quality information (fig. (1)) such as a Measurement Report (Measurement Report) transmitted from a terminal (User Equipment, UE), prepares handover ((2) in the figure), and transmits a handover command to the terminal (fig. (3)).

However, when the terminal passes through a point (point) of appropriate handover in preparation for handover in the network side, there are cases where the terminal directly migrates to the target radio base station ((4) in the drawing) without receiving a handover command from the source radio base station (S-gNB) (may also be referred to as a "too late HO"). Therefore, the terminal cannot recognize the setting related to the target radio base station, and a momentary interruption of the radio link occurs.

To solve this problem, a process called Conditional HO (may also be abbreviated CHO) was studied. In the Conditional HO, a handover candidate cell and a handover condition for the candidate cell are set in advance for a terminal. Thus, the terminal can connect to the target radio base station without waiting for an instruction (handover command) from the network, and can avoid an instantaneous interruption of the radio link.

Specifically, as shown in fig. 4, preparation for handover is performed in advance between the source radio base station (S-gNB) and the target radio base station (T-gNB) (fig. (1)), and the terminal is notified of the setting content of the Conditional HO including the transition condition for the target radio base station (fig. (2)). When the terminal determines to connect to the target radio base station by moving, etc., (3) in the figure, the terminal starts an RA procedure with the target radio base station according to the setting contents of the Conditional HO.

In addition, the "handover command" may be referred to as a reconfiguration withsync in NR and may be referred to as RRC connection reconfiguration (including mobility) in LTE.

(3.2) recovery from Radio Link Failure (RLF) Using Conditional HO

Next, a procedure of recovery from Radio Link Failure (RLF) using the above-described Conditional HO will be described. Specifically, as an example of the RLF, a case will be described in which a terminal (UE) fails handover to a current target cell (cell a) (referred to as a handover failure (HOF)) and switches to another target cell (cell B).

Fig. 5 is an explanatory diagram of a procedure for recovering from a handover failure (HOF) using a reconnection procedure in an RRC layer, and fig. 6 is an explanatory diagram of a procedure for recovering from a handover failure (HOF) using a Conditional HO.

In fig. 5 and 6, an example is shown in which the terminal attempts handover to cell a ((1) in the figure), but moves into cell B ((2) in the figure) before the handover is completed and performs handover to cell B ((3) in the figure).

When the terminal (UE) detects RLF in the case of recovery from the HOF using the reconnection procedure, it searches for an optimal cell (best cell) as a migration destination (handover destination) at that time. If the optimal cell can be searched, the terminal initiates a reconnection procedure (RRC connection re-establishment) with the cell and performs a connection with the cell (3GPP TS38.331, etc.).

On the other hand, in the case of recovery from the HOF using the Conditional HO (CHO), a process according to the Conditional HO is executed instead of the reconnection process ((3) in the figure). This can reduce the instantaneous interruption time of the wireless link.

Hereinafter, the details will be further described. Fig. 7 shows a recovery timing from a handover failure (HOF) using a reconnection procedure in the RRC layer. Fig. 8 shows a recovery sequence from a handover failure (HOF) using the Conditional HO.

As shown in fig. 7 and 8, in either of the restoration from the HOF using the reconnection procedure and the restoration from the HOF using the Conditional HO, the terminal (UE) is notified of the settings (HO configuration/CHO configuration) related to the handover in accordance with the preparation for the handover (HO preparation/CHO preparation).

As described above, information on CHO, such as a handover candidate cell and a transition condition for the handover candidate cell, is set in advance in the terminal in the case of the Conditional HO.

The terminal transmits a Random Access Preamble (RA) to the target cell (cell a, see fig. 5 and 6), but moves to cell B, and therefore the RA procedure fails, and determines the HOF.

After that, in the case of recovery from the HOF using the reconnection procedure shown in fig. 7, the terminal and cell B (here, denoted as gNB #2) start the RA procedure. In addition, a reconnection procedure in the RRC layer is performed, and a user plane (user plane) is turned on (established).

On the other hand, in the case of the recovery from the HOF using the Conditional HO shown in fig. 8, after performing the RA procedure with the cell B (gNB #2), the terminal transmits only the RRC reconfiguration complete, and the user plane is turned on (established). In Conditional HO, since the terminal identifies a candidate destination cell for handover and a migration condition for the candidate destination cell, it is not necessary to perform a reconnection procedure in the RRC layer. Thereby, the instantaneous interruption time is shortened compared to the recovery from the HOF using the reconnection procedure (fig. 7).

Further, RRC reconfiguration complete (RRC reconfiguration complete) in the case of recovery from the HOF using the Conditional HO may also be omitted. This is because the cell B (gNB #2) can implicitly recognize that the terminal has migrated due to the Conditional HO.

The terminal can operate as follows. Specifically, when RLF occurs, the terminal performs cell selection, and in the case where the selected cell is a CHO candidate cell, CHO is attempted. Otherwise, the terminal performs a reconnection procedure in the RRC layer. Further, the terminal performs cell selection in case of a conventional HO failure (expiration of T304) or an access failure to a CHO candidate cell, and attempts CHO in case of the selected cell being a CHO candidate cell.

(3.3) layer 2 control at Handover/reconnection

The control of the layer 2 in the case where the above-described conventional RLF occurs includes control based on the Conditional HO, and there are the following problems.

Specifically, there may occur (i) a case where there is no opportunity to start a random access procedure (RA procedure) with the target radio base station, and (ii) a case where there is no opportunity to restart (resume) the radio bearer (that is, UL data cannot be transmitted).

Thus, it is desirable to specify a radio bearer restart opportunity after occurrence of the HOF and layer 2 control for RA procedure start.

The layer 2 control at the time of handover and reconnection can be performed as follows from the viewpoint of RA procedures and UL data transmission.

Views of the RA procedure

In the conventional HO and reconnection procedures, it is prescribed that the RA procedure is started for the target cell and synchronization is established

In the medium access control layer (MAC) layer, the initiation of this RA procedure is performed according to a buffer status report (in particular, a Regular BSR) based on the occurrence of high priority data (3GPP TS38.321)

That is, the intra-terminal operation procedure becomes (i) high-priority data generation, (ii) Regular BSR is triggered, (iii) Scheduling Request (SR) is triggered, and (iv) RA procedure is started.

In the case of HO, all data of the SRB1 is discarded when RLF occurs, and then an RRC reconfiguration complete is generated (RRC reconfiguration complete). The RRC reconfiguration complete is handled as occurrence of "high priority data".

Further, upon reconnection to the target radio base station, an RRC Request (transmitted by SRB0 (CCCH)) is generated. The RRC Request is handled as "high priority data" occurring.

Viewpoint of UL data

In the case of HO, all data of the radio bearer can be sent after HO

When re-connection with the target radio base station is performed by RLF, transmission is once reserved (suspend) in RLF, and after re-connection establishment, re-start (resume) is initiated with the following trigger

SRB 1: when RRC Request is transmitted

SRB 2/DRB: after the reception processing of RRC Reconfiguration

Fig. 9 and 10 show images of interaction (interaction) between layers within the terminal related to the start of the RA procedure. Specifically, fig. 9 shows an image of interaction between layers within the terminal at the time of handover, and fig. 10 shows an image of interaction between layers within the terminal at the time of reconnection toward the target wireless base station.

As shown in fig. 9 and 10, the terminal (UE200) has a radio resource control layer (RRC), a Packet Data Convergence Protocol layer (PDCP), a radio link control layer (RLC), and a medium access control layer (MAC).

As shown in fig. 9, in case of HO, when a terminal (RRC) receives a handover command, a Reset (Reset) is requested for MAC and a re-establishment (re-est) is requested for PDCP/RLC. In addition, the RRC notifies the PDCP/RLC of an RRC reconfiguration complete.

The PDCP/RLC notifies display of a Buffer Status (BS) to the MAC. The MAC initiates the RA procedure according to the BS's display.

On the other hand, as shown in fig. 10, when the terminal (RRC) detects RLF in the case of performing reconnection to the target radio base station, all radio bearers except for the SRB0 are stopped (suspend). In addition, the RRC requests the MAC to be reset, and notifies the PDCP/RLC of an RRC Request (RRC Reestablishment Request).

Thereafter, as in fig. 9, the PDCP/RLC notifies the MAC of the display of the Buffer Status (BS). The MAC initiates the RA procedure according to the BS's display.

Fig. 11 shows an image of suspension/resume of a radio bearer during reconnection to a target radio base station. Specifically, fig. 11 shows a relationship between data transmittable/non-transmittable periods of SRBs 0, SRBs 1, and SRBs 2/DRBs and transmission/reception timings of messages in RRC.

As shown in fig. 11, the terminal detects RLF, and when the RA procedure is started, SRB1 and SRB2/DRB are stopped (suspend).

Thereafter, when the terminal transmits an RRC Request, the SRB1 is restarted (resume). In addition, the terminal receives RRC Reconfiguration, and when transmitting RRC Reconfiguration complete, SRB2/DRB is also restarted. In addition, the SRB0 is not particularly stopped and restarted, and data transmission is always possible as a state.

(3.4) random Access procedure

As described above, in the case where the control of layer 2 in the case of the conventional RLF occurrence includes control by Conditional HO (CHO), there may be no chance of starting the random access procedure (RA procedure) with the target radio base station.

More specifically, in the CHO, the terminal transmits an RRC reconfiguration complete to the target radio base station, but since the buffer status report (Regular BSR) is not triggered, there is a case where an RA procedure for Scheduling Request (SR) transmission is not started.

Fig. 12 shows an example in which an RA procedure is not initiated in a case where a Conditional HO is applied to a Radio Link Failure (RLF).

Specifically, when there is data already transmitted by the SRB1, for example, there may be an RRC reconfiguration complete (RRC reconfiguration complete) transmitted by the initial HO. As shown in fig. 12, the RRC reconfiguration complete is also maintained after the RLF.

More specifically, in the terminal, in a state where there is already data of the same priority (not transmitted), the Regular BSR is not triggered even if additional data occurs (refer to 3GPP TS 38.321). Further, in the usual case, once the Regular BSR is triggered, then the Regular BSR can be triggered due to the expiration of the retxsbsr-timer, but in such a case as described above, the retxsbsr-timer is stopped due to the MAC reset at RLF.

Thus, in the present embodiment, the terminal starts the RA procedure at a predetermined timing. Fig. 13 is a diagram showing an example in which an RA procedure is initiated in the case where the problem shown in fig. 12 is eliminated and a Conditional HO is applied to a Radio Link Failure (RLF).

Specifically, the terminal may initiate the RA procedure by any of the methods described below.

(method 1): directly starting RA procedure at predetermined timing

Method 2: at CHO after RLF, data for SRB1 is discarded

In the case of (method 1), the predetermined timing may be any timing described below.

Timing of the layer PDCP/RLC, etc. receiving the indication from the RRC layer

When a specific message (for example, RRC reconfiguration complete) is generated in the RRC layer, or when the message is transmitted from the RRC layer to a lower layer such as PDCP/RLC

The case where a specific message arrives at the PDCP/RLC buffer, or the case where a PDCP PDU, an RLC PDU, or a MAC sub PDU is generated from the message

The RRC reconfiguration complete may be referred to by another name, for example, a CHO reconfiguration message (CHO completion information). Further, the "predetermined timing" may be any timing after the RLF or after the CHO. Alternatively, the timing may be the timing when the Radio Bearer (RB) is stopped or the timing when the MAC reset is transmitted.

Furthermore, the initiation of the RA procedure may be achieved by any of the methods described below (either an indication from the RRC or autonomously performed by the MAC layer).

Triggering the RA procedure directly (which can also be considered triggered)

Trigger Scheduling Request (SR) (which can also be considered triggered)

Triggering Regular BSR (can also be considered triggered)

In the case of (method 2), the discard of the data may be at least one of the following.

By discarding the data, the RRC reconfiguration complete generated thereafter is regarded as "high priority data", and the RA procedure based on the past Regular BSR is started. Alternatively, the data may not be discarded but may be regarded as having no (transmittable) data.

The following arbitrary case may be used for discarding (timing when data is deemed to be missing)

RLF detection (defined as the case where CHO is set)

CHO-based migration to a new cell

Application of settings (preset by CHO settings) corresponding to the new cell

After a lapse of a predetermined time from the above-mentioned arbitrary timing

Discarding can be achieved by any of the following methods

Consider as if discardOnPDCP (for SRB)

Perform PDCP data recovery or PDCP re-estimation on SRB1

When discardOnPDCP is notified to PDCP, a terminal (UE) discards at least one or both of transmission data and reception data staying at that time point with respect to a SRB. When the PDCP is notified of the PDCP data recovery or the PDCP re-acknowledgement, the terminal discards at least one or both of the transmission data and the reception data for the radio bearer. Thus, the discarding of the data is performed while using the existing procedure.

(4) Action and Effect

According to the above embodiment, the following operational effects can be obtained. Specifically, the UE200 (terminal) supports a Conditional HO of "migration to a target radio base station without using a reconnection procedure in the RRC layer", and when migration to the target radio base station is performed in accordance with the Conditional HO without using the reconnection procedure in association with a Radio Link Failure (RLF), the UE200 can start a random access procedure (RA procedure) with the target radio base station at a predetermined timing.

Further, the UE200 can decide to discard data transmitted via the signaling radio bearer when migrating to the target radio base station in accordance with the Conditional HO without using the reconnection procedure with the Radio Link Failure (RLF).

Specifically, the UE200 starts an RA procedure with the target radio base station according to the Buffer Status Report (BSR) after discarding data.

In the Conditional HO, the UE200 transmits an RRC reconfiguration complete to the target radio base station, but in a specific case such as a case where there is data already transmitted via the SRB1, the Buffer Status Report (BSR) is not triggered, and therefore, an RA procedure for transmitting a Scheduling Request (SR) to the radio base station may not be started.

Thus, by including control based on Conditional HO (CHO) for layer 2 control at the time of occurrence of the conventional RLF, it is possible to reduce, for example, the instantaneous interruption time at the time of HOF and avoid a communication delay due to the RA procedure not being started.

(5) Other embodiments

While the present invention has been described with reference to the embodiments, it will be apparent to those skilled in the art that the present invention is not limited to the descriptions, and various modifications and improvements can be made.

For example, in the above-described embodiment, NR was described as an example, but Conditional HO can be applied to LTE, and similar operations can be performed in LTE. Furthermore, the present invention can also be applied to addition/modification of a Primary SCell (PSCell) of a Multi-RAT Dual Connectivity (MR-DC).

In the above-described embodiments, the Conditional HO was described as an example, but any other procedure may be applied instead of the Conditional HO as long as the procedure is a transition procedure to the target radio base station without using the reconnection procedure in the RRC layer. For example, the present invention can be generally applied to a case where a terminal autonomously selects a cell and executes a random access procedure, such as a case where the terminal transitions from an IDLE state (IDLE state) or an Inactive state (Inactive state) to a Connected state (Connected state). That is, the random access procedure is not necessarily limited to the case of moving to the target radio base station without using the reconnection procedure, and may be executed as described above when moving from the predetermined state to the target radio base station.

In the above-described embodiment, the case of discarding the data transmitted via the SRB1 was described as an example, but the terminal (UE200) may determine to discard the data transmitted via the Data Radio Bearer (DRB) or the signaling radio bearer other than the SRB 1.

Note that the block diagram (fig. 2) used in the description of the above embodiment shows blocks in units of functions. These functional blocks (components) are realized by any combination of at least one of hardware and software. The method of implementing each functional block is not particularly limited. That is, each functional block may be implemented by one device that is physically or logically combined, or may be implemented by two or more devices that are physically or logically separated and that are directly or indirectly (for example, wired or wireless) connected and implemented by these plural devices. The functional blocks may also be implemented by a combination of software and one or more of the above-described devices.

The functions include judgment, decision, judgment, calculation, processing, derivation, investigation, search, confirmation, reception, transmission, output, access, resolution, selection, establishment, comparison, assumption, expectation, viewing, broadcasting (broadcasting), notification (notification), communication (communicating), forwarding (forwarding), configuration (configuring), reconfiguration (reconfiguring), allocation (allocating, mapping), assignment (assigning), and the like, but are not limited thereto. For example, a function block (a configuration unit) that functions transmission is referred to as a transmission unit (transmitter) or a transmitter (transmitter). In short, as described above, the method of implementation is not particularly limited.

The UE200 described above may also function as a computer that performs the processing of the wireless communication method of the present disclosure. Fig. 14 is a diagram showing an example of the hardware configuration of the UE 200. As shown in fig. 14, the UE200 may be configured as a computer device including a processor 1001, a memory 1002(memory), a storage 1003(storage), a communication device 1004, an input device 1005, an output device 1006, a bus 1007, and the like.

In the following description, the term "device" may be replaced with "circuit", "device", "unit", and the like. The hardware configuration of the apparatus may include one or more of the illustrated apparatuses, or may be configured not to include a part of the apparatuses.

Each functional block of the UE200 (see fig. 2) can be implemented by any hardware element of the computer device or a combination of the hardware elements.

Further, each function in the UE200 is realized by the following method: when predetermined software (program) is read into hardware such as the processor 1001 and the memory 1002, the processor 1001 performs an operation to control communication of the communication device 1004 or at least one of reading and writing of data in the memory 1002 and the storage 1003.

The processor 1001 operates, for example, an operating system to control the entire computer. The processor 1001 may be a Central Processing Unit (CPU) including an interface with a peripheral device, a control device, an arithmetic device, a register, and the like.

Further, the processor 1001 reads out a program (program code), a software module, data, or the like from at least one of the memory 1003 and the communication device 1004 to the memory 1002, and executes various processes in accordance therewith. As the program, a program that causes a computer to execute at least a part of the operations described in the above-described embodiments is used. The various processes described above may be executed by one processor 1001, or may be executed by two or more processors 1001 simultaneously or sequentially. The processor 1001 may also be mounted by more than one chip. In addition, the program may also be transmitted from the network via a telecommunication line.

The Memory 1002 is a computer-readable recording medium, and may be configured by at least one of a ROM (Read Only Memory), an EPROM (Erasable Programmable ROM), an EEPROM (Electrically Erasable Programmable ROM), a RAM (Random Access Memory), and the like. Memory 1002 may also be referred to as registers, cache, main memory (primary storage), etc. The memory 1002 can store a program (program code), a software module, and the like that can execute the method according to one embodiment of the present disclosure.

The storage 1003 is a computer-readable recording medium, and may be constituted by at least one of an optical disk such as a CD-rom (compact Disc rom), a hard disk drive, a flexible disk, a magneto-optical disk (for example, a compact Disc, a digital versatile Disc, a Blu-ray (registered trademark) Disc, a smart card, a flash memory (for example, a card, a stick, a Key drive), a Floppy (registered trademark) Disc, a magnetic stripe, and the like.

The communication device 1004 is hardware (a transmitting/receiving device) for performing communication between computers via at least one of a wired network and a wireless network, and may be referred to as a network device, a network controller, a network card, a communication module, or the like.

Communication apparatus 1004 may be configured to include a high-Frequency switch, a duplexer, a filter, a Frequency synthesizer, and the like, for example, in order to realize at least one of Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD).

The input device 1005 is an input device (for example, a keyboard, a mouse, a microphone, a switch, a key, a sensor, and the like) that receives an input from the outside. The output device 1006 is an output device (for example, a display, a speaker, an LED lamp, or the like) that outputs to the outside. The input device 1005 and the output device 1006 may be integrally formed (for example, a touch panel).

The processor 1001 and the memory 1002 are connected to each other via a bus 1007 for communicating information. The bus 1007 may be configured by using a single bus, or may be configured by using different buses for each device.

The apparatus may include hardware such as a microprocessor, a Digital Signal Processor (DSP), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), and an FPGA (Field Programmable Gate Array), and a part or all of each functional block may be realized by the hardware. For example, the processor 1001 may also be installed using at least one of these hardware.

Further, the notification of information is not limited to the form/embodiment described in the present disclosure, and may be performed using other methods. For example, the Information may be notified by physical layer signaling (e.g., DCI (Downlink Control Information), UCI (Uplink Control Information)), higher layer signaling (e.g., RRC (Radio Resource Control) signaling, MAC (Medium Access Control) signaling, broadcast Information (MIB (Master Information Block), SIB (System Information Block)), other signals, or a combination thereof).

The forms/embodiments described in this disclosure can also be applied to at least one of Long Term Evolution (LTE), LTE-Advanced (LTE-a), SUPER 3G, IMT-Advanced, fourth generation Mobile communication system (4th generation Mobile communication system: 4G), fifth generation Mobile communication system (5th generation Mobile communication system: 5G), Future Radio Access (Future Radio Access: FRA), New Radio (NR), W-CDMA (registered trademark), GSM (registered trademark), CDMA 2000, Ultra Mobile Broadband (Ultra Mobile Broadband: UMB), IEEE 802.11(Wi-Fi (registered trademark)), IEEE 802.16(WiMAX (registered trademark)), IEEE 802.20, UWB (Ultra-wide band), Bluetooth (registered trademark), and extended systems using other suitable systems. Furthermore, a plurality of systems (for example, a combination of 5G and at least one of LTE and LTE-a) may be combined and applied.

For the processing procedures, timings, flows, and the like of the respective forms/embodiments described in the present disclosure, the order may be changed without contradiction. For example, for the methods described in this disclosure, elements of the various steps are suggested using an illustrative sequence, but are not limited to the particular sequence suggested.

In the present disclosure, a specific operation performed by a base station is sometimes performed by its upper node (upper node) depending on the situation. In a network including one or more network nodes (network nodes) having a base station, it is obvious that various operations performed for communication with a terminal may be performed by at least one of the base station and a network node other than the base station (for example, an MME, an S-GW, or the like is considered, but not limited thereto). In the above, the case where there is one network node other than the base station is exemplified, but the other network node may be a combination of a plurality of other network nodes (e.g., MME and S-GW).

Information, signals (information), and the like can be output from an upper layer (or a lower layer) to a lower layer (or an upper layer). Or may be input or output via a plurality of network nodes.

The input or output information and the like may be stored in a specific location (for example, a memory) or may be managed using a management table. The input or output information and the like may be rewritten, updated, or appended. The output information and the like may also be deleted. The inputted information and the like may also be transmitted to other apparatuses.

The determination may be made by a value (0 or 1) represented by 1 bit, may be made by a Boolean value (true or false), or may be made by comparison of values (for example, comparison with a predetermined value).

The aspects and embodiments described in the present disclosure may be used alone or in combination, or may be switched depending on execution. Note that the notification of the predetermined information is not limited to be performed explicitly (for example, notification of "X") but may be performed implicitly (for example, notification of the predetermined information is not performed).

Software, whether referred to as software, firmware, middleware, microcode, hardware description languages, or by other names, should be construed broadly to mean commands, command sets, code segments, program code, programs (routines), subroutines, software modules, applications, software packages, routines, subroutines (subroutines), objects, executables, threads of execution, procedures, functions, and the like.

Further, software, commands, information, and the like may be transmitted and received via a transmission medium. For example, where software is transmitted from a web page, server, or other remote source using at least one of wired technology (coaxial cable, fiber optic cable, twisted pair, Digital Subscriber Line (DSL), etc.) and wireless technology (infrared, microwave, etc.), at least one of these is included within the definition of transmission medium.

Information, signals, and the like described in this disclosure may also be represented using any of a variety of different technologies. For example, data, commands, instructions (commands), information, signals, bits, symbols (symbols), chips (chips), etc., that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or photons, or any combination thereof.

Further, terms described in the present disclosure and terms necessary for understanding the present disclosure may be replaced with terms having the same or similar meanings. For example, at least one of the channel and the symbol may be a signal (signaling). Further, the signal may also be a message. In addition, a Component Carrier (CC) may be referred to as a Carrier frequency, a cell, a frequency Carrier, and the like.

The terms "system" and "network" and the like as used in this disclosure may be used interchangeably.

Further, information, parameters, and the like described in the present disclosure may be expressed using absolute values, may be expressed using relative values to predetermined values, and may be expressed using other corresponding information. For example, the radio resource may also be indicated by an index.

The names used for the above parameters are in no way limiting. Further, the numerical expressions and the like using these parameters may be different from those explicitly shown in the present disclosure. Various channels (e.g., PUCCH, PDCCH, etc.) and information elements may be identified by appropriate names, and thus the various names assigned to these various channels and information elements are not limiting in any respect.

In the present disclosure, terms such as "Base Station (BS)", "wireless Base Station", "fixed Station", "NodeB", "enodeb (enb)", "gnnodeb (gnb)", "access point", "transmission point", "reception point", "cell", "sector", "cell group", "carrier", "component carrier" and the like may be used interchangeably. A base station may also be referred to as a macrocell, a smallcell, a femtocell, a picocell, or the like.

A base station can accommodate one or more (e.g., 3) cells. When a base station accommodates a plurality of cells, the entire coverage area of the base station can be divided into a plurality of smaller areas, and each smaller area can also be provided with a communication service by a base station subsystem (e.g., an indoor small Radio Head (RRH)).

The term "cell" or "sector" refers to a part or the whole of the coverage area of at least one of a base station and a base station subsystem that performs communication service within the coverage area.

In the present disclosure, terms such as "Mobile Station (MS)", "User terminal (User terminal)", "User Equipment (UE)", "terminal" and the like may be used interchangeably.

For a mobile station, those skilled in the art will sometimes also refer to the following terms: a subscriber station, a mobile unit, a subscriber unit, a wireless unit, a remote unit, a mobile device, a wireless communication device, a remote device, a mobile subscriber station, an access terminal, a mobile terminal, a wireless terminal, a remote terminal, a handset, a user agent (user agent), a mobile client, a client, or some other suitable terminology.

At least one of the base station and the mobile station may also be referred to as a transmitting apparatus, a receiving apparatus, a communication apparatus, or the like. At least one of the base station and the mobile station may be a device mounted on a mobile body, the mobile body itself, or the like. The moving body may be a vehicle (e.g., an automobile, an airplane, etc.), may be a moving body that moves in an unmanned manner (e.g., an unmanned aerial vehicle, an autonomous automobile, etc.), or may be a robot (manned or unmanned). At least one of the base station and the mobile station includes a device that does not necessarily move during a communication operation. For example, at least one of the base station and the mobile station may be an IoT (Internet of Things) device such as a sensor.

In addition, the base station in the present disclosure may also be replaced with a mobile station (user terminal, the same applies hereinafter). For example, the embodiments and embodiments of the present disclosure may be applied to a configuration in which communication between a base station and a mobile station is replaced with communication between a plurality of mobile stations (for example, a configuration may be referred to as D2D (Device-to-Device) or V2X (Vehicle-to-all system), and in this case, the mobile station may have a function of the base station.

Likewise, the mobile station in the present disclosure may be replaced with a base station. In this case, the base station may have a function of the mobile station.

A radio frame may be composed of one or more frames in the time domain. One or more individual frames in the time domain may also be referred to as subframes. A subframe may also be composed of one or more slots in the time domain. The subframe may be a fixed time length (e.g., 1ms) independent of a parameter set (numerology).

The parameter set may also be a communication parameter applied to at least one of transmission and reception of a certain signal or channel. The parameter set may indicate, for example, at least one of SubCarrier Spacing (SCS), bandwidth, symbol length, cyclic prefix length, Transmission Time Interval (TTI), number of symbols per TTI, radio frame structure, specific filtering processing performed by the transceiver in the frequency domain, specific windowing processing performed by the transceiver in the Time domain, and the like.

A slot may be composed of one or more symbols in a time domain (OFDM (Orthogonal Frequency Division Multiplexing) symbol, SC-FDMA (Single Carrier Frequency Division Multiple Access) symbol, etc.). The time slot may be a time unit based on a parameter set.

A timeslot may also contain multiple mini-slots. Each mini-slot may be formed of one or more symbols in the time domain. In addition, a mini-slot may also be referred to as a sub-slot. A mini-slot may also be made up of fewer symbols than a slot. The PDSCH (or PUSCH) transmitted in a time unit larger than the mini slot may also be referred to as PDSCH (or PUSCH) mapping type a. PDSCH (or PUSCH) transmitted using mini-slots may also be referred to as PDSCH (or PUSCH) mapping type B.

The radio frame, subframe, slot, mini-slot, and symbol all represent a unit of time when a signal is transmitted. Other names respectively corresponding to radio frame, subframe, slot, mini-slot and symbol may be used.

For example, 1 subframe may be referred to as a Transmission Time Interval (TTI), a plurality of consecutive subframes may be referred to as TTIs, and 1 slot or 1 mini-slot may be referred to as a TTI. That is, at least one of the subframe and TTI may be a subframe (1ms) in the conventional LTE, may be a period shorter than 1ms (for example, 1-13 symbols), or may be a period longer than 1 ms. Note that the unit indicating TTI may be referred to as a slot, a mini slot, or the like, instead of a subframe.

Here, the TTI refers to, for example, the minimum time unit of scheduling in wireless communication. For example, in the LTE system, the base station performs scheduling for allocating radio resources (frequency bandwidths, transmission powers, and the like that can be used by each user terminal) to each user terminal in units of TTIs. In addition, the definition of TTI is not limited thereto.

The TTI may be a transmission time unit of a channel-coded data packet (transport block), code block, code word, or the like, or may be a processing unit of scheduling, link adaptation, or the like. When a TTI is assigned, the time interval (for example, the number of symbols) to which the transport block, code word, and the like are actually mapped may be shorter than the TTI.

When 1 slot or 1 mini-slot is referred to as TTI, 1 or more TTIs (i.e., 1 or more slots or 1 or more mini-slots) may constitute the minimum time unit for scheduling. In addition, the number of slots (the number of mini-slots) constituting the minimum time unit of the schedule may be controlled.

A TTI having a time length of 1ms may be referred to as a normal TTI (TTI in LTE rel.8-12), a normal TTI, a long TTI, a normal subframe, a long subframe, a slot, etc. A TTI shorter than a normal TTI may be referred to as a shortened TTI, a short TTI, a partial TTI, a shortened subframe, a short subframe, a mini-slot, a sub-slot, a slot, etc.

Further, a long TTI (e.g., normal TTI, subframe, etc.) may be replaced with a TTI having a time length exceeding 1ms, and a short TTI (e.g., shortened TTI, etc.) may be replaced with a TTI having a TTI length less than that of the long TTI and greater than 1 ms.

A Resource Block (RB) is a resource allocation unit in the time domain and the frequency domain, and may include one or more consecutive subcarriers (subcarriers) in the frequency domain. The number of subcarriers included in the RB may be the same regardless of the parameter set, and may be 12, for example. The number of subcarriers included in the RB may be determined based on the set of parameters.

In addition, the time domain of the RB may include one or more symbols, and may have a length of 1 slot, 1 mini-slot, 1 subframe, or 1 TTI. The 1TTI, 1 subframe, and the like may be configured by one or more resource blocks.

In addition, one or more RBs may also be referred to as Physical Resource Blocks (PRBs), Sub-Carrier groups (SCGs), Resource Element Groups (REGs), PRB pairs, RB peers, and so on.

In addition, a Resource block may also be composed of one or more Resource Elements (REs). For example, 1RE may be a radio resource region of 1 subcarrier and 1 symbol.

The Bandwidth Part (BWP: Bandwidth Part) (which may be referred to as partial Bandwidth) may represent a subset of consecutive common rbs (common resource blocks) for a certain parameter set in a certain carrier. Here, the common RB may also be determined by an index of an RB with reference to a common reference point of the carrier. PRBs may also be defined by a certain BWP and numbered within that BWP.

The BWP may include UL BWP (UL BWP) and DL BWP (DL BWP). For the UE, one or more BWPs may also be set within 1 carrier.

At least one of the set BWPs may be active, and the UE may not assume to transmit and receive a predetermined signal/channel outside the active BWP. In addition, "cell", "carrier", and the like in the present disclosure may also be replaced with "BWP".

The above-described structures of radio frames, subframes, slots, mini slots, symbols, and the like are merely examples. For example, the number of subframes included in the radio frame, the number of slots per subframe or radio frame, the number of mini-slots included in a slot, the number of symbols and RBs included in a slot or mini-slot, the number of subcarriers included in an RB, the number of symbols in a TTI, the symbol length, the Cyclic Prefix (CP) length, and the like can be variously changed.

The terms "connected" and "coupled" or any variation thereof are intended to mean that two or more elements are directly or indirectly connected or coupled to each other, and may include one or more intermediate elements between two elements that are "connected" or "coupled" to each other. The combination or connection between the elements may be physical, logical, or a combination of these. For example, "connect" may be replaced with "Access". As used in this disclosure, two elements may be considered to be "connected" or "coupled" to each other by using at least one of one or more wires, cables, and printed electrical connections, and by using electromagnetic energy, such as electromagnetic energy having wavelengths in the radio frequency domain, the microwave domain, and the optical (both visible and invisible) domain, as some non-limiting and non-inclusive examples.

The reference signal may be referred to as rs (reference signal) for short, or may be referred to as Pilot (Pilot) according to the applied standard.

As used in this disclosure, a statement "according to" is not intended to mean "solely according to" unless explicitly stated otherwise. In other words, the expression "according to" means both "according to" and "at least according to".

The "unit" in the configuration of each device described above may be replaced with a "section", "circuit", "device", or the like.

Any reference to an element using the designations "first", "second", etc. used in this disclosure is not intended to limit the number or order of such elements. These terms are used in the present disclosure as a convenient way to distinguish between two or more elements. Thus, references to first and second elements do not imply that only two elements are possible here or that in any case the first element must precede the second element.

Where the disclosure uses the terms "including", "comprising" and variations thereof, these terms are meant to be inclusive in the same way as the term "comprising". Also, the term "or" used in the present disclosure means not exclusive or.

In the present disclosure, where articles are added by translation, for example, as in the english language a, an, and the, the present disclosure also includes the case where nouns following the articles are plural.

Terms such as "determining" and "determining" used in the present disclosure may include various operations. The terms "determining" and "decision" may include, for example, the case where the determination (judging), calculation (calculating), processing (processing), derivation (deriving), investigation (investigating), search (looking up) (for example, searching in a table, a database, or another data structure), and confirmation (ascertaining) are regarded as being performed. The "determination" and "decision" may include a case where an event of reception (e.g., reception), transmission (e.g., transmission), input (input), output (output), and access (e.g., access to data in the memory) is regarded as an event of "determination" and "decision". The "judgment" and "decision" may include matters regarding the solution (resolving), selection (selecting), selection (breathing), establishment (evaluating), comparison (comparing), and the like as the "judgment" and "decision". That is, the terms "determining" and "deciding" may include any action. The "determination (decision)" may be replaced by "assumption", "expectation", "consideration", and the like.

In the present disclosure, the phrase "a and B are different" may also mean "a and B are different from each other". The term "A and B are different from C" may be used. The terms "separate", "coupled", and the like may also be construed as "different" in a similar manner.

While the present disclosure has been described in detail, it should be apparent to those skilled in the art that the present disclosure is not limited to the embodiments described in the present disclosure. The present disclosure can be implemented as modifications and alterations without departing from the spirit and scope of the present disclosure as defined by the claims. Accordingly, the disclosure is intended to be illustrative, and not limiting.

Description of reference numerals:

10 radio communication system

100A～100C gNB

200 UE

210 wireless transmission unit

220 wireless receiving part

230 RA procedure execution unit

240 data discarding part

250 control part

1001 processor

1002 internal memory

1003 memory

1004 communication device

1005 input device

1006 output device

1007 bus

Claims

1. A terminal, wherein,

the terminal has a control section that migrates to a target radio base station without using a reconnection procedure,

the control unit starts a random access procedure with the target radio base station at a predetermined timing when the radio link failure occurs and the radio base station moves to the target radio base station.

2. A terminal, wherein,

the control unit determines to discard data transmitted via a signaling radio bearer when the radio link failure occurs and the radio link is transferred to the target radio base station.

3. The terminal of claim 2, wherein,

the control unit starts a random access procedure with the target radio base station based on the buffer status report after discarding the data.