Method and apparatus for performing handover procedure in a wireless communication system

Abstract

and determining, based on the determination result, whether a dual active protocol stack (DAPS) handover has been set for at least one bearer. Based on the determination result, maintaining a connection with the source base station and performing a DAPS handover to the target base station, suspending a signaling radio bearer (SRB) set in the source base station, and establishing an SRB for the target base station. If it is determined that the DAPS handover has failed, determining whether a radio link failure (RLF) has occurred in a connection with the source base station. Based on the determination result, if no RLF has occurred in a connection with the source base station, resuming the SRB set in the source base station and reporting a failure of the DAPS handover.

Description

Technical Field

[0001] The present invention relates to a method and apparatus for performing a handover procedure in a wireless communication system.

Background Art

[0002] Due to the commercialization of 4G communication systems and the increase in multimedia services, an improved 5G communication system or pre-5G communication system has been developed to meet the explosively increasing demand for wireless data traffic. For this reason, 5G communication systems or pre-5G communication systems are referred to as systems after the 4G network (beyond 4G network) or systems after the LTE (long term evolution) system (post LTE).

[0003] In order to increase the data transmission rate, the 5G communication system is considered to be implemented in the millimeter wave (mmWave) band (such as the 60 gigahertz (60 GHz) band). In order to mitigate the path loss of radio waves in the millimeter wave band and extend the transmission distance of radio waves, in the 5G communication system, technologies such as beamforming, massive MIMO (multi input multi output), FD-MIMO (full dimensional MIMO), array antenna, analog beam-forming, and large scale antenna are being discussed.

[0004] In addition, for improving the network performance of the system, in the 5G communication system, technologies such as evolved small cells, improved small cells (advanced small cells), cloud radio access networks (cloud RAN), ultra-dense networks, device-to-device communication (D2D), wireless backhaul, moving networks, cooperative communication, CoMP (coordinated multi-points), and interference cancellation have been developed.

[0005] In addition, in the 5G system, FQAM (hybrid FSK and QAM modulation) and SWSC (sliding window superposition coding), which are advanced coding modulation (ACM) methods, and advanced connection technologies such as FBMC (filter bank multi carrier), NOMA (non orthogonal multiple access), and SCMA (sparse code multiple access) have been developed.

[0006] On the other hand, the Internet is evolving into an Internet of Things (IoT) network that exchanges and processes information among distributed components such as things in a human-centered connection network where people generate and consume information. The Internet of Everything (IoE) technology, which combines IoT technology with big data processing technology through connections to cloud servers and the like, has also been proposed. To implement the IoT, technical elements such as sensing technology, wired and wireless communication and network infrastructure, service interface technology, and security technology are required. Recently, technologies such as sensor networks for connecting things, machine to machine (M2M), and machine type communication (MTC) have been studied.

[0007] In the IoT environment, intelligent IT (internet technology) services can be provided that collect and analyze data generated by connected things and create new value for people's lives. The IoT is also applied to fields such as smart homes, smart buildings, smart cities, smart cars or connected cars, smart grids, healthcare, smart home appliances, and advanced medical services through the integration and combination of existing IT (information technology) technologies and various industries.

[0008] Accordingly, various attempts have been made to apply the 5G communication system to the IoT network. For example, technologies such as sensor networks, machine to machine (M2M), and MTC are implemented by techniques such as beamforming, MIMO, and array antennas, which are 5G communication technologies. The application of cloud radio access network (cloud RAN) as the aforementioned big data processing technology can also be said to be one example of the integration of 5G technology and IoT technology. As described above, with the development of wireless communication systems, various services can be provided, and in particular, a solution is required to smoothly support services related to terminal handover.

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] The present invention has been made in view of the problems in the above-described conventional wireless communication system, and an object of the present invention is to provide an apparatus and a method capable of effectively providing a service in a mobile communication system. **Means for Solving the Problem**

[0010] According to one aspect of the present invention, a terminal in a wireless communication system performed by The method includes receiving a message including "Reconfiguration with sync" from a source base station, a step of starting a first timer; when a DAPS (dual active protocol stack) bearer is set based on the message, establishing an RLC (radio link control) entity related to the target base station and stopping an SRB (signaling radio bearer) related to the source base station; when the first timer expires and the radio connection between the terminal and the source base station is valid, releasing the RLC entity related to the target base station, restarting the stopped SRB related to the source base station, and transmitting a DAPS handover failure report to the source base station; when the first timer expires and the radio connection between the terminal and the source base station is not valid, performing an RRC (radio resource control) re-establishment procedure, and having which is characterized in that.

[0011] the previous third timer When conditions for starting are satisfied, second timer while is being driven, it is preferable to further include the step of starting the third timer . previous When the DAPS bearer is set and an "out-of-sync" indicator is received from a lower layer entity, the first timer while is being driven, it is preferable to further include the step of starting the at the source base station related second timer . The second timer while is being driven, when an "in-sync "finger indicator is received from the lower layer entity, the second timer is abort caused, third timer if is being driven, it is preferable to further include the step of causing abort . When the third timer is completed, the it is preferable to further include the step of starting an RRC re-establishment procedure. Preferably, the first timer is a T304 timer, the second timer is a T310 timer, and the third timer is a T312 timer.

[0012] According to one aspect of the present invention, a terminal in a wireless communication system includes a transceiver and at least one processor connected to the transceiver. The at least one processor receives a message including "Reconfiguration with sync" from a source base station, Start the first timer. When a DAPS (dual active protocol stack) bearer is set based on the message, establish an RLC (radio link control) entity related to the target base station, stop the SRB (signaling radio bearer) related to the source base station. When the first timer expires and the radio connection between the terminal and the source base station is valid, release the RLC entity related to the target base station, restart the stopped SRB related to the source base station, transmit a DAPS handover failure report to the source base station. When the first timer expires and the radio connection between the terminal and the source base station is not valid, perform an RRC (radio resource control) re-establishment procedure which is characterized by the above.

[0013] previous the at least one processor, third timer when the condition for starting is satisfied, second timer while [a certain entity] is running, it is preferable to start the third timer . previous the at least one processor, when the DAPS bearer is set and an "out-of-sync" indicator is received from a lower layer entity, the first timer while [a certain entity] is running, it is preferable to start [a certain operation] at the source base station. related second timer While the at least one processor is driving the second timer, if an "in-sync" indicator is received from the lower layer entity, it is preferable to stop the second timer and, if a third timer is driving, to stop it. When the third timer expires, it is preferable that the RRC re-establishment procedure is performed. Preferably, the first timer is a T304 timer, the second timer is a T310 timer, and the third timer is a T312 timer.

[0014] According to one aspect of the present invention, a source base station in a wireless communication system performed by The method includes transmitting a message including "Reconfiguration with sync" to a terminal, When the first timer is started at the terminal and a DAPS (dual active protocol stack) bearer is set based on the message, an RLC (radio link control) entity related to the target base station is established at the terminal, the SRB (signaling radio bearer) related to the source base station is stopped at the terminal, the first timer expires, and if the radio connection between the terminal and the source base station is valid, the RLC entity related to the target base station is released at the terminal, the stopped SRB related to the source base station is restarted at the terminal, a DAPS handover failure report is transmitted to the source base station at the terminal, the first timer expires, and if the radio connection between the terminal and the source base station is not valid, an RRC (radio resource control) re-establishment procedure is executed at the terminal which is characterized by the above.

[0016] According to one aspect of the present invention, a source base station in a wireless communication system includes a transceiver and at least one processor connected to the transceiver. The at least one processor transmits a message including "Reconfiguration with sync" to a terminal, When a first timer is started at the terminal and a DAPS (dual active protocol stack) bearer is configured based on the message, an RLC (radio link control) entity related to the target base station is established at the terminal, an SRB (signaling radio bearer) related to the source base station is stopped at the terminal, the first timer expires, and the radio connection between the terminal and the source base station is valid, the RLC entity related to the target base station is released at the terminal, the stopped SRB related to the source base station is restarted at the terminal, a DAPS handover failure report is transmitted to the source base station at the terminal, the first timer expires, and the radio connection between the terminal and the source base station is not valid, an RRC (radio resource control) re-establishment procedure is executed at the terminal which is characterized by the above.

Advantages of the Invention

[0017] According to the method and apparatus for performing handover procedures in a wireless communication system according to the present invention, in a mobile communication system, an apparatus and a method for effectively providing services are provided.

Brief Description of the Drawings

[0018]

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Mode for Carrying Out the Invention

[0019] Hereinafter, with reference to the accompanying drawings, the operation principle of the present invention will be described in detail. In the following description of the present invention, when it is determined that a specific description related to a related known function or configuration will obscure the gist of the present invention, the detailed description thereof will be omitted. And the terms described below are terms defined in consideration of the functions in the present invention, and they also vary depending on the intention or convention of the user, operator, etc. Therefore, their definitions must be based on the content throughout this specification. In the following description of the present invention, when it is determined that a specific description related to a related known function or configuration will obscure the gist of the present invention, the detailed description thereof will be omitted. Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0020] Terms for identifying connection nodes (nodes) used in the following description, terms for referring to network entities, terms for referring to messages, terms for referring to network entity interfaces, terms for referring to various identification information, etc. are exemplified for the convenience of explanation. Therefore, the present invention is not limited to the terms described hereinafter, and other terms referring to objects having equivalent technical meanings may be used.

[0021] Hereinafter, for convenience of explanation, the present invention uses terms and names defined in the 3GPP (registered trademark) LTE (3rd generation partnership project long term evolution) standard. However, the present invention is not limited by the above-mentioned terms and names, and is equally applicable to systems based on other standards. In the present invention, for convenience of explanation, eNB may also be used interchangeably with gNB. That is, the base station described as eNB may indicate gNB.

[0022] The present invention relates to a handover method and an apparatus therefor in a next-generation mobile communication system for minimizing or making zero the interruption time of data transmission and reception due to handover. Specifically, the efficient handover method proposed in the present invention may have at least one of the following plurality of features.

[0023] · When a terminal that performs data transmission or reception (data transmission and reception on the uplink or downlink) via a source base station and each protocol layer "entity" (hereinafter referred to as "entity") of a plurality of first bearers (PHY layer entity, MAC layer entity, RLC layer entity, or PDCP layer entity) receives a handover command message (for example, a handover command message or an RRC Reconfiguration message) from the source base station, the terminal sets new protocol layer entities of a plurality of second bearers corresponding to the protocol layer entities of the plurality of first bearers (for example, those having the same bearer identifier), and continues to maintain data transmission or reception (data transmission and reception on the uplink or downlink) via the source base station and the plurality of first bearers without interruption, and performs data transmission and reception (data transmission and reception on the uplink or downlink).

[0024] · After the terminal receives the handover command message, the newly set protocol layer entities of the plurality of second bearers (PHY layer entity, MAC layer entity, RLC layer entity, or PDCP layer entity) are set for data transmission and reception with the target base station based on the bearer configuration information or protocol layer entity information included in the handover command message. · While performing data transmission or reception (data transmission and reception on the uplink or downlink) with the source base station using the protocol layer entities of the plurality of first bearers, the terminal performs a random access procedure with the target base station using the protocol layer entities of the plurality of second bearers (for example, the MAC layer entity). The random access procedure includes preamble transmission, random access response reception, message 3 transmission, or message 4 reception (for example, Contention resolution MAC CE or uplink transmission resource reception).

[0025] · The terminal uses the protocol layer devices of the first plurality of bearers to transmit or receive data with the source base station, and uses the protocol layer devices of the second plurality of bearers (e.g., MAC layer devices) to complete the random access procedure with the target base station, and uses the protocol layer devices of the second plurality of bearers to transmit a handover completion message to the target base station. · The terminal uses the protocol layer devices of the first plurality of bearers to transmit or receive data with the source base station, and uses the protocol layer devices of the second plurality of bearers (e.g., MAC layer devices) to complete the random access procedure with the target base station, and uses the protocol layer devices of the second plurality of bearers to transmit a handover completion message to the target base station and perform data transmission and reception (uplink or downlink).

[0026] · When the terminal successfully completes the random access procedure with the target base station and receives the uplink transmission resource from the target base station for the first time, it stops transmitting data with the source base station via the protocol layer devices of the first plurality of bearers, switches the uplink transmission, and transmits data to the target base station via the second plurality of bearers. · When the terminal receives a handover command message, it maintains transmitting and receiving data (data transmission and data reception on the uplink or downlink) with the source base station using the protocol layer devices of the first plurality of bearers, and executes the random access procedure with the target base station via the protocol layer devices of the second plurality of bearers. Also, when the terminal successfully completes the random access procedure and receives the uplink transmission resource from the target base station for the first time, it interrupts the transmission of uplink data to the source base station via the protocol layer devices of the first plurality of bearers, and performs uplink data transmission only to the target base station via the protocol layer devices of the second plurality of bearers. At this time, the protocol layer devices of the first plurality of bearers continue to receive downlink data from the source base station, and the protocol layer devices of the second plurality of bearers continue to receive downlink data from the target base station.

[0027] · The first bearer and the second bearer are configured in the structure of the second PDCP layer device. The structure of the second PDCP layer device is such that one PDCP layer device is connected to both the first bearer (e.g., RLC layer device, MAC layer device, or PHY layer device) for the source base station and the second bearer (e.g., RLC layer device, MAC layer device, or PHY layer device) for the target base station. Uplink data is transmitted via one of the first bearer or the second bearer through the PDCP layer device. That is, the terminal transmits uplink data via the first bearer until it executes the random access procedure for the target base station, successfully completes the random access procedure, and receives the uplink transmission resource from the target base station for the first time. If the terminal executes the random access procedure for the target base station, successfully completes the random access procedure, and receives the uplink transmission resource from the target base station for the first time, the terminal interrupts and switches the data transmission via the first bearer, and transmits the uplink data to the target base station via the second bearer. However, in the second PDCP layer device structure, the terminal receives downlink data from the source base station or the target base station via the first bearer or the second bearer.

[0028] Based on the above features, the present invention provides a method and an apparatus for performing an efficient handover procedure without an interruption time for data transmission and reception. In addition, when the terminal executes an efficient handover method without interruption time for data transmission and reception, if the terminal fails in the handover, the present invention provides a method of falling back to the source base station and further establishing a connection with the source base station. According to an embodiment of the present invention, when the terminal executes a handover procedure, it can maintain a connection with the source base station, and even if the handover fails, it can utilize the radio connection with the existing source base station and fall back.

[0029] FIG. 1A is a diagram showing the structure of an LTE system according to an embodiment of the present invention. Referring to FIG. 1A, as shown in the figure, the radio access network of the LTE system is composed of evolved Node B (hereinafter referred to as ENB, Node B or base station) (1a-05, 1a-10, 1a-15, 1a-20), MME (mobility management entity) (1a-25), and S-GW (serving-gateway) (1a-30). The user equipment (UE or terminal) (1a-35) is connected to an external network via the ENB (1a-05 to 1a-20) and the S-GW (1a-30).

[0030] In FIG. 1A, the ENB (1a-05 to 1a-20) corresponds to the existing Node B of the UMTS (universal mobile telecommunication system). The ENB is connected to the UE (1a-35) through a radio channel and performs a more complex role than the existing Node B. In the LTE system, all user traffic, including real-time services such as VoIP (voice over IP) via the Internet protocol, is served via a shared channel. Therefore, a device that performs scheduling by combining state information such as the buffer state, available transmission power state, and channel state of the UE is required, and the ENB (1a-05 to 1a-20) is responsible for this.

[0031] One ENB usually controls multiple cells. For example, to implement a transmission speed of 100 Mbps, the LTE system uses orthogonal frequency division multiplexing (OFDM) as a radio connection technology in a 20 MHz bandwidth, for example. In addition, an adaptive modulation & coding (AMC) scheme that determines the modulation scheme and the channel coding rate according to the channel state of the terminal is applied. The S-GW (1a-30) is a device that provides a data bearer and generates or removes a data bearer under the control of the MME (1a-25). The MME, which is responsible for the mobility management function for the terminal, is connected to a plurality of base stations in a device that is responsible for various control functions, needless to say.

[0032] Figure 1B is a diagram showing the radio protocol structure of the LTE system according to an embodiment of the present invention. Referring to Figure 1B, the radio protocol of the LTE system consists of PDCP (packet data convergence protocol) (1b-05, 1b-40), RLC (radio link control) (1b-10, 1b-35), and MAC (medium access control) (1b-15, 1b-30) in the terminal and the ENB, respectively. PDCP (packet data convergence protocol) (1b-05, 1b-40) is responsible for operations such as IP header compression / decompression. The main functions of PDCP are summarized as follows. · Function of header compression and decompression: ROHC (robust header compression) only · Function of user data transfer · Function of in-sequence delivery (at the PDCP re-establishment procedure for RLC AM, delivering upper layer PDUs in sequence) · Function of order re-alignment (for split bearers in DC (only support for RLC AM): PDCP PDU routing for transmission and PDCP PDU reordering for reception) · Function of duplicate detection (detecting duplicates of lower layer SDUs at the PDCP re-establishment procedure for RLC AM) · Function of retransmission (retransmitting PDCP SDUs at handover and for split bearers in DC, and retransmitting PDCP PDUs at the PDCP data-recovery procedure for RLC AM) · Function of ciphering and deciphering · Function of timer-based SDU discard in uplink

[0033] Radio Link Control (RLC) (1b-10, 1b-35) reconstructs the "PDCP PDU" (Packet Data Unit) to an appropriate size and performs operations such as ARQ operations. The main functions of RLC are summarized as follows.

[0034] · Data transfer function (transfer of upper layer PDUs) · ARQ function (error correction through ARQ (only for AM data transfer)) · Concatenation, segmentation and reassembly function (concatenation, segmentation and reassembly of RLC SDUs (only for UM and AM data transfer)) · Re-segmentation function (re-segmentation of RLC data PDUs (only for AM data transfer)) · Reordering function (reordering of RLC data PDUs (only for UM and AM data transfer)) · Duplicate detection function (duplicate detection (only for UM and AM data transfer)) · Protocol error detection function (protocol error detection (only for AM data transfer)) · RLC SDU discard function (RLC SDU discard (only for UM and AM data transfer)) · RLC re-establishment function (RLC re-establishment)

[0035] MAC (1b-15, 1b-30) is connected to various RLC layer devices configured in one terminal and performs operations of multiplexing the "RLC PDU" into the "MAC PDU" and de-multiplexing the "RLC PDU" from the "MAC PDU". The main functions of the MAC are summarized as follows.

[0036] · Mapping function (mapping between logical channels and transport channels) · Multiplexing and demultiplexing functions (multiplexing / demultiplexing of MAC SDUs belonging to one or different logical channels into / from TB (transport blocks) delivered to / from the physical layer on transport channels) · Scheduling information reporting function · HARQ function (error correction through HARQ) · Logical channel priority adjustment function (priority handling between logical channels of one UE) · Inter-terminal priority adjustment function (priority handling between UEs by means of dynamic scheduling) · MBMS service identification function · Transmission format selection function · Padding function

[0037] The physical layer (1b-20, 1b-25) performs operations of channel coding and modulating the upper layer data into OFDM symbols for transmission on the radio channel, or demodulating the OFDM symbols received via the radio channel and performing channel decoding to transmit to the upper layer.

[0038] Figure 1C is a diagram showing the structure of a next-generation mobile communication system according to an embodiment of the present invention. Referring to FIG. 1C, as shown in the figure, the radio access network of the next-generation mobile communication system (hereinafter referred to as NR or 5G) is composed of a next-generation base station (hereinafter referred to as "NR gNB" (New Radio Node B) or NR base station) (1c-10) and a "NR CN" (new radio core network) (1c-05). The user terminal (hereinafter referred to as "NR UE" (new radio user equipment) or terminal) (1c-15) connects to an external network via the "NR gNB" (1c-10) and the "NR CN" (1c-05).

[0039] In FIG. 1C, the "NR gNB" (1c-10) corresponds to the eNB (Evolved Node B) of the existing LTE system. The "NR gNB" is connected to the "NR UE" (1c-15) through a radio channel and provides a higher-quality service than the existing Node B. In the next-generation mobile communication system, since all user traffic is served via a shared channel, a device that schedules by combining state information such as the buffer state, available transmission power state, and channel state of the UE is required, and the "NR gNB" (1c-10) is responsible for it.

[0040] One "NR gNB" usually controls a plurality of cells. Currently, compared with LTE, in order to implement ultra-high-speed data transmission, it has a bandwidth greater than the existing maximum bandwidth, uses orthogonal frequency division multiplexing (OFDM) as the radio access technology, and further integrates beamforming technology. In addition, an adaptive modulation & coding (AMC) scheme that determines the modulation scheme and the channel coding rate according to the channel state of the terminal is applied. "NR CN" (1c-05) performs functions such as mobility support, bearer configuration, and QoS configuration. The mobility management function of "NR CN" for the terminal is, needless to say, connected to a plurality of base stations in the device responsible for various control functions. Also, the next-generation mobile communication system is interlocked with the existing LTE system, and "NR CN" is connected to the MME (1c-25) via a network interface. The MME is connected to the eNB (1c-30), which is an existing base station.

[0041] FIG. 1D is a diagram showing the radio protocol structure of the next-generation mobile communication system according to an embodiment of the present invention. Referring to FIG. 1D, the radio protocol of the next-generation mobile communication system consists of "NR SDAP" (1d-01, 1d-45), "NR PDCP" (1d-05, 1d-40), "NR RLC" (1d-10, 1d-35), and "NR MAC" (1d-15, 1d-30) in the terminal and the NR base station, respectively.

[0042] The main functions of "NR SDAP" (1d-01, 1d-45) also include some of the following functions including. · Transfer function of user plane data · Mapping function between a "QoS flow" related to the uplink and the downlink and a data bearer (mapping between a Qos flow and a DRB for both DL and UL) · Marking function of "QoS flow ID" related to the uplink and the downlink (marking QoS flow ID in both DL and UL packets) · Function of mapping a "reflective QoS flow" to a data bearer in the uplink "SDAP PDU" (reflective QoS flow to DRB mapping for the ULSDAP PDUs).

[0043] Regarding the SDAP layer device, the terminal is configured with the RRC message to indicate whether to use the header of the SDAP layer device for each PDCP layer device, for each bearer, or for each logical channel, or whether to use the function of the SDAP layer device. When the SDAP header is configured, the terminal can update or reset the "QoS flow" of the uplink and downlink and the mapping information related to the data bearer by the "NAS QoS" reflection setting 1-bit indicator (NAS reflective QoS) and the "AS QoS" reflection setting 1-bit indicator (AS reflective QoS) of the SDAP header. The SDAP header also includes "QoS flow" ID information indicating QoS. The QoS information is also used as data processing priorities, scheduling information, etc. to support smooth services.

[0044] The main functions of "NR PDCP" (1d-05, 1d-40) also include some of the following functions. · Function of header compression and decompression (Header compression and decompression: ROHC only) · Function of user data transmission (transfer of user data) · Function of in-sequence delivery (in-sequence delivery of upper layer PDUs) · Function of out-of-sequence delivery (out-of-sequence delivery of upper layer PDUs) · Function of PDCP PDU reordering for reception (PDCP PDU reordering for reception) · Function of duplicate detection of lower layer SDUs (duplicate detection of lower layer SDUs) · Function of retransmission of PDCP SDUs (retransmission of PDCP SDUs) ·Ciphering and deciphering functions - Timer-based SDU discard in uplink

[0045] The reordering function of the "NR PDCP" device refers to the function of sequentially reordering the "PDCP PDUs" received at the lower layer based on the "PDCP SN" (sequence number), and includes the function of transmitting the data to the upper layer in the reordered order, or includes the function of transmitting immediately without considering the order, includes the function of recording the lost "PDCP PDUs" after reordering the order, includes the function of making a status report related to the lost "PDCP PDUs" to the transmitting side, and also includes the function of requesting retransmission for the lost "PDCP PDUs".

[0046] The main functions of "NR RLC" (1d-10, 1d-35) also include some of the following functions. · Transfer of upper layer PDUs · In-sequence delivery of upper layer PDUs · Out-of-sequence delivery of upper layer PDUs · Error correction through ARQ · Concatenation, segmentation and reassembly of RLCSDUs · Re-segmentation of RLC data PDUs · Reordering of RLC data PDUs · Duplicate detection · Protocol error detection · "RLC SDU" deletion function (RLC SDU discard) · RLC re-establishment function

[0047] The in-sequence delivery function of the "NR RLC" device refers to the function of sequentially delivering the "RLC SDU" received from the lower layer to the upper layer. When one "RLC SDU" is originally received divided into several "RLC SDU", it includes the function of reassembling and delivering it, and also includes the function of re-aligning the received "RLC PDU" based on the "RLC SN" (sequence number) or "PDCP SN" (sequence number). Also, the in-sequence delivery function of the "NR RLC" device includes the function of re-aligning the order and recording the lost "RLC PDU", the function of sending a status report related to the lost "RLC PDU" to the transmitting side, the function of requesting retransmission for the lost "RLC PDU", and when there is a lost "RLC SDU", it includes the function of sequentially delivering only the "RLC SDU" before the lost "RLC SDU" to the upper layer, or even if there is a lost "RLC SDU", if a predetermined timer expires, it includes the function of sequentially delivering all the "RLC SDU" received before the timer started to the upper layer, or even if there is a lost "RLC SDU", if a predetermined timer expires, it includes the function of sequentially delivering all the "RLC SDU" received so far to the upper layer.

[0048] Also, the "NR RLC" device processes the received "RLC PDU" in the order of arrival (regardless of the order of the sequence number), and can also deliver it to the PDCP device out-of-sequence. When it is a segment, it can be stored in the buffer or receive the segments received subsequently, reconstitute them into a complete "RLC PDU" and then process and deliver it to the PDCP device. The "NR RLC" layer does not include a concatenation function, and the concatenation function can be executed at the "NR MAC" layer or replaced by the multiplexing function of the "NR MAC" layer.

[0049] The out-of-sequence delivery function of the "NR RLC" device refers to the function of immediately transmitting the RLC SDU received from the lower layer to the upper layer regardless of the order. Originally, when one "RLC SDU" is received after being split into several "RLC SDUs", it also includes the function of reassembling and transmitting them, saving the "RLC SN" or "PDCP SN" of the received "RLC PDU" to align the order, and recording the lost "RLC PDU".

[0050] "NR MAC" (1d-15, 1d-30) is connected to various "NR RLC" layer devices configured in one terminal, and the main functions of "NR MAC" also include some of the following functions. · Mapping function between logical channels and transport channels · Multiplexing and demultiplexing functions of MAC SDUs · Scheduling information reporting function · HARQ function (error correction through HARQ) · Priority handling function between logical channels of one UE · Priority handling function between UEs by means of dynamic scheduling · MBMS service identification function · Transport format selection function · Padding function

[0051] The "NR PHY" layer (1d-20, 1d-25) performs operations of channel coding and modulating the upper layer data into OFDM symbols for transmission over the radio channel, or demodulating the OFDM symbols received via the radio channel and performing channel decoding for transmission to the upper layer.

[0052] FIG. 1E is a diagram for explaining a procedure in which a terminal according to an embodiment of the present invention sets a connection with a network. Specifically, FIG. 1E is a diagram for explaining a procedure in which, in the present invention, a terminal switches from the RRC idle mode to the RRC connected mode and sets a connection with a network.

[0053] In FIG. 1E, when a terminal (UE) that transmits and receives data in the RRC connected mode does not transmit and receive data for a predetermined reason or for a certain period of time, the base station (gNB) sends a "RRCConnectionRelease" message to the terminal to convert the terminal to the RRC idle mode (reference numeral 1e-01). A terminal for which a connection is not currently set (hereinafter, "idle mode UE") performs a "RRC connection establishment" process with the base station if data to be transmitted subsequently occurs. The terminal establishes reverse transmission synchronization with the base station via a random access process and transmits a "RRCConnectionRequest" message to the base station (reference numeral 1e-05). The "RRCConnectionRequest" message contains an identifier of the terminal and a reason for establishing the connection (establishment cause), etc. The base station transmits an "RRCConnectionSetup" message (reference 1e-10) so that the terminal sets up an RRC connection.

[0054] The message includes configuration information for each service / bearer / each RLC device, for each logical channel, or for each bearer, whether to use ROHC (robust header compression) for each bearer / logical channel, ROHC configuration information (e.g., ROHC version, initial information, etc.), "statusReportRequired" information (information for the base station to instruct the terminal to send a "PDCP Status report"), and "drb-ContinueROHC" information (configuration information to maintain the ROHC configuration information and use it as it is, which can be included in the PDCP layer device configuration information (pdcp-config) and transmitted). In addition, the "RRCConnectionSetup" message stores RRC connection configuration information and the like. The bearer for RRC connection is also called an SRB (signaling radio bearer) and is used for transmitting and receiving RRC messages, which are control messages between the terminal and the base station.

[0055] The terminal that has set up an RRC connection transmits an "RRCConnetionSetupComplete" message to the base station (reference 1e-15). The "RRCConnetionSetupComplete" message includes a control message "SERVICE REQUEST" in which the terminal requests the MME to set up a bearer for a predetermined service. The base station transmits the "SERVICE REQUEST" message stored in the "RRCConnetionSetupComplete" message to the MME or AMF (access and mobility management function) (reference 1e-20), and the MME or AMF determines whether to provide the service requested by the terminal.

[0056] If, as a result of the determination, it is decided to provide the service requested by the terminal, the MME or AMF transmits a message "INITIAL CONTEXT SETUP REQUEST" to the base station (reference numeral 1e-25). The "INITIAL CONTEXT SETUP REQUEST" message includes information such as QoS (quality of service) information applied when setting up a DRB (data radio bearer), and security-related information (e.g., Security Key, Security Algorithm) applied to the DRB.

[0057] Also, if the base station cannot receive the terminal's capability information from the MME or AMF, the base station transmits a terminal capability information request message to the terminal to confirm the terminal's capability information (reference numeral 1e-26). If the terminal receives the terminal capability information request message, the terminal constructs and generates a terminal capability information message and reports it to the base station (reference numeral 1e-27). The terminal capability information message includes information on what types of handover methods the terminal supports. For example, the terminal reports its capabilities to the base station via an indicator indicating whether the terminal supports the efficient handover method (Dual Active Protocol Stack: DAPS) proposed in the present invention. If the base station confirms the terminal capabilities information, when the base station instructs the terminal to perform a handover, the base station defines an indicator for each handover method in the handover command message indicating which handover to instruct and instructs the terminal.

[0058] For example, the efficient handover method (DAPS handover method) proposed in the present invention can be instructed to the terminal, and as another method, the DAPS handover method can be set for each bearer (DRB or SRB) of the terminal. When the base station sets the DAPS handover method for the terminal, it can be instructed together with other handover methods to prevent data loss or transmission delay that may occur during handover. For example, as a handover method that the base station can set for the terminal together with the DAPS handover method, there may be a conditional handover method or a handover method without a random access procedure. The conditional handover method is a method in which a plurality of target cells are set, a plurality of conditions are set for the terminal, and if the terminal satisfies the conditions set in the cell selection or reselection procedure, the terminal executes a handover procedure to one target cell.

[0059] The terminal executes a handover procedure to the target base station according to the handover method indicated by the handover command message. The base station exchanges a "SecurityModeCommand" message (sign 1e-30) and a "SecurityModeComplete" message (sign 1e-35) to set security with the terminal. If the security setting is completed, the base station transmits an "RRCConnectionReconfiguration" message to the terminal (sign 1e-40).

[0060] The above message includes configuration information for each service / bearer / each RLC device, for each logical channel, or for each bearer. It includes whether to use ROHC for each bearer / logical channel, ROHC configuration information (such as ROHC version, initial information, etc.), "statusReportRequired" information (information for the base station to instruct the terminal to send a "PDCP Status report"), and "drb-ContinueROHC" information (configuration information to maintain the ROHC configuration information and use it as it is, which can be included in the PDCP layer device configuration information (pdcp-config) and transmitted). In addition, the above message stores RRC connection configuration information and the like. The bearer for RRC connection, also referred to as SRB (signaling radio bearer), is used for transmitting and receiving RRC messages, which are control messages between the terminal and the base station.

[0061] In addition, the above message contains the setting information of the DRB for processing user data. The terminal applies the above information to set up the DRB and transmits an "RRCConnectionReconfigurationComplete" message to the base station (reference numeral 1e-45). The terminal and the base station that have completed the DRB setting transmit an "INITIAL CONTEXT SETUP COMPLETE" message to the MME or AMF (reference numeral 1e-50). The received MME or AMF exchanges an "S1 BEARER SETUP" message and an "S1 BEARER SETUP RESPONSE" message to set up the S1 bearer with the "S-GW" (reference numerals 1e-055, 1e-60).

[0062] The S1 bearer is a data transmission connection set up between the "S-GW" and the base station and corresponds one-to-one with the DRB. If all of the above steps are completed, the terminal transmits and receives data to and from the base station via the "S-GW" (reference numerals 1e-65, 1e-70). In this way, generally speaking, the data transmission process is mainly composed of three steps: RRC connection setting, security setting, and DRB setting. In addition, the base station transmits an "RRCConnectionReconfiguration" message to the terminal to newly set, add, or change the settings for certain reasons (reference numeral 1e-75).

[0063] In the present invention, the bearer also means including SRB and DRB. SRB means "signaling radio bearer", and DRB means "data radio bearer". The SRB is mainly used to transmit and receive RRC messages of the RRC layer device, and the DRB is mainly used to transmit and receive user layer data. And, "UM DRB" means a DRB that uses an RLC layer device operating in the UM (unacknowledged mode) mode, and "AM DRB" means a DRB that uses an RLC layer device operating in the AM (acknowledged mode) mode.

[0064] FIG. 1F is a diagram showing a signaling procedure for a terminal to perform handover in a next-generation mobile communication system according to an embodiment of the present invention. A terminal (1f-01) in the RRC connection mode state currently performs cell measurement information (measurement report) periodically to the source eNB (1f-02) or when a specific event is satisfied (reference numeral 1f-05).

[0065] Based on the cell measurement information, the source base station determines whether the terminal proceeds with a handover to an adjacent cell. A handover changes the source base station that provides services to a terminal in the connected mode state to another base station (or another cell of the same base station). If the source base station determines a handover, the source base station transmits a "handover (hereinafter, HO) request" message (for example, "Handover Preparation Information" message) to a new base station that provides services to the terminal, that is, a target eNB (1f-03), and requests a handover (reference numeral 1f-10).

[0066] If the target base station accepts the handover request, it transmits an "HOrequestAck message (for example, HandoverCommand message) to the source base station (reference numeral 1f-15). The source base station that has received the "HO request Ack" message transmits a handover command message (the "HO command" message, the "RRCReconfiguration" message included in the DCCH (dedicated control channel) of the "HO request Ack" message) to the terminal (reference numeral 1f-20). The handover command (HO command) message is extracted by the source base station from the message received from the target base station and transmitted to the terminal using the "RRCConnectionReconfiguration" message (reference numeral 1f-20).

[0067] In the present invention, when the source base station transmits a "Handover Preparation information" message (reference numeral 1f-10) and, in response thereto, the target base station transmits a "HandoverCommand" message (reference numeral 1f-15) to the source base station, embodiments for determining a DAPS handover method using the above two messages are provided.

[0068] A first example for executing a DAPS handover procedure according to an embodiment of the present invention is as follows. In the first example, the entity that determines the DAPS handover method is the source base station. Also, in the first example, when the source base station requests a DAPS handover, the target base station instructs or executes a DAPS handover.

[0069] · The source base station defines a new indicator in the "Handover Preparation information" message, and such an indicator instructs and requests the target base station that the source base station executes a DAPS handover procedure. The "Handover Preparation information" message includes the current bearer configuration information of the terminal, security key information, cell group configuration information, or terminal capability information, etc. The source base station shares the capabilities of the target base station in advance, knows in advance whether the target base station supports the DAPS handover method, and instructs the target base station that the source base station will execute the DAPS handover method. Thereby, the source base station notifies the target base station that the source base station can execute data forwarding (early data forwarding) quickly or early, and instructs the target base station to receive the data forwarding and prepare to process it quickly. The source base station can also request the DAPS handover method separately for each bearer (DRB or SRB).

[0070] · When the target base station receives the "Handover Preparation information" message and confirms that the "Handover Preparation information" message contains an indicator requesting the DAPS handover method, when the target base station constructs the "RRCReconfiguration" message for instructing the terminal to perform handover, it includes an indicator for instructing the DAPS handover method, and when the terminal executes the DAPS handover method, it constructs an "RRCReconfiguration" message including the necessary bearer configuration information, bearer configuration information, security key information, cell group configuration information or system information. Then, the target base station includes the constructed "RRCReconfiguration" message in the "DL-DCCH" message of the "Handover command" message and transmits it to the source base station. The target base station can also execute the instruction for the DAPS handover method separately for each bearer (DRB or SRB).

[0071] · If the source base station receives a "Handover command" message, it extracts the "RRCReconfiguration" message contained in the "Handover command" message or transmits the "RRCReconfiguration" message to the terminal to instruct handover. The source base station can confirm the instructed DAPS handover method for each bearer and execute the DAPS handover method for each bearer (DRB or SRB).

[0072] A second example for executing the DAPS handover procedure according to an embodiment of the present invention is as follows. In the second example, the entity that determines the DAPS handover method is the target base station. Also, in the second example, when the source base station requests the target base station for the DAPS handover method with an indicator, the target base station can reject the request, accept it, or indicate another handover method in the "Handover command" message and instruct the source base station.

[0073] · The source base station defines a new indicator in the "Handover Preparation information" message, and such an indicator instructs and requests the target base station that the source base station executes the DAPS handover procedure. The "Handover Preparation information" message includes the current bearer configuration information of the terminal, security key information, cell group configuration information, or terminal capability information, etc. The source base station shares the capabilities of the target base station in advance, knows in advance whether the target base station supports the DAPS handover method, and instructs the target base station to execute the DAPS handover method. Thereby, the source base station notifies the target base station that the source base station can perform early data forwarding quickly, and instructs the target base station to receive the data forwarding and be prepared to process it quickly. The source base station can also request requirements for the DAPS handover method separately for each bearer (DRB or SRB).

[0074] · When the target base station receives the "Handover Preparation information" message and confirms that the "Handover Preparation information" message contains an indicator requesting the DAPS handover method, the target base station determines whether the target base station can support the DAPS handover method, and based on the current amount of transmission resources or scheduling, rejects or accepts the request for the DAPS handover, or instructs another handover method. The target base station includes in the "Handover command" message an indicator for rejecting the request for the DAPS handover request, an indicator for accepting it, or an indicator for instructing another type of handover method and transmits it. When the target base station configures the "RRCReconfiguration" message for instructing the terminal to perform handover, if it accepts the DAPS handover request, it includes an indicator for instructing the DAPS handover method. When the target base station rejects the DAPS handover request, when configuring the "RRCReconfiguration" message for instructing the terminal to perform handover, it includes an indicator for instructing another handover method, and when the terminal executes the DAPS handover method or another handover method, it includes the necessary bearer configuration information, security key information, cell group configuration information or system information, and configures the "RRCReconfiguration" message. Then, the target base station includes the configured "RRCReconfiguration" message in the "DL-DCCH" message of the "Handover command" message and transmits it to the source base station. The target base station can also execute the instruction for the DAPS handover method separately for each bearer (DRB or SRB).

[0075] Upon receiving the "Handover command" message, the source base station checks the indicator contained in the "Handover command" message and confirms whether the request for the DAPS handover method has been accepted. If the request for the DAPS handover method has been accepted, the source base station also executes the DAPS handover procedure, extracts the "RRCReconfiguration" message contained in the "Handover command" message, or transmits the "RRCReconfiguration" message to the terminal to instruct the handover. However, if the source base station checks the indicator contained in the "Handover command" message and the request for the DAPS handover method is rejected, or if another handover method is instructed, the source base station executes the other handover method instructed by the target base station. Then, the source base station extracts the "RRCReconfiguration" message contained in the "Handover command" message, or transmits the "RRCReconfiguration" message to the terminal to instruct the handover.

[0076] According to another embodiment of the present invention, even if there is no separate indicator in the "Handover command" message, the source base station checks the "RRCReconfiguration" message included in the "Handover command" message, checks what kind of handover method the handover method instructed by the target base station is, checks whether the requirement for the DAPS handover method is accepted, and the source base station also executes the handover method (for example, the DAPS handover method or another handover method) instructed by the "RRCReconfiguration" message. The source base station can also check the instructed DAPS handover method for each bearer and execute the DAPS handover method for each bearer (DRB or SRB).

[0077] A third example for executing the DAPS handover procedure according to an embodiment of the present invention is as follows. In the third example, the entity that determines the DAPS handover method is the target base station. Also, in the third example, the target base station checks the terminal's capabilities and determines whether the target base station can support the DAPS handover method, or determines the handover method (for example, the DAPS handover method) based on the current amount of transmission resources or scheduling.

[0078] · The source base station transmits a "Handover Preparation information" message to request a handover to the target base station, including the terminal's current bearer configuration information, security key information, cell group configuration information, or terminal capability information, etc. in the "Handover Preparation information" message. The source base station shares the capabilities of the target base station in advance, knows in advance whether the target base station can support the DAPS handover method, and if it is instructed that the target base station will perform the DAPS handover procedure, the source base station will execute early data forwarding quickly or early.

[0079] · The target base station receives the "Handover Preparation information" message, and the target base station determines the handover method (e.g., DAPS handover) based on the terminal's capability information, whether the target base station can support the DAPS handover method, or the amount or scheduling of the current transmission resources. If the target base station determines the DAPS handover method, it includes an indicator indicating the DAPS handover method in the "Handover command" message and transmits it. When the target base station configures the "RRCReconfiguration" message to instruct the terminal to perform a handover, if the target base station determines the DAPS handover, the "RRCReconfiguration" message includes an indicator indicating the DAPS handover method; if the target base station determines another handover method other than DAPS handover, the "RRCReconfiguration" message includes an indicator indicating the other handover method. Also, when the terminal performs the DAPS handover method or another handover method, the target base station includes the necessary bearer configuration information, bearer configuration information, security key information, cell group configuration information, or system information to configure the "RRCReconfiguration" message. Then, the configured "RRCReconfiguration" message is included in the "DL-DCCH" message of the "Handover command" message and transmitted to the source base station. The target base station can also execute instructions for the DAPS handover method separately for each bearer (DRB or SRB).

[0080] · If the source base station receives a "Handover command" message, it checks the indicator contained in the "Handover command" message to confirm whether DAPS handover has been determined. If the DAPS handover method is indicated, the source base station also executes the DAPS handover method, extracts the "RRCReconfiguration" message contained in the "Handover command" message, or transmits the "RRCReconfiguration" message to the terminal to instruct the handover. However, if the source base station checks the indicator contained in the "Handover command" message and the DAPS handover method has not been determined or another handover method has been indicated, the source base station also executes the other handover method indicated by the target base station. Then, the source base station extracts the "RRCReconfiguration" message contained in the "Handover command" message or transmits the "RRCReconfiguration" message to the terminal to instruct the handover. As another method, even if there is no separate indicator in the "Handover command" message, the source base station checks the "RRCReconfiguration" message contained in the "Handover command" message to confirm what handover method is indicated by the target base station and whether a decision related to the DAPS handover method has been made. If another handover method is indicated, the indicated handover method can also be executed. The source base station can check the instructed DAPS handover method for each bearer and execute the DAPS handover method for each bearer (DRB or SRB).

[0081] It is also possible to combine the methods of the first embodiment, the second embodiment, or the third embodiment for executing the DAPS handover procedure according to an embodiment of the present invention to execute a new embodiment. In the "RRCReconfiguration" message, the base station instructs the terminal of the handover method (DAPS handover method) according to an embodiment of the present invention. As another method, the DAPS handover method can also be set for each bearer (DRB or SRB) of the terminal.

[0082] For example, in the RRC message, a new indicator for instructing the above-mentioned efficient handover method (DAPS handover method) is defined in the bearer configuration information, PDCP configuration information, or RLC configuration information according to the bearer identifier or the logical channel identifier. The base station uses the indicator to instruct the terminal of the efficient handover method according to the bearer or the logical channel identifier. When the base station sets the DAPS handover method for the terminal, it can be instructed together with other handover methods to prevent data loss or transmission delay that may occur during handover. For example, as a handover method that the base station can set for the terminal together with the DAPS handover method, there may be a conditional handover method or a handover method without a random access procedure.

[0083] The conditional handover method is a method in which a plurality of target cells are set, a plurality of conditions are set for the terminal, and if the terminal satisfies the conditions set in the cell selection or cell reselection procedure, the terminal executes the handover procedure to one target cell. If the terminal receives the above message, it stops or continues data transmission / reception with the source base station according to the set handover method, and starts the T304 timer. If the terminal fails to successfully hand over to the target base station within the pre-set time (for example, when the T304 timer expires), it returns to its original settings and switches to the "RRC Idle" state. Then, the RRC connection re-establishment procedure is triggered. As another method, if an efficient handover method is set and the connection with the source base station is valid, it falls back and reports the handover failure to the source base station.

[0084] The source base station transmits a series of serial number statuses (SN (sequence number) status) related to uplink / downlink data for each bearer (for example, RLC UM bearer or RLC AM bearer), and if there is downlink data or uplink data, it transmits it to the target base station (codes 1f-30, 1f-35). The terminal attempts random access to the target cell indicated by the source base station (code 1f-40). Random access is used to inform the target cell that the terminal is moving through handover and to align the uplink synchronization. For random access, the terminal transmits a preamble corresponding to the preamble ID provided by the source base station or a randomly selected preamble ID to the target cell.

[0085] After transmitting the preamble, the terminal monitors whether a random access response message (RAR) is transmitted from the target cell after a certain number of subframes have passed. The time interval for monitoring is called the random access response window (RAR window (random access response window)). If a Random Access Response (RAR) is received during a specific time period (reference numeral 1f-45), the terminal transmits a Handover Complete (HO complete) message to the target base station as an "RRC Reconfiguration Complete" message (reference numeral 1f-55). If the terminal successfully receives a random access response from the target base station, it stops or terminates timer T304 (reference numeral 1f-50).

[0086] The target base station requests a path modification (reference numerals 1f-60, 1f-65) to modify the path of the bearer set in the source base station, and notifies the source base station to delete the UE context of the terminal (reference numeral 1f-70). Then, the target base station transmits an RRC message (e.g., an "RRC Reconfiguration" message (reference numeral 1f-71)) to the terminal, and uses the indicator in the RRC message to instruct the terminal to release the connection with the source base station.

[0087] As another method, the target base station can also transmit MAC control information, RLC control information, or PDCP control information to instruct the terminal to release the connection with the source base station. Therefore, the terminal attempts to receive data from the start point of the "RAR window" for the target base station. After receiving the RAR, it transmits an "RRC Reconfiguration Complete" message, receives a downlink transmission resource or an uplink transmission resource, and starts data transmission and reception with the target base station. The terminal sets a plurality of bearers with the source base station and can perform data transmission and reception (uplink or downlink data transmission and data reception) through each protocol layer device (PHY layer device, MAC layer device, RLC layer device, or PDCP layer device) of each bearer. For the sake of convenience of explanation, the case where the terminal has one bearer will be described. That is to say, it goes without saying that the embodiments of the present invention can also be applied when the terminal has a plurality of bearers.

[0088] FIG. 1G is a diagram for explaining a first embodiment of a handover method for minimizing data interruption time due to handover according to an embodiment of the present invention. Referring to FIG. 1G, in stage (reference numeral 1g-01), the terminal (1g-20) is transmitting and receiving data with the source base station (1g-05). If the terminal receives a handover command message from the source base station (1g-05), the terminal disconnects the connection with the source base station (1g-05) by the handover method indicated by the handover command message (for example, an “RRCReconfiguration” message), executes a random access procedure with the target base station (1g-10), and executes a handover procedure. The terminal can also continue to transmit and receive data with the source base station (1g-05) in order to minimize the data interruption time that occurs during handover by the handover method indicated by other methods.

[0089] In stage (reference numeral 1g-02), when the terminal (1g-20) executes a random access procedure with the target base station (1g-10), transmits a preamble, or uses PUCCH transmission resources or PUSCH transmission resources to transmit data to the uplink transmission resources for the first time by the handover method indicated by the handover command message, the terminal interrupts data transmission and reception (uplink data transmission and downlink data reception) with the source base station (1g-05). In stage (reference numeral 1g-03), the terminal (1g-20) completes the random access procedure with the target base station (1g-10), transmits a handover completion message, and starts data transmission and reception (uplink data transmission and downlink data reception) with the target base station (1g-10).

[0090] FIG. 1H is a diagram for explaining a second embodiment of a handover method for minimizing data interruption time due to handover according to an embodiment of the present invention. In the (1h-20) terminal, in the (1h-01) stage, data is transmitted and received with the source base station (1h-05). When receiving a handover command message from the source base station (1h-05), if the handover command message indicates the handover method according to the above-described second embodiment (for example, the DAPS handover method), or if it is indicated for each bearer, even if the terminal (1h-20) receives the handover command message, in order to minimize the data interruption time that occurs during handover, data is continuously transmitted and received via the source base station (1h-05) and the protocol layer device (1h-22) of the first bearer.

[0091] Then, in the handover command message received by the RRC layer device, when confirming an instruction for the handover method (for example, the DAPS handover method) according to the second embodiment of the present invention, or when confirming an indicator related to the DAPS handover method for each bearer, the RRC layer device transmits the indicator to the PDCP layer device corresponding to each bearer or the bearer for which the DAPS handover method is indicated. If the PDCP layer device receives the indicator, it converts from the structure of the first PDCP layer device (1i-11 or 1i-12) to the structure of the second PDCP layer device (1i-20). For this purpose, the terminal (1h-20) receives a handover command message (``RRCReconfiguration'' message) from the base station.

[0092] Also, when the terminal (1h-20) converts to the second PDCP layer device structure according to the settings included in the received handover command message, it pre-sets or establishes the protocol layer device (PHY layer device, MAC layer device, RLC layer device, or PDCP layer device) (1h-21) of the second bearer for the target base station (1h-10), derives and updates the security key for the target base station (1h-10), and configures the header (or data) compression context for the target base station (1h-10). Then, when the terminal (1h-20) receives a handover command message, and in the handover command message, when indicating the DAPS handover method according to an embodiment of the present invention, when indicating the DAPS handover method for a specific bearer, or when the PDCP reordering timer value is newly set, the terminal (1h-20) converts from the structure or function of the first PDCP layer device (1i-11 or 1i-12) to the structure or function of the second PDCP layer device (1i-20) for each bearer or for the bearer for which the DAPS handover method is indicated. When doing so, the terminal (1h-20) updates the variable for reordering to the PDCP sequence number or COUNT value expected to be received next, stops the reordering timer, and restarts the reordering timer.

[0093] Then, when a handover command message (e.g., "RRCReconfiguration" message) is received, the RRC layer device of the terminal starts a first timer (e.g., T304 timer). Then, the first timer executes a random access procedure for the target base station in order to perform a handover. When the random access procedure is successfully completed (e.g., when the first condition proposed in the present invention is satisfied), it is stopped. If the handover fails and the first timer expires, if the connection to the source base station is valid, a fallback is performed, the handover failure is reported to the source base station, an attempt is made to recover the connection, and if the connection to the source base station is not valid, an RRC connection reestablishment procedure is executed.

[0094] The handover command message can be set and established for each bearer so that the second bearer has the same bearer identifier as the first bearer to avoid data interruption time. Also, in the second embodiment, the PDCP layer device of the first bearer and the PDCP layer device of the second bearer can operate logically as one PDCP layer device. The specific operation method will be described with reference to FIG. 1I.

[0095] Also, in the second embodiment, when the terminal (1h-20) is to transmit uplink data to both the source base station (1h-05) and the target base station (1h-10), in order to prevent the problem of coverage reduction due to insufficient transmission power of the terminal (1h-20), or the problem (link selection) of having to determine which base station to request transmission resources from and transmit the uplink data to when transmitting uplink data, the transmission of uplink data is performed to only one of the source base station (1h-05) and the target base station (1h-10).

[0096] Specifically, in the second embodiment, if the terminal does not have the ability (dual uplink transmission) to simultaneously transmit uplink data to different base stations at different frequencies or the same frequency at the same time, the transmission of uplink data is performed to only one of the source base station and the target base station in one time unit. Therefore, the terminal (1h-20) executes a scheduling request to only one of the source base station (1h-05) or the target base station (1h-10), transmits a report related to the size of the data to be transmitted to the PDCP layer device (for example, buffer status report transmission) to only one of the source base station (1h-05) or the target base station (1h-10), receives uplink transmission resources, and transmits uplink data to only one base station. Also, even if the terminal (1h-20) receives a handover command message from the source base station (1h-05), it can continue data transmission and reception to prevent data loss due to HARQ retransmission. Also, the MAC layer device of the first bearer is not initialized so that the HAQR retransmission described above can continue. Also, the RLC layer device in the AM mode can continue to perform data transmission for RLC retransmission.

[0097] As another method, in the handover command message, when the second embodiment (DAPS handover method) of the efficient handover method proposed in the present invention is indicated for each bearer, in the handover command message, only for the PDCP layer device, RLC layer device, or MAC layer device corresponding to the bearer or logical channel identifier for which the second embodiment (DAPS handover method) is indicated, or only for the data corresponding to the bearer or logical channel identifier, it is also possible to continue transmitting or receiving data with the source base station. Also, even when the first condition proposed in the present invention is satisfied (for example, when switching the uplink data transmission to the target base station), in the handover command message, only for the PDCP layer device, RLC layer device, or MAC layer device corresponding to the bearer or logical channel identifier for which the second embodiment (DAPS handover method) is indicated, it is also possible to continue transmitting or receiving RLC control data (RLC status report), PDCP control data (ROHC feedback or PDCP status report), or HARQ retransmission to / from the source base station. Or, in the handover command message, when the second embodiment (DAPS handover method) of the efficient handover method proposed in the present invention is indicated for each bearer, or when it is indicated, for the PDCP layer device, RLC layer device, or MAC layer device corresponding to the bearer or logical channel identifier for which the second embodiment (DAPS handover method) is not indicated in the handover command message, it is also possible to stop data transmission or data reception with the source base station.

[0098] Then, when the terminal (1h-20) receives a handover command message and the DAPS handover method is indicated in the handover command message, for a specific bearer, when the DAPS handover method is indicated, or when the "QoS flow" and bearer mapping information are newly set, the terminal (1h-20) converts from the structure or function of the first SDAP layer device (1j-10) to the structure or function of the second SDAP layer device (1j-20) for each bearer or for the bearer for which the DAPS handover method is indicated according to an embodiment of the present invention. Then, in the structure of the second SDAP layer device, the existing "first QoS flow" and bearer mapping information for the source base station (1h-05) are maintained, the uplink data transmitted to the source base station (1h-05) and the downlink data received from the source base station (1h-05) are processed, and the "second QoS flow" and bearer mapping information newly set in the handover command message are set for the target base station (1h-10) and used to process the uplink data transmitted to the target base station (1h-10) and the downlink data received from the target base station (1h-10).

[0099] That is, in the structure of the second SDAP layer device according to an embodiment of the present invention, the "first QoS flow" and bearer mapping information, or the "second QoS flow" and bearer mapping information for the source base station (1h-05) are maintained, and the data for the source base station (1h-05) and the data for the target base station (1h-10) are processed separately. In the structure of the second SDAP layer device, the SDAP layer device distinguishes whether the data received from the lower layer is data received from the source base station (1h-05) or data received from the target base station (1h-10) via a 1-bit indicator in the SDAP header, a 1-bit indicator in the PDCP header, or information indicated by the PDCP layer device.

[0100] When the source base station (1h-05) or the target base station (1h-10) instructs the terminal (1h-20) on the DAPS handover method for each bearer in the handover command message, for the default bearer (default DRB), the DAPS handover method shall be instructed. During the DAPS handover procedure, when data is generated in a new "QoS flow" that does not correspond to the "QoS flow" and bearer mapping information, the default bearer shall be used to transmit uplink data. If the DAPS handover method is not set for the default bearer, since uplink data transmission for the new "QoS flow" generated during handover is not possible, a data interruption time may occur.

[0101] As another method, when a handover command message (for example, an "RRCReconfiguration" message) is received and the second embodiment (DAPS handover method) is instructed, and in the RRC message, the SDAP layer device configuration information for the target base station or the "second QoS flow" and bearer mapping information are set, the terminal can also apply the SDAP layer device configuration information or the "second QoS flow" and bearer mapping information when the first condition proposed in the present invention is satisfied. Or if the second embodiment (DAPS handover method) is instructed for each bearer in the handover command message, when the "first QoS flow" and bearer mapping information for the source base station are maintained, the terminal shall only maintain and apply the "first QoS flow" and bearer mapping information corresponding to the bearer instructed by the second embodiment, and the "first QoS flow" and bearer mapping information corresponding to the bearer not instructed by the second embodiment can be released or not applied. Also, in the RRC message, when the SDAP layer device configuration information for the target base station or the "second QoS flow" and bearer mapping information are configured, the terminal can also apply the SDAP layer device configuration information or the "second QoS flow" and bearer mapping information for data transmission or reception with the target base station when the first condition proposed in the present invention is satisfied.

[0102] In step (1h-02), the terminal (1h-20) also continues to transmit or receive data (uplink data transmission or downlink data reception) with the source base station (1h-05) via the protocol layer device of the first bearer when the terminal (1h-20) executes a random access procedure via the protocol layer device of the second bearer to the target base station (1h-10) indicated by the handover command message. In step (1h-02), the terminal (1h-20) executes a cell selection or cell reselection procedure and performs a random access procedure to the target cell indicated by the handover command message ("RRCReconfiguration" message) received from the source base station (1h-05).

[0103] In step (1h-03), if the terminal (1h-20) satisfies the first condition, it interrupts the transmission of uplink data to the source base station (1h-05) via the protocol layer device (1h-22) of the first bearer, transmits uplink data to the target base station (1h-10) via the protocol layer device (1h-21) of the second bearer, and continues to receive downlink data from the source base station (1h-05) and the target base station (1h-10) via the protocol layer devices of the first bearer and the second bearer. In the (1h-03) stage, the terminal (1h-20) satisfies the first condition and can switch the uplink transmission from the source base station (1h-05) to the target base station (1h-10). Specifically, until the terminal (1h-20) satisfies the first condition, it transmits uplink data to the source base station (1h-05) via the first bearer. If the first condition is satisfied, it interrupts transmitting uplink data to the source base station (1h-05) via the first bearer and starts transmitting uplink data to the target base station (1h-10) via the second bearer.

[0104] Specifically, in the second PDCP layer device structure proposed in the present invention, when the PDCP layer device is transmitting uplink data via the first bearer and satisfies the first condition, and receives an indicator from a lower layer device (when the random access procedure is successful from the MAC layer device to the target base station) or an upper layer device (when the first timer expires in the RRC layer device), it stops the uplink data transmission via the first bearer, switches to the second bearer, and starts the uplink data transmission via the second bearer. Also, similar to the PDCP layer device structure proposed in FIG. 1I, the receiving PDCP layer device (1h-21) of the second bearer is driven together with the receiving PDCP layer device (1h-22) of the first bearer, and uses information such as the stored transmission and reception data, sequence number information, or the context of header compression and decompression to continuously perform uninterrupted data reception from the source base station (1h-05) or the target base station (1h-10).

[0105] The first condition is one of the following conditions. The first condition proposed below is for proposing the uplink data transmission switching time point that can most efficiently use the transmission resources and minimize the data interruption time. · When the terminal successfully completes the random access procedure via the second bearer's hierarchical device (e.g., the MAC hierarchical device) to the target base station, or when the terminal (1h-20) successfully completes the random access procedure via the second bearer's hierarchical device (e.g., the MAC hierarchical device) to the target base station (1h-10) and is allocated the first uplink transmission resource from the target base station (1h-10), or when the uplink transmission resource is first indicated to the terminal (1h-20), the terminal (1h-20) determines that it satisfies the first condition.

[0106] · For example, if the terminal (1h-20) receives a handover command message from the source base station (1h-05) and is instructed to perform random access to the target base station (1h-10), and if the instructed random access is a contention-free random access procedure (CFRA (contention free random access)) (e.g., if a pre-specified preamble or a terminal cell identifier (e.g., C-RNTI) is allocated), · When the terminal (1h-20) transmits the preamble pre-specified for the cell of the target base station (1h-10) and receives a random access response (RAR (random access response)) message, it can be considered that the random access procedure has been successfully completed. When the first uplink transmission resource allocated, included, or indicated in the random access response message is received, it is determined that the first condition is satisfied. As another method, it can also be determined that the first condition is satisfied when the uplink transmission resource is received for the first time after receiving the RAR.

[0107] · If the terminal (1h-20) receives a handover command message from the source base station (1h-05) and is instructed to perform a random access to the target base station (1h-10), and if the instructed random access is a contention-based random access procedure (CBRA) (for example, if no preamble or terminal cell identifier (for example, C-RNTI) has been previously assigned), · The terminal 1h-20 transmits a preamble (for example, any preamble) to the cell of the target base station (1h-10) and receives a random access response (RAR) message. At that time, if the terminal (1h-20) uses the uplink transmission resources assigned, included, or indicated in the random access response message to transmit message 3 (for example, a handover completion message), and receives from the target base station (1h-10) in message 4 a "MAC CE" (Contention resolution MAC CE) indicating that contention has been resolved, or receives uplink transmission resources by means of a PDCCH corresponding to the "C-RNTI" of the terminal, then the terminal (1h-20) can consider that the random access procedure to the target base station (1h-10) has been successfully completed. Therefore, after that, when the terminal monitors the PDCCH and receives uplink transmission resources for the first time by means of a PDCCH corresponding to the C-RNTI of the terminal, or is instructed to use uplink transmission resources for the first time, it is determined that the first condition is satisfied. As another method, if the size of the uplink transmission resources assigned in the random access response message is sufficient and the terminal (1h-20) can transmit message 3 and add and transmit uplink data, it can be determined that the uplink transmission resources have been received for the first time, and it can also be determined that the first condition is satisfied. That is, when the RAR is received, the terminal (1h-20) determines that the uplink transmission resources have been received for the first time and determines that the first condition is satisfied.

[0108] · If both a handover method that does not require a random access procedure (RACH-less handover) is indicated in the handover command message received by the terminal (1h-20), · If the handover command message includes uplink transmission resources related to the target base station (1h-10), · If the terminal (1h-20) transmits a Message 3 (e.g., a handover completion message or an "RRC Reconfiguration Complete" message) using the uplink transmission resources of the target base station (1h-10) and receives a UE Identity Confirmation MAC CE with Message 4 from the target base station (1h-10), or receives uplink transmission resources with a PDCCH corresponding to the "C-RNTI" of the terminal, it is determined that the random access procedure has been successfully completed, and it is determined that the first condition is satisfied. As another method, after the random access procedure is successfully completed, the terminal (1h-20) can also perform PDCCH monitoring and determine that the first condition is satisfied when it receives the first uplink transmission resources with a PDCCH corresponding to the C-RNTI of the terminal (1h-20).

[0109] · If the handover command message does not include uplink transmission resources related to the target base station 1h-10, · The terminal (1h-20) monitors the PDCCH for the target base station (1h-10) (or cell). When the terminal (1h-20) receives uplink transmission resources with the PDCCH corresponding to its "C-RNTI", or transmits Message 3 (e.g., handover completion message or "RRC Reconfiguration Complete" message) with the uplink transmission resources, and receives the terminal identifier confirmation "MAC CE" (UE Identity Confirmation MAC CE) from the base station, or if it receives uplink transmission resources with the PDCCH corresponding to the "C-RNTI" of the terminal, it determines that the random access procedure has been successfully completed and determines that the first condition is satisfied. As another method, after the random access procedure is successfully completed, PDCCH monitoring is performed. When the terminal (1h-20) receives the first uplink transmission resources with the PDCCH corresponding to its "C-RNTI", it can also be determined that the first condition is satisfied.

[0110] According to an embodiment of the present invention, in the DAPS handover method, a method for switching uplink data from a source base station to a target base station is provided. Whether the above-mentioned first condition is satisfied can be confirmed or detected by one of the following methods in the MAC layer device or RRC layer device for the target base station corresponding to the second bearer, and a new method can be applied by combining the following methods.

[0111] · First method: For example, when DAPS handover is indicated in the "RRCReconfiguration" message received by the terminal, the terminal sets the MAC layer device for the target base station corresponding to the second bearer, and the MAC layer device executes the random access procedure to confirm whether the first condition is satisfied. And if the first condition is satisfied, the MAC layer device instructs the upper layer device (e.g., the PDCP layer device) using an indicator to switch the transmission of uplink data from the source base station via the first bearer to the target base station via the second bearer in the DAPS handover method.

[0112] · Second method: For example, when DAPS handover is indicated in the "RRCReconfiguration" message received by the terminal, the terminal configures the MAC layer device for the target base station corresponding to the second bearer, and the MAC layer device executes a random access procedure to check whether the first condition is satisfied. And if the first condition is satisfied, the MAC layer device notifies the upper layer device (e.g., the RRC layer device) that the first condition is satisfied. And the upper layer device (e.g., the RRC layer device) instructs the lower layer device (e.g., the PDCP layer device) using an indicator to switch the uplink data transmission from the source base station via the first bearer to the target base station via the second bearer in the DAPS handover method. When the first condition proposed in the present invention is satisfied, or when the random access procedure to the target base station is successfully executed, the upper layer device (e.g., the RRC layer device) stops the first timer. When the first timer is stopped, the RRC layer device instructs the PDCP layer device to switch to the indicator.

[0113] · Third method: When DAPS handover is indicated in the "RRCReconfiguration" message received by the terminal, the terminal configures the MAC layer device for the target base station corresponding to the second bearer. At this time, when an indicator indicating that the RRC layer device of the terminal performs DAPS handover is transmitted to the lower layer device (e.g., the MAC layer device), the MAC layer device executes a random access procedure to check whether the first condition is satisfied. And if the first condition is satisfied, in the DAPS handover method, the MAC layer device instructs the upper layer device (e.g., the PDCP layer device) to switch the uplink data transmission from the source base station via the first bearer to the target base station via the second bearer, using an indicator.

[0114] · Fourth method: When DAPS handover is indicated in the "RRCReconfiguration" message received by the terminal, the terminal configures the MAC layer device for the target base station corresponding to the second bearer. At this time, when an indicator indicating that the RRC layer device of the terminal performs DAPS handover is transmitted to the lower layer device (e.g., the MAC layer device), the MAC layer device executes a random access procedure to confirm whether the first condition is satisfied. And if the first condition is satisfied, the MAC layer device notifies the upper layer device (e.g., the RRC layer device) that the first condition is satisfied. If the indicator is confirmed, when the first condition proposed in the present invention is satisfied, or when the random access procedure to the target base station is successfully executed, the upper layer device (e.g., the RRC layer device) stops the first timer. And the upper layer device (e.g., the RRC layer device) instructs the lower layer device (e.g., the PDCP layer device) to switch the uplink data transmission from the source base station via the first bearer to the target base station via the second bearer in the DAPS handover method, using an indicator.

[0115] By the first method, the second method, the third method, or the fourth method, from a higher layer device (e.g., an RRC layer device) or a lower layer device (e.g., an MAC layer device), if the PDCP layer device receives an indicator that the first condition is satisfied, or an indicator to switch the uplink data transmission from the source base station to the target base station (e.g., when the DAPS handover method is indicated), the PDCP layer device performs the following operations to effectively execute the switching of the uplink data transmission and may perform one or more of the following operations to prevent data loss due to the uplink data transmission. The following operations are also applicable to a PDCP layer device connected to an RLC layer device operating in the "AM DRB" or "UM DRB" (an RLC layer device operating in the AM mode or an RLC layer device operating in the UM mode).

[0116] If there is data to be transmitted to the buffer before the PDCP layer device satisfies the first condition or receives an indicator that the first condition is satisfied, the PDCP layer device indicates the size or amount of the data to be transmitted (e.g., "PDCP data volume") to the MAC layer device of the first bearer for the source base station, notifies that there is data to be transmitted, and causes the source base station to perform uplink data transmission. Thereby, the MAC layer device of the first bearer for the source base station performs a scheduling request or buffer status report procedure to be allocated uplink transmission resources by the source base station. However, if the first condition is satisfied or the PDCP layer device receives an indicator that the first condition is satisfied, the PDCP layer device switches the uplink data transmission to the target base station as follows.

[0117] · The PDCP layer device indicates that the size or amount of the data transmitted to the MAC layer device of the first bearer is "0" (or, that there is none) in order to switch the uplink data transmission from the first bearer for the source base station to the second bearer for the target base station. That is, it is indicated to the MAC layer device of the first bearer that the data volume of the PDCP layer device is "0", indicating that there is no more data to be transmitted (in fact, even if there is data to be transmitted in the buffer, in order to switch the uplink data transmission, it is indicated to the MAC layer device of the first bearer for the source base station that there is no data to be transmitted.). However, as proposed in the present invention, when the handover method of the second embodiment (DAPS handover method) is indicated, or in the case of a bearer for which the handover method of the second embodiment (DAPS handover method) is indicated, when RLC control data (RLC status report) or PDCP control data (PDCP status report or ROHC feedback) is generated, the data volume corresponding to the RLC control data or PDCP control data is indicated to the MAC layer device, and data transmission is executed to the source base station.

[0118] · The PDCP layer device connected to the "AM DRB" (RLC layer device operating in the AM mode) discards all the "PDCP PDUs" that were previously stored (for example, in order to prevent the loss of the original data, the "PDCP SDU" is not discarded), and before satisfying the first condition from the first data (for example, "PDCP SDU") for which successful transmission from the lower layer (for example, the RLC layer device corresponding to the first bearer for the source base station) has not been confirmed, or before receiving an indicator that the first condition has been satisfied, the following procedure is executed on the data ("PDCP SDU" in the buffer) in ascending order of the COUNT value (or PDCP serial number) that was assigned.

[0119] Specifically, the PDCP layer device connected to the "AM DRB" (the RLC layer device operating in the AM mode) executes a new header compression procedure on the data (the "PDCP SDU" in the buffer) based on the header context for the target base station, applies the security key for the target base station, further executes an integrity procedure or an encryption procedure, constructs a PDCP header, transmits it to the lower layer device (the RLC layer device of the second bearer for the target base station), and retransmits or transmits the data. That is, cumulative retransmission of data is performed starting from the first data for which successful transmission has not been confirmed. As another method, when retransmitting data, retransmission can also be performed only on the data for which successful transmission has not been confirmed from the lower layer (for example, the RLC layer device of the first bearer for the source base station).

[0120] More specifically, the PDCP layer device connected to the "AM DRB" (or the RLC layer device operating in the AM mode) discards all the "PDCP PDUs" stored for transmission to the source base station via the first protocol layer device that was already connected to the PDCP layer device (for example, to prevent loss of the original data, the "PDCP SDU" is not discarded), and only for the data (for example, the "PDCP SDU") for which successful transmission has not been confirmed from the lower layer (for example, the RLC layer device), which is the first protocol layer device for the source base station, before satisfying the first condition or before receiving an indicator that the first condition has been satisfied, based on the COUNT value (or the PDCP sequence number) assigned previously, the following procedure is executed.

[0121] Specifically, the PDCP layer device connected to the "AM DRB" (RLC layer device operating in the AM mode) applies the header compression (or data compression) protocol context or security key corresponding to the target base station to the data for which successful transmission has not been confirmed (the "PDCP SDU" in the buffer), executes a new header or data compression procedure, further executes an integrity procedure or an encryption procedure, constructs a PDCP header, transmits the data to the lower layer device which is the second protocol layer device, and performs retransmission or transmission to the target base station. That is, in order to prevent waste of transmission resources, selective retransmission is performed only for the data for which successful transmission has not been confirmed. The above-described transmission operation or retransmission operation is also executed even if the lower layer (for example, the transmitting RLC layer device, the receiving RLC layer device, or the MAC layer device) which is the first protocol layer device for transmitting data to the source base station is released.

[0122] If the transmission procedure or the retransmission procedure is extended to the "UM DRB" (unacknowledged mode data radio bearer), the PDCP layer device connected to the RLC layer device operating in the UM mode receives from the upper layer device or regards as newly received data that has not yet been transmitted to the lower layer device, data for which the PDCP discard timer has not expired, or data for which a PDCP sequence number (or COUNT value) has already been assigned, does not restart the PDCP discard timer associated with each data, performs header (or data) compression on the data with the header (or data) compression context or security key for the target base station, or executes an encryption or integrity protection procedure, generates and joins a PDCP header, performs transmission or retransmission, processes the data in ascending order of the COUNT value assigned before the above procedure is triggered, and performs transmission or retransmission. Then, the window state variable of the PDCP layer device connected to the "UM DRB" or "AM DRB" (acknowledged mode data radio bearer) is not initialized and is maintained or used as it is.

[0123] · If there is data to be transmitted to the buffer, the PDCP layer device indicates the size or amount of the data to be transmitted (e.g., PDCP data volume) to the MAC layer device of the second bearer for the target base station, notifies that there is data to be transmitted, and performs uplink data transmission switching to the target base station. Thereby, the MAC layer device of the second bearer for the target base station executes a scheduling request or buffer status report procedure to be allocated uplink transmission resources by the target base station.

[0124] In a second embodiment of the handover method according to an embodiment of the present invention (e.g., DAPS handover method), even after the terminal receives a handover command message (e.g., "RRCReconfiguration" message), it continues to receive downlink data from the source base station or the target base station via the protocol layer device of the first bearer for the source base station or the second bearer for the target base station. Also, in order for the terminal to smoothly receive downlink data from the source base station (or the target base station), or for the source base station (or the target base station) to smoothly transmit downlink data, for the AM bearer, via the protocol layer device of the first bearer (or the second bearer), an RLC status report that is not data is allowed to be transmitted to the source base station (or the target base station) for uplink transmission.

[0125] That is, even if the first condition is satisfied and the terminal switches the uplink data transmission to the target base station, if the RLC status report, "HARQ ACK" or "HARQ NACK", or PDCP control data ("PDCP ROHC" feedback or PDCP status report) must be transmitted to the source base station, it is allowed to transmit data via the first bearer for the source base station. This is because, in the case of an AM bearer, after transmitting data at the transmitting end, if successful transmission is not indicated in the RLC status report (i.e., if the RLC status report is not received), data cannot be continuously transmitted thereafter.

[0126] Specifically, referring to FIG. 1H, in the (1h-03) stage of the second embodiment, even if the terminal (1h-20) satisfies the first condition and interrupts the transmission of uplink data to the source base station (1h-05) via the protocol layer device (1h-22) of the first bearer and switches to the target base station (1h-10) and starts transmitting uplink data to the target base station (1h-10) via the protocol layer device (1h-21) of the second bearer, the terminal (1h-20) can smoothly receive downlink data from the source base station (1h-05) (or the target base station (1h-10)), or the source base station (1h-05) (or the target base station (1h-10)) can smoothly transmit downlink data. Transmit "HARQ ACK" information or "HARQ NACK" information via the protocol layer device of the first bearer (or the second bearer), or continuously transmit the RLC status report (ACK information or NACK information) or PDCP control data (e.g., PDCP status report or ROHC feedback information).

[0127] Also, referring to FIG. 1H, in the (1h-03) stage of the second embodiment, when the terminal (1h-20) satisfies the first condition and interrupts the transmission of uplink data to the source base station (1h-05) via the protocol layer device (1h-22) of the first bearer, and switches to the target base station (1h-10) and starts transmitting uplink data to the target base station (1h-10) via the protocol layer device (1h-21) of the second bearer, the terminal (1h-20) also continues to perform data transmission by HARQ retransmission of the MAC layer device or data transmission by retransmission of the AM mode RLC layer device in order to avoid data loss to the source base station (1h-05).

[0128] If the terminal (1h-20) satisfies the first condition, interrupts the transmission of uplink data to the source base station (1h-05) via the protocol layer device (1h-22) of the first bearer, switches to the target base station (1h-10), and starts transmitting uplink data to the target base station (1h-10) via the protocol layer device (1h-21) of the second bearer, the source base station (1h-05) or the target base station (1h-10) allocates transmission resources to the terminal (1h-20) at different times so that the uplink transmission resources to the target base station (1h-10) and the uplink transmission resources to the source base station (1h-05) do not collide. If the uplink transmission resources to the target base station (1h-10) and the uplink transmission resources to the source base station (1h-05) overlap, the terminal (1h-20) gives priority to the uplink transmission resources to the source base station (1h-05) and performs data transmission to the source base station (1h-05) in order to maintain the downlink data transmission from the source base station (1h-05) or to continue receiving without problems.

[0129] As another method, if the uplink transmission resources to the target base station (1h-10) and the uplink transmission resources to the source base station (1h-05) collide and overlap, in order to maintain the downlink data transmission from the target base station (1h-10), the terminal (1h-20) can prioritize the uplink transmission resources to the target base station (1h-10) and perform data transmission to the target base station (1h-10).

[0130] Specifically, when the terminal (1h-20) receives a handover command message, if a handover corresponding to the second embodiment (for example, DAPS handover) is indicated in the handover command message, or if it is indicated for each bearer, the terminal (1h-20) or the bearer for which DAPS handover is indicated shall, until the first condition is satisfied, make a scheduling request via the first protocol layer device, transmit a buffer status report to the source base station (1h-05), receive uplink transmission resources, transmit uplink data, and receive downlink data from the source base station (1h-05). However, if the first condition is satisfied, the terminal (1h-20) shall no longer transmit data to the source base station (1h-05), switch the uplink to the target base station (1h-10), make a scheduling request via the second protocol layer device, transmit a buffer status report to the target base station (1h-10), receive uplink transmission resources, and transmit uplink data to the target base station (1h-10).

[0131] However, the terminal (1h-20) shall continue to receive downlink data from the source base station (1h-05), and continue to transmit the corresponding "HARQ ACK" or "HARQ NACK", RLC status report, or PDCP control data (for example, PDCP status report or ROHC feedback information) for the downlink data even after the uplink transmission is switched. Also, even if the first condition is satisfied, the terminal (1h-20) continues to receive downlink data from the source base station (1h-05) or the target base station (1h-10).

[0132] In the (1h-04) stage, if the terminal (1h-20) satisfies the second condition, the terminal (1h-20) interrupts receiving downlink data from the source base station (1h-05) or releases the connection with the source base station (1h-05) via the protocol layer device (1h-22) of the first bearer. The second condition can be one of the following conditions. Also, the PDCP layer device (1h-21) of the second bearer uses information such as the transmission data or reception data stored in the PDCP layer device (1h-22) of the first bearer, the sequence number information, or the context of header compression and header decompression, and continues to perform data transmission or data reception without interruption with the target base station (1h-10).

[0133] · When the terminal (1h-20) executes a random access procedure with the target base station (1h-10) via the layer device (1h-21) of the second bearer and receives a random access response, it is determined that the second condition is satisfied. · When the terminal (1h-20) executes a random access procedure with the target base station (1h-10) via the layer device of the second bearer, receives a random access response, and configures and transmits a handover completion message to the target base station (1h-10), it is determined that the second condition is satisfied. · When the terminal (1h-20) completes a random access procedure with the target base station (1h-10) via the layer device of the second bearer and transmits data for the first time using PUCCH uplink transmission resources or PUSCH uplink transmission resources, or when the terminal receives PUCCH uplink transmission resources or PUSCH uplink transmission resources from the base station for the first time, it is determined that the second condition is satisfied. · When the source base station (1h-05) separately sets a timer for the terminal (1h-20) with an RRC message and the timer expires, it is determined that the second condition is satisfied. · The timer is started when the terminal (1h-20) receives a handover command message from the source base station (1h-05), or when it starts a random access to the target base station (1h-10) (when transmitting a preamble), when it receives a random access response from the target base station (1h-10), when transmitting a handover completion message to the target base station (1h-10), or when first transmitting data using PUCCH uplink transmission resources or PUSCH uplink transmission resources.

[0134] · When the terminal (1h-20) executes a random access procedure to the target base station (1h-10) via the layer device of the second bearer, receives a random access response, constructs and transmits a handover completion message to the target base station (1h-10), and then the successful transmission of the handover completion message is confirmed by the MAC layer device (HARQ ACK) or the RLC layer device (RLC ACK), it is determined that the second condition is satisfied. · When the terminal (1h-20) executes a random access procedure to the target base station (1h-10) via the layer device of the second bearer, receives a random access response, or constructs and transmits a handover completion message at the target base station (1h-10), and then the target base station (1h-10) first allocates uplink transmission resources or when uplink transmission resources are first indicated, it is determined that the second condition is satisfied.

[0135] · When performing a handover procedure, the source base station (1h-05) determines when to interrupt the transmission of downlink data to the terminal (1h-20) or when to release the connection with the terminal (1h-20). For example, when to interrupt the downlink data transmission to the terminal (1h-20) or when to release the connection with the terminal (1h-20) is determined based on when a predetermined timer expires, after a handover instruction, when timer start is possible), or when the source base station (1h-05) receives an instruction from the target base station (1h-10) indicating that the terminal (1h-20) has successfully executed a handover to the target base station (1h-10). If the terminal (1h-20) does not receive downlink data from the source base station (1h-05) for a predetermined period of time, it determines that the second condition is satisfied, determines that the connection with the source base station (1h-05) has been released, and releases the connection.

[0136] · The terminal (1h-20) receives an instruction from the target base station (1h-10) to release the connection with the source base station (1h-05). The terminal (1h-20) determines that the second condition is satisfied, for example, when it receives an RRC message (e.g., "RRCReconfiguration" message), "MAC CE", "RLC control PDU", or "PDCP control PDU". · When the terminal (1h-20) receives an instruction to release the connection with the source base station (1h-05) from the source base station (1h-05) (e.g., an RRC message (e.g., "RRCReconfiguration" message), "MAC CE", "RLC control PDU", or "PDCP control PDU"), it determines that the second condition is satisfied. · If the terminal (1h-20) is unable to receive downlink data from the source base station (1h-05) for a predetermined period of time, it determines that the second condition is satisfied.

[0137] · When the terminal successfully completes the random access procedure via the second bearer's hierarchical device (e.g., the MAC hierarchical device) to the target base station, the terminal (1h - 20) successfully completes the random access procedure at the target base station (1h - 10) via the second bearer's hierarchical device. When the first uplink transmission resource is allocated from the target base station (1h - 10) or when the uplink transmission resource is first indicated to the terminal (1h - 20), it is determined that the second condition is satisfied. · For example, if the terminal (1h - 20) receives a handover command message from the source base station (1h - 05) and is instructed to perform a random access to the target base station (1h - 10), if the instructed random access is a contention - free random access procedure (CFRA (contention free random access)) (e.g., if a pre - specified preamble or a terminal cell identifier (e.g., C - RNTI) is allocated), · When the terminal (1h - 20) transmits a pre - specified preamble to the cell of the target base station (1h - 10) and receives a random access response (RAR (random access response)) message, since the random access procedure can be regarded as successfully completed, when the first uplink transmission resource allocated, included, or indicated in the random access response message is received, it is determined that the second condition is satisfied. As another method, when the terminal (1h - 20) receives the uplink transmission resource for the first time after receiving the RAR, it can also be determined that the second condition is satisfied.

[0138] · If the terminal (1h-20) receives a handover command message from the source base station (1h-05) and is instructed to perform a random access to the target base station (1h-10), and if the instructed random access is a contention-based random access procedure (CBRA) (for example, if a pre-specified preamble or a terminal cell identifier (e.g., C-RNTI) is not assigned), · The terminal transmits a preamble (e.g., any preamble) to the cell of the target base station, receives a random access response (RAR) message, and uses the uplink transmission resources assigned, included, or indicated in the random access response message to transmit message 3 (e.g., a handover completion message). Also, if the terminal receives a "MAC CE" (Contention resolution MAC CE) indicating that contention has been resolved by message 4 from the target base station, or if the terminal receives uplink transmission resources by means of a PDCCH corresponding to the terminal's "C-RNTI", it can be considered that the random access procedure to the target base station has been successfully completed. Thereafter, when the terminal monitors the PDCCH and receives uplink transmission resources for the first time by means of a PDCCH corresponding to the terminal's "C-RNTI", or when it is first instructed, it is determined that the second condition is satisfied. As another method, if the size of the uplink transmission resources assigned in the random access response message is sufficient and the terminal can transmit message 3 and add and transmit uplink data, it can be determined that the uplink transmission resources have been received for the first time, and it can also be determined that the second condition is satisfied. That is, when the terminal receives an RAR, it can be determined that the uplink transmission resources have been received for the first time, and it can also be determined that the second condition is satisfied.

[0139] · If both a handover method that does not require a random access procedure (RACH-less handover) are indicated in the handover command message received by the terminal, · If the handover command message includes uplink transmission resources related to the target base station, · If the terminal transmits Message 3 (e.g., handover completion message or "RRC Reconfiguration Complete" message) using the uplink transmission resources of the target base station, receives the UE Identity Confirmation MAC CE from the base station as Message 4, or receives uplink transmission resources via the PDCCH corresponding to the terminal's "C-RNTI", it determines that the random access procedure has been successfully completed and determines that the second condition is satisfied. As another method, after the random access procedure is successfully completed, the terminal can perform PDCCH monitoring and determine that the second condition is satisfied when it receives the first uplink transmission resources via the PDCCH corresponding to the terminal's C-RNTI.

[0140] · If the handover command message does not include uplink transmission resources related to the target base station, · The terminal performs PDCCH monitoring on the target base station (or cell) and determines that the random access procedure has been successfully completed and the second condition is satisfied when it receives uplink transmission resources via the PDCCH corresponding to the terminal's C-RNTI, or transmits Message 3 (e.g., handover completion message or "RRC Reconfiguration Complete" message) using the uplink transmission resources and receives the UE Identity Confirmation MAC CE from the base station, or receives uplink transmission resources via the PDCCH corresponding to the terminal's "C-RNTI". As another method, after the random access procedure is successfully completed, PDCCH monitoring is performed. When the first uplink transmission resource is received by a PDCCH corresponding to the "C-RNTI" of the terminal, it can also be determined that the second condition is satisfied.

[0141] When the terminal executes the second embodiment of the handover method according to an embodiment of the present invention (for example, the DAPS handover method), if it is confirmed that the RRC layer device, MAC layer device, or RLC layer device of the first bearer for the source base station of the terminal, or the RRC layer device, MAC layer device, or RLC layer device of the second bearer for the target base station satisfies the second condition, the terminal executing the DAPS handover method, or the PDCP layer device of the bearer is instructed with an indicator that the second condition is satisfied. If the PDCP layer device of the terminal receives an indicator indicating that the second condition is satisfied from a lower layer device or an upper layer device, at least one or more procedures within the following procedures are executed, and the handover procedure according to the second embodiment of the present invention can be successfully completed.

[0142] · The terminal releases the first bearer for the source base station and releases the connection with the source base station. Before the terminal releases the first bearer for the source base station, the terminal executes an RLC re-establishment procedure on the RLC layer device corresponding to the first bearer for the source base station (for example, if the reordering timer is running, stop or initialize the timer, if the received data is stored in the buffer, process the stored data and transmit it to the upper layer device. Also, if the data to be transmitted is in the buffer, discard it.), or initializes the MAC layer device. · After the terminal releases the connection with the source base station, in order to report the reception status of the downlink data received from the source base station to the target base station, the terminal triggers a PDCP status reporting procedure, configures a PDCP status report, and transmits the PDCP status report to the target base station.

[0143] · When the terminal satisfies the second condition, it converts from the structure or function of the second PDCP layer device (1i-20) to the structure or function of the first PDCP layer device (1i-11 or 1i-12) for each bearer or for the bearer for which the DAPS handover method is indicated, initializes the variables for reordering, stops and initializes the reordering timer, and for reordering, applies the security key for the source base station or the header decompression context to the data stored in the buffer (e.g., the data received from the source base station), performs the decryption procedure or header (or data) decompression, and then discards the security key for the source base station or the header decompression context. Then, the processed data is transmitted to the upper layer in ascending order. That is, when the terminal satisfies the second condition, for reordering, it applies the security key for the source base station or the header decompression context to the data stored in the buffer (e.g., the data received from the source base station), performs the decryption procedure or header (or data) decompression, and then discards the security key for the source base station or the header decompression context.

[0144] As another method, when the terminal satisfies the second condition, it converts from the structure or function of the second PDCP layer device (1i-20) to the structure or function of the third PDCP layer device (1i-30) for each bearer or for the bearer for which the DAPS handover method is indicated, and stops or does not initialize the variables for reordering and the reordering timer, and continues to use them as they are. However, it is also possible to apply the security key for the source base station or the header decompression context to the data stored in the buffer (e.g., the data received from the source base station) for reordering, perform the decryption procedure or header (or data) decompression, and then discard the security key for the source base station or the header decompression context. Then, the processed data is transmitted to the upper layer in ascending order. That is, when the terminal satisfies the second condition, for the data stored in the buffer for reordering (for example, the data received from the source base station), apply the security key for the source base station or the header decompression context, perform the decoding procedure or header (or data) decompression, and then discard the security key for the source base station or the header decompression context. The terminal releases the QoS mapping information of the SDAP layer device for the source base station, the security key information of the PDCP layer device for the source base station, the header (or data) compression context information for the source base station, the RLC layer device for the source base station, or the MAC layer device.

[0145] · The terminal is executing the DAPS handover method. If the second condition is satisfied, for each bearer or for the bearer indicated by the DAPS handover method, the structure and function (1j-20) of the second SDAP layer device that was applied are released for the first bearer of the source base station and further converted to the structure and function (1j-10) of the first SDAP layer device. Then, when the terminal satisfies the second condition, it converts from the structure or function (1j-20) of the second SDAP layer device to the structure or function (1j-10) of the first SDAP layer device, for each bearer or for the bearer indicated by the DAPS handover method. Before releasing the mapping information between the second bearer or "second QoS flow" for the target base station and the bearer, apply the mapping information between the "first QoS flow" and the bearer to the data received from the source base station (for example, all the data received from the source base station). After completing the data processing, release the mapping information between the "first QoS flow" and the bearer or the first bearer. Then, the processed data is transmitted to the upper layer in ascending order.

[0146] That is, when the terminal satisfies the second condition, for the data stored in the buffer (for example, the data received from the source base station), the "first QoS flow" for the source base station and the mapping information of the bearer are applied, and the data is processed (for example, reading the SDAP header information, updating the mapping information, or constructing the SDAP header, or routing or transmitting to a suitable upper-layer device or lower-layer device based on the "first QoS flow" and the mapping information of the bearer), and then the "first QoS flow" for the source base station and the mapping information of the bearer are discarded.

[0147] The SDAP layer device defines a 1-bit indicator of the new SDAP header, a 1-bit indicator of the PDCP header, SDAP control data (for example, the downlink "End marker"), or the information indicated by the PDCP layer device, and based on this, determines what the last data received from the source base station is. Therefore, after performing data processing by applying the "first QoS flow" for the source base station and the mapping information of the bearer to the last data received from the source base station, the "first QoS flow" for the source base station and the mapping information of the bearer are discarded. Then, the SDAP layer device continues to maintain the "second QoS flow" and the mapping information of the bearer, and based on this, processes the uplink data or downlink data to the target base station.

[0148] In the foregoing FIG. 1F, when the base station transmits a handover command message (1f-20) to the terminal, in the handover command message (for example, an "RRCReconfiguration" message), an indicator related to the foregoing embodiment is defined, and the terminal is instructed as to which handover procedure corresponding to any embodiment is to be triggered. The terminal executes the handover procedure by the handover method indicated in the handover command message, executes the handover method (DAPS handover method) according to the second embodiment of the present invention, and performs a handover to the target base station while minimizing the data interruption time.

[0149] In other ways, in the handover command message, an indicator related to the foregoing embodiment can be defined for each bearer, and it can be further specifically indicated as to which embodiment is to be applied to which bearer during handover. For example, it can be indicated that the second embodiment is to be applied only to the AM bearer driven by the RLC layer device driven in the AM mode, or it can also be applied to the UM bearer driven by the RLC layer device driven in the UM mode. Also, it is assumed that the embodiments of the present invention are applied to DRBs. However, if necessary (for example, when the terminal maintains the SRB related to the source base station, fails to hand over to the target base station, and can report or recover the handover failure message with the SRB related to the source base station), the foregoing embodiments can also be applied to SRBs.

[0150] In the embodiments of the present invention, when the terminal performs data transmission and reception with the source base station via the protocol layer device of the first bearer and performs data transmission and reception with the target base station via the protocol layer device of the second bearer, the MAC layer device of the first bearer and the MAC layer device of the second bearer each operate a separate discontinuous reception (DRX) cycle, which can reduce the battery consumption of the terminal. That is, when the terminal transmits and receives data via the protocol layer device of the first bearer, even if it receives a handover command message, it continues to apply the DRX cycle of the MAC layer device and can stop DRX according to the first condition or the second condition. In addition, separately, the application of the DRX cycle to the MAC layer device of the second bearer is operated according to the instruction of the target base station.

[0151] In addition, when the terminal interrupts the uplink transmission to the source base station and interrupts the downlink data reception from the source base station via the protocol layer device of the first bearer, it means that the terminal re-establishes, initializes, or releases the protocol layer device (PHY layer device, MAC layer device, RLC layer device, or PDCP layer device) of the first bearer. In the embodiments of the present invention, for the sake of convenience of explanation, it is described that the first bearer for the source base station or the second bearer for the target base station is set for the terminal, and the same applies when a plurality of first bearers for the source base station or a plurality of second bearers for the target base station are set for the terminal.

[0152] In addition, the foregoing embodiments are equally applicable when a plurality of bearers for a plurality of target base stations are set. For example, when a handover procedure is executed to the first target base station, the second bearer is set, and the handover fails, a handover procedure is executed to the second target base station, the second bearer is set, and the terminal among the plurality of target base stations searches for and determines a cell that satisfies a predetermined condition (for example, a signal strength of a certain level or more) by itself, determines one cell, and can execute a handover procedure.

[0153] FIG. 1I is a diagram for explaining a second example of the structure of an efficient PDCP layer device applied in a DAPS handover method and a handover method applying the structure according to an embodiment of the present invention. Referring to FIG. 1I, a specific structure and function of an efficient PDCP layer device applied in the DAPS handover method according to the second embodiment of the present invention are provided. At this time, when the DAPS handover procedure is executed, the following PDCP layer device structures apply different PDCP layer structures for each bearer at different times.

[0154] For example, before the terminal receives a handover command message from the base station, for each bearer, it applies the structure and function (1i-11) or (1i-12) of the first PDCP layer device to process and transmit or receive data (reference numeral 1i-01). However, if the terminal receives a handover command message from the base station and the DAPS handover method proposed in the present invention is indicated in the handover command message, or if the DAPS handover method is indicated for a specific bearer, the terminal applies the structure and function (1i-20) of the second PDCP layer device for each bearer or for the bearer for which the DAPS handover method is indicated to process and transmit or receive data (reference numeral 1i-02). That is, when the terminal receives a handover command message and the DAPS handover method is indicated in the handover command message, or when the DAPS handover method is indicated for a specific bearer, it converts from the structure or function (1i-11) or (1i-12) of the first PDCP layer device used for each bearer to the structure or function (1i-20) of the second PDCP layer device for each bearer or for the bearer for which the DAPS handover method is indicated.

[0155] As another method, when the terminal satisfies the first condition, it can also convert from the structure or function (1i-11) or (1i-12) of the first PDCP layer device used for each bearer to the structure or function (1i-20) of the second PDCP layer device for each bearer or for the bearer for which the DAPS handover method is indicated (reference numeral 1i-02). Then, when the terminal receives a handover command message and the DAPS handover method is indicated in the handover command message, or when the DAPS handover method is indicated for a specific bearer, or when the PDCP reordering timer value is newly set, the terminal converts from the structure or function (1i-11) or (1i-12) of the first PDCP layer device to the structure or function (1i-20) of the second PDCP layer device proposed in the present invention for each bearer or for the bearer for which the DAPS handover method is indicated. When converting, the variables for reordering are updated to the PDCP sequence number or COUNT value that is expected to be received next, the reordering timer is stopped and restarted.

[0156] Then, when the terminal is executing the DAPS handover method and if the second condition is satisfied, for each bearer or for the bearer for which the DAPS handover method is indicated, the first bearer for the source base station is released for the structure and function (1i-20) of the second PDCP layer device that was being applied, and it is further converted and applied to the structure and function (1i-11) or (1i-12) of the first PDCP layer device. Then, when the terminal satisfies the second condition, when converting from the structure or function (1i-20) of the second PDCP layer device to the structure or function (1i-11) or (1i-12) of the first PDCP layer device for each bearer or for the bearer for which the DAPS handover method is indicated, the variables for reordering are initialized, the reordering timer is stopped and initialized. For reordering, the security key or header decompression context for the source base station is applied to the data stored in the buffer (for example, the data received from the source base station), and after performing the decryption procedure or header (or data) decompression, the security key or header decompression context for the source base station is discarded. Then, the processed data is transmitted to the upper layer in ascending order. That is, when the terminal satisfies the second condition, for reordering, a security key or a header decompression context for the source base station is applied to the data stored in the buffer (e.g., the data received from the source base station), and after performing the decryption procedure or header (or data) decompression, the security key or the header decompression context for the source base station is discarded.

[0157] As another method, when the terminal is executing the DAPS handover method and if the second condition is satisfied, for each bearer or for the bearers for which the DAPS handover method is indicated, the structure and function (1i-20) of the second PDCP layer device that was applied are released for the bearer for the source base station, and it is also possible to convert to the structure and function (1i-30) of the third PDCP layer device. Then, when the terminal satisfies the second condition and converts from the structure or function (1i-20) of the second PDCP layer device to the structure or function (1i-30) of the third PDCP layer device for each bearer or for the bearers for which the DAPS handover method is indicated, the variables and the reordering timer for reordering are not stopped or initialized, but are continuously used as they are.

[0158] However, for reordering, a security key or a header decompression context for the source base station is applied to the data stored in the buffer (e.g., the data received from the source base station), and after performing the decryption procedure or header (or data) decompression, the security key or the header decompression context for the source base station is discarded. And the processed data is transmitted to the upper layer in ascending order. That is, when the terminal satisfies the second condition, for reordering, a security key or a header decompression context for the source base station is applied to the data stored in the buffer (e.g., the data received from the source base station), and after performing the decryption procedure or header (or data) decompression, the security key or the header decompression context for the source base station is discarded.

[0159] Referring to FIG. 1I, when the terminal applies different structures and functions of the first PDCP layer device (1i-11 or 1i-12), the second PDCP layer device (1i-20), or the third PDCP layer device (1i-30) for different bearers at different times and performs handover, data loss can be avoided and the data interruption time can be minimized. The structure of the first PDCP layer device (1i-11 or 1i-12) in FIG. 1I has the following first-1 PDCP layer device structure, first-2 PDCP layer device structure, first-3 PDCP layer device structure, or first-4 PDCP layer device structure, and may have the following characteristics.

[0160] 1) (If it is the structure of the first-1 PDCP layer device) For example, if the terminal applies the structure and function (1i-11) of the first PDCP layer device to a PDCP layer device (for example, "E-UTRA PDCP" layer device or "LTE PDCP" layer device) connected to an "AM RLC" layer device (for example, "E-UTRA AM RLC" layer device), it has the following characteristics. 2) The receiving PDCP layer device first performs data detection outside the window or duplicate data detection on the received data. ("RLC AM" has retransmissions, and since the sizes of "LTE RLC SN" and "PDCP SN" are different, duplicate data and data outside the window are received. The window indicates the area of the PDCP sequence number or COUNT value where valid data is received.) 3) Before discarding the data outside the window or duplicate data, the terminal performs a decoding procedure and a header decompression procedure and then discards it. (Because useful information (for example, IR package or header compression information) for the header decompression procedure is included, it is confirmed and discarded.) 2) The data received without being discarded is immediately decoded without sequencing and a header decompression procedure is executed. This is because the "E-UTRA AM RLC" layer device sequences the data and transmits it to the PDCP layer device. 2) When the data is transmitted to the upper layer, it is transmitted in ascending order of the COUNT value.

[0161] 1) (If it is the structure of the first - second PDCP layer device) For example, if a PDCP layer device (e.g., an "E - UTRA PDCP" layer device or an "LTE PDCP" layer device) connected to a UMRLC layer device (e.g., an "E - UTRA UM RLC" layer device) applies the structure and functions (1i - 11) of the first PDCP layer device, it has the following characteristics. 2) The data detection outside the window or the duplicate data detection procedure is not executed. This is because there is no re - transmission procedure in the "UME - UTRA RLC" layer device. 2) Then, the terminal immediately executes the decoding procedure and the header decompression procedure on the received data. 2) Then, after the re - alignment procedure, the terminal transmits it to the upper layer (e.g., in ascending order).

[0162] 1) (If it is the structure of the first - third PDCP layer device) For example, if a PDCP layer device (e.g., an "E - UTRA PDCP" layer device or an "LTE PDCP" layer device) configured for a split bearer, a packet duplication bearer, or an LWA bearer applies the structure and functions (1i - 11) of the first PDCP layer device, it always applies the sequence re - alignment procedure and the re - alignment timer and has the following characteristics. 2) The terminal first performs data detection outside the window or duplicate data detection on the received data (because re - transmission of "RLC AM" is performed, or data is received at different times from different RLC layer devices, and the sizes of "LTE RLC SN" and "PDCP SN" are different, so data outside the window or duplicate data is received). 3) The terminal executes the decryption procedure. However, the header decompression procedure is not executed (because for "E-UTRA PDCP", header compression protocol configuration is not possible for split bearers or LWQ bearers.). 3) If the integrity protection or verification procedure is executed and the integrity verification procedure fails, the received data mentioned above is discarded. If the integrity verification procedure fails, discard the data and report it to the upper layer device. 3) Data outside the window or duplicate data is discarded. 2) If the data has not been discarded, immediately execute the decryption procedure on the received data without sorting. And if integrity protection or verification is set, integrity verification is performed. If the integrity protection or verification procedure is executed, discard the data after execution. If the integrity verification procedure fails, discard the data and report it to the upper layer device. 2) Then, if the received data is sorted and continuously sorted in ascending order without a gap in the PDCP sequence number or COUNT value, the header compression procedure is executed on the data (if the header compression procedure or header decompression procedure is set), and the data is transmitted to the upper layer in ascending order. 2) If the reordering timer is running, 3) If the data corresponding to the COUNT value that has the same value as the value obtained by subtracting "1" from the value maintained by the variable for reordering is transmitted to the upper layer device, or if any data is transmitted to the upper layer without a gap in the PDCP sequence number (or COUNT value), 4) The terminal stops and initializes the reordering timer. 2) If the reordering timer is not running, 3) If there is data stored in the buffer without being transmitted to the upper layer device, or if a gap occurs in the PDCP sequence number (or COUNT value), 4) The terminal starts the reordering timer. 4) Then, the terminal updates the variable for reordering to the PDCP sequence number or COUNT value that is expected to be received next. 2) If the reordering timer expires, 3) If the decompression procedure is set for the saved data in ascending order of the PDCP sequence number or COUNT value to a value smaller than the reordering variable value, execute the decompression procedure and transmit the data to the upper layer device. 3) If the decompression procedure is set for the saved data in ascending order of the PDCP sequence number or COUNT value to a value that is the same as or greater than the reordering variable value so as to be continuous, execute the decompression procedure and transmit it to the upper layer device. 3) Then, update the PDCP sequence number or COUNT value of the last transmitted data to the variable value related to the data last transmitted to the upper layer. 3) If there is data saved in the buffer without being transmitted to the upper layer device, or if a gap occurs in the PDCP sequence number (or COUNT value), 4) Start the reordering timer. 4) Then, update the variable for reordering to the PDCP sequence number or COUNT value that is expected to be received next.

[0163] 1) (If it is the structure of the first to fourth PDCP layer devices) For example, if the terminal applies the structure and functions (1i - 12) of the first PDCP layer device to the "NR PDCP" layer device, the reordering procedure and the reordering timer are always applied and have the following characteristics. 2) First, execute the decoding procedure on the received data. 2) If the integrity protection or verification procedure is set, execute the integrity protection or verification procedure for the received data. If the integrity verification procedure fails, discard the data and report it to the upper layer device. 2) Detect data outside the window or duplicate data in the received data (perform data detection outside the window or duplicate data detection after the decryption procedure. The terminal performs data detection outside the window or duplicate data detection after the decryption procedure only when the integrity protection or verification procedure is set. If the integrity protection or verification procedure is not set, the terminal performs data detection outside the window or duplicate data detection and then executes the decryption procedure only for the data that has not been discarded). 3) Discard data outside the window or duplicate data. 2) If the data has not been discarded, sort the received data. If the received data is sorted in ascending order continuously without gaps in the PDCP sequence number or COUNT value, execute the header compression procedure (when the header compression procedure or header decompression procedure is set), and transmit the data to the upper layer in ascending order. 2) Then, when transmitting to the upper layer, transmit in ascending order of the COUNT value. 2) If the reordering timer is running, 3) If the data corresponding to the COUNT value that has the same value as the value obtained by subtracting 1 from the value maintained by the variable for reordering is transmitted to the upper layer device, or if any data is transmitted to the upper layer without gaps in the PDCP sequence number (or COUNT value), or if the value of the variable that stores the PDCP sequence number or COUNT value of the data transmitted to the upper layer is greater than or equal to the value of the variable for reordering, 4) Stop and initialize the reordering timer. 2) If the reordering timer is not running, 3) If there is data stored in the buffer that has not been transmitted to the upper layer device, or if a gap occurs in the PDCP sequence number (or COUNT value), or if the value of the variable that stores the COUNT value of the first data that has not been transmitted to the upper layer is smaller than the value of the variable for reordering, 4) Then, update the variable for reordering to the PDCP sequence number or COUNT value that is expected to be received next. 4) Start the reordering timer. 2) If the reordering timer expires, 3) If the decompression procedure is set for the saved data in ascending order of the PDCP sequence number or COUNT value to a value smaller than the reordering variable value, execute the decompression procedure and transmit it to the upper layer device. 3) If the decompression procedure is set for the saved data in ascending order of the PDCP sequence number or COUNT value to be the same as or greater than the reordering variable value so as to be continuous, execute the decompression procedure and transmit it to the upper layer device. 3) Then, update the variable value related to the first data not transmitted to the upper layer to the PDCP sequence number or COUNT value of the first data not transmitted to the upper layer. 3) If there is data saved in the buffer without being transmitted to the upper layer device, a gap occurs in the PDCP sequence number (or COUNT value), or the value of the variable storing the COUNT value of the first data not transmitted to the upper layer is smaller than the value of the variable for reordering, 4) Then, update the variable for reordering to the PDCP sequence number or COUNT value expected to be received next. 4) Start the reordering timer.

[0164] The structure of the second PDCP layer device (1i - 20) in FIG. 1I has the following second - 1 PDCP layer device structure or second - 2 PDCP layer device structure proposed by the present invention and has the following characteristics. In the present invention, as shown by (reference numeral 1i - 20), an efficient structure of the second PDCP layer device is provided in handover. The structure of the second PDCP layer device applies the second embodiment of the efficient handover method for minimizing the data interruption time proposed by the present invention. In the structure of the second PDCP layer device, the terminal performs data transmission or reception with the source base station (1i-21) via the protocol layer device of the first bearer (for example, the SDAP layer device, the PDCP layer device, the RLC layer device, or the MAC layer device), and performs data transmission or reception with the target base station (1i-22) via the protocol layer device of the second bearer (for example, the SDAP layer device, the PDCP layer device, the RLC layer device, or the MAC layer device).

[0165] The PDCP layer device of the first bearer and the PDCP layer device of the second bearer are each set in the terminal, but logically operate as one PDCP layer device, such as (reference numeral 1i-20). Specifically, the aforementioned one PDCP layer device divides the functions of the PDCP layer device into the functions of the upper PDCP layer device (for example, the function of assigning a sequence number, the reordering function, the in-sequence delivery function, or the duplicate detection function), and the functions of two lower PDCP layer devices for each source base station and each target base station (for example, the function of decoding or encrypting, the function of header (or data) compression or header (or data) decompression, or the function of integrity protection or verification or its duplicate detection function).

[0166] Also, as proposed above, in the DAPS handover method, the terminal transmits the uplink data to the source base station, and if the first condition is satisfied, switches to the target base station, and the downlink data continues to be received from the source base station and the target base station. Therefore, for the header (or data) compression protocol context, only one context for the source base station or the target base station is maintained and applied for the uplink, and two contexts for the source base station or the target base station are maintained and applied for the downlink.

[0167] Based on the second PDCP layer structure, the second - 1 PDCP layer structure (for example, the "E - UTRA PDCP" layer device for the DAPS handover method) has the following characteristics. The upper - layer transmitting PDCP layer device function plays a role of assigning PDCP sequence numbers to the data received from the upper - layer device. Then, in the two lower - layer transmitting PDCP layer device functions (1i - 21, 1i - 22) for each source base station and each target base station, separate security keys set with each source base station and each target base station are used. For the data to be transmitted to the source base station, the header (or data) compression context or security key set with the source base station is applied. For the data to be transmitted to the target base station, the header (or data) compression context or security key set with the target base station is applied. When a header (or data) compression procedure is set, the header (or data) compression procedure is applied. When integrity protection is set, the integrity protection procedure is applied to the PDCP header and data (PDCP SDU) and then an encryption procedure is applied. The data to be transmitted to the source base station is transmitted to the transmitting RLC layer device of the first bearer, and the data to be transmitted to the target base station is transmitted to the transmitting RLC layer device of the second bearer for transmission.

[0168] In the two lower - layer transmitting PDCP layer device functions (1i - 21, 1i - 22), in order to accelerate the data - processing speed, parallel data processing for performing header compression, integrity protection, or encryption procedures in parallel is carried out. In the two lower - layer transmitting PDCP layer device functions, different security keys are used to perform integrity protection or encryption procedures. Also, logically, within one transmitting PDCP layer device, different compression contexts, security keys, or security algorithms are applied, and compression, integrity protection, or encryption procedures are performed for different data.

[0169] The receiving PDCP layer device function performs, on the data received from each lower layer device (e.g., the data received from two RLC layer devices for each source base station and each target base station), the lower receiving PDCP layer device functions (1i-21, 1i-22) for the source base station or the target base station, independently for the data received from each RLC layer device, a procedure for detecting data outside the window or detecting duplicate data based on the PDCP sequence number or COUNT value.

[0170] As another method, for the convenience of implementation, a procedure for detecting data outside the window or detecting duplicate data based on the PDCP sequence number or COUNT value can also be performed on the overall data received without dividing each RLC layer device. As yet another method, for more accurate duplicate detection, a procedure for detecting data outside the window based on the PDCP sequence number or COUNT value is performed on the overall data received without dividing each RLC layer device, and the duplicate detection procedure is independently performed for the data received from each RLC layer device. As yet another method, when data received from different base stations overlap with each other, in order to prevent data loss for the header compression protocol, a procedure for detecting data outside the window based on the PDCP sequence number or COUNT value is performed on the overall data received without dividing each RLC layer device, and the duplicate detection procedure can also be performed on the overall data after receiving the decryption procedure, integrity protection procedure, or header (or, data) decompression procedure for the data received from each RLC layer device. The lower function of the receiving PDCP layer device immediately applies a decryption procedure to the data received using a separate header (or, data) compression context or security key set for each source base station and each target base station, and when integrity protection is set, an integrity verification procedure is applied to the PDCP header and the data (PDCP SDU).

[0171] In the structure of the 2-1 PDCP layer device, the data received from the RLC layer device of the first bearer for each source base station immediately undergoes a decompression procedure for the header (or data) without reordering, and the data received from the RLC layer device of the second bearer for each target base station also immediately undergoes a decompression procedure for the header (or data) without reordering. Also, the 2-1 PDCP layer device defines an indicator for each piece of data in order to distinguish between the data received from the RLC layer device of the first bearer for each source base station and the data received from the RLC layer device of the second bearer for each target base station, so as to distinguish whether it is data received from a source base station or data received from a target base station.

[0172] As another method, the 2-1 PDCP layer device can also define a 1-bit indicator in the PDCP header, SDAP header, or RLC header to distinguish whether it is data received from a source base station or data received from a target base station. Furthermore, the 2-1 PDCP layer device performs a duplicate detection procedure (for each PDCP sequence number or COUNT value, only leaving one piece of data (applying including previously received data or data transmitted to the upper layer) and discarding all others) on the entirety of the data received from the RLC layer device of the first bearer for the source base station and the data received from the RLC layer device of the second bearer for the target base station, based on the PDCP sequence number or COUNT value. Then, the 2-1 PDCP layer device performs a reordering procedure in ascending order on the entirety of the data received from the RLC layer device of the first bearer for the source base station and the data received from the RLC layer device of the second bearer for the target base station, based on the PDCP sequence number or COUNT value, and transmits the data to the upper layer device in sequence. One PDCP layer device always performs a reordering procedure so that it can receive data from different base stations, i.e., from the first bearer or the second bearer, regardless of the order.

[0173] Two lower receiving PDCP layer device functions perform parallel data processing (parallel processing) to accelerate the data processing speed by executing header compression, integrity protection, or encryption procedures in parallel, respectively based on the PDCP sequence number reference or the COUNT value. Also, different header (or data) compression contexts or security keys are used, and integrity protection, encryption procedures, or decompression procedures are executed. Also, logically, within one transmitting PDCP layer device, different header (or data) compression contexts, security keys, or security algorithms are applied, and integrity protection, encryption procedures, or decompression procedures are executed for different data. Also, in the lower receiving PDCP layer device function, an out-of-sequence deciphering or integrity verification procedure is executed for each received data regardless of the order of the PDCP sequence number or the COUNT value.

[0174] When one PDCP layer device distinguishes between the layer device of the first bearer and the layer device of the second bearer, it considers whether it is connected to different MAC layer devices, whether it has different logical channel identifiers, whether it is different RLC layer devices connected to different MAC layer devices, or whether it uses different encryption keys, and distinguishes between the layer device of the first bearer (or the first RLC layer device) and the layer device of the second bearer (or the second RLC layer device), and performs encryption or decryption procedures with different security keys for the uplink data and the downlink data, and uses different compression protocol contexts to compress or decompress.

[0175] Based on the foregoing second PDCP layer structure, the second - 2 PDCP layer structure (for example, the "NR PDCP" layer device for the DAPS handover method) has the following characteristics. The transmitting PDCP layer device function serves to assign PDCP sequence numbers to the data received from the upper - layer device. Then, in the two lower - layer transmitting PDCP layer device functions (1i - 21, 1i - 22) for each source base station and each target base station, separate security keys set with each source base station and each target base station are used. For the data to be transmitted to the source base station, the header (or data) compression context or security key set with the source base station is applied. For the data to be transmitted to the target base station, the header (or data) compression context or security key set with the target base station is applied. When a header (or data) compression procedure is set, the header (or data) compression procedure is applied. When integrity protection is set, the integrity protection procedure is applied to the PDCP header and data ("PDCP SDU"). When an encryption procedure is applied, the data to be transmitted to the source base station is transmitted to the transmitting RLC layer device of the first bearer, and the data to be transmitted to the target base station is transmitted to the transmitting RLC layer device of the second bearer to perform data transmission.

[0176] In the two lower - layer transmitting PDCP layer device functions (1i - 21, 1i - 22), in order to accelerate the data - processing speed, parallel data processing for performing header compression, integrity protection, or encryption procedures in parallel is carried out. In the two lower - layer transmitting PDCP layer device functions, different security keys are used to perform integrity protection or encryption procedures. Also, logically, within one transmitting PDCP layer device, different compression contexts, security keys, or security algorithms are applied, and compression, integrity protection, or encryption procedures are performed on different data.

[0177] The receiving PDCP layer device function independently performs procedures for detecting out-of-window data or detecting data duplication on the data received from each lower-layer device, specifically on the data received from two RLC layer devices for each source base station and each target base station, and on the data received from the lower receiving PDCP layer device functions (1i-21, 1i-22) for the source base station or the target base station, based on the PDCP sequence number or COUNT value, for the data received from each RLC layer device.

[0178] As another method, for convenience of implementation, the receiving PDCP layer device can also perform procedures for detecting out-of-window data or detecting data duplication on the overall data received without distinguishing each RLC layer device, based on the PDCP sequence number or COUNT value. As yet another method, for more accurate duplication detection, the receiving PDCP layer device performs out-of-window data detection on the overall data received without distinguishing each RLC layer device, based on the PDCP sequence number or COUNT value, and can independently perform the duplication detection procedure on the data received from each RLC layer device. As yet another method, when the data received from different base stations overlap with each other, the receiving PDCP layer device performs out-of-window data detection on the overall data received without distinguishing each RLC layer device, based on the PDCP sequence number or COUNT value, to prevent data loss for the header compression protocol. The duplication detection procedure can be performed on the overall data after receiving the decryption procedure, integrity protection procedure, or header (or data) decompression procedure for the data received from each RLC layer device, respectively, on the data received from each RLC layer device.

[0179] The lower function of the receiving PDCP layer device immediately applies the decryption procedure to the data received using a separately set header (or data) compression context or security key for each source base station and each target base station, and applies the integrity verification procedure to the PDCP header and data (PDCP SDU) if integrity protection is set. In the 2-2 PDCP layer device structure, after performing an order re-alignment procedure on the entirety of the data received from the RLC layer device of the first bearer for each source base station and the data received from the RLC layer device of the second bearer for each target base station, for each piece of data received from each base station (source base station or target base station), apply the header (or, data) compression context of each base station (source base station or target base station) in ascending order of the PDCP sequence number or COUNT value, and perform a header (or, data) decompression procedure.

[0180] Also, the 2-2 PDCP layer device defines an indicator for each piece of data in order to distinguish between the data received from the RLC layer device of the first bearer for each source base station and the data received from the RLC layer device of the second bearer for each target base station, and distinguishes whether it is data received from the source base station or data received from the target base station.

[0181] As another method, the 2-2 PDCP layer device can also define a 1-bit indicator in the PDCP header, SDAP header, or RLC header to distinguish whether it is data received from the source base station or data received from the target base station. Furthermore, the 2-2 PDCP layer device can perform a duplicate detection procedure (a procedure in which for each PDCP sequence number or COUNT value, only one piece of data (which can be applied including previously received data or data transmitted to the upper layer) is left and all others are discarded) on the entirety of the data received from the RLC layer device of the first bearer for the source base station after completing the header (or, data) compression procedure and the data received from the RLC layer device of the second bearer for the target base station, based on the PDCP sequence number or COUNT value. Then, the 2-2 PDCP layer device transmits the data to the upper layer device in ascending order based on the PDCP sequence number or COUNT value for the entirety of the data received from the RLC layer device of the first bearer for the source base station and the data received from the RLC layer device of the second bearer for the target base station. One PDCP layer device always performs a reordering procedure so that it can receive data from different base stations, i.e., from the first bearer or the second bearer, regardless of the order.

[0182] Two lower receiving PDCP layer device functions perform parallel data processing (parallel processing) to accelerate the data processing speed by performing procedures of header compression, integrity protection, or encryption in parallel based on the PDCP sequence number or COUNT value respectively, and utilize different header (or data) compression contexts or security keys to perform integrity protection, encryption procedures, or decompression procedures. Also, logically, within one transmitting PDCP layer device, different header (or data) compression contexts, security keys, or security algorithms are applied to perform integrity protection, encryption procedures, or decompression procedures on different data. Also, in the lower receiving PDCP layer device function, an out-of-sequence deciphering or integrity verification procedure is performed on each received data regardless of the order of the PDCP sequence number or COUNT value.

[0183] When distinguishing the layer device of the first bearer from the layer device of the second bearer, one PDCP layer device takes into account the fact that it is connected to different MAC layer devices, or has different logical channel identifiers, or considers the fact that it is connected to different RLC layer devices which are connected to different MAC layer devices, or uses different encryption keys, and distinguishes the layer device of the first bearer (or the first RLC layer device) from the layer device of the second bearer (or the second RLC layer device), and performs encryption procedures or decryption procedures on the uplink data and the downlink data with different security keys, and utilizes different compression protocol contexts to compress or decompress.

[0184] In the present invention, a structure of a third PDCP layer device for executing a handover procedure is provided, such as (reference sign 1i-30). The structure of the third PDCP layer device applies a second embodiment of a handover method for minimizing data interruption time. Also, in the structure of the third PDCP layer device, the functions of the PDCP layer device are the same as those of the second PDCP layer device. However, the structure of the third PDCP layer device is characterized in that, in the structure of the second PDCP layer device, the first bearer for the source base station is released.

[0185] Specifically, the structure of the third PDCP layer device has the same functions as the structure of the second PDCP layer device, but has a structure in which the first bearer (for example, an SDAP layer device or a PDCP layer device, an RLC layer device, or a MAC layer device) for the source base station is released. Therefore, the structure of the third PDCP layer device is characterized in that the QoS mapping information of the SDAP layer device for the source base station, the security key information for the source base station of the PDCP layer device, the header (or data) compression context information for the source base station, the RLC layer device for the source base station, or the MAC layer device is released.

[0186] FIG. 1J is a diagram for explaining a second embodiment of an efficient SDAP layer device structure applied in a DAPS handover method and a handover method applying the structure according to an embodiment of the present invention. Referring to FIG. 1J, a specific structure and functions of an efficient SDAP layer device applied in a DAPS handover method, which is a second embodiment of a handover method according to an embodiment of the present invention, are provided. When executing a DAPS handover procedure, the structure of the SDAP layer device applies different SDAP layer structures for each bearer at different times.

[0187] For example, before the terminal receives a handover command message from the base station, for each bearer, it applies the structure and functions (1j-10) of the first SDAP layer device proposed in the present invention to process and transmit or receive data (reference numeral 1j-01). In the structure and functions of the first SDAP layer device, the SDAP layer device maintains and applies one "first QoS flow" for the source base station and the mapping information of the bearer to the uplink data to be transmitted or the downlink data to be received (for example, the data received from the source base station), and processes the data (for example, reads the SDAP header information, updates the mapping information, constructs the SDAP header, or performs procedures for routing or transmitting to a suitable upper layer device or lower layer device based on the "first QoS flow" and the mapping information of the bearer).

[0188] However, if the terminal receives a handover command message from the base station and the handover command message indicates the DAPS handover method proposed in the present invention, or if the DAPS handover method is indicated for a specific bearer, the terminal applies the structure and functions (1j-20) of the second SDAP layer device proposed in the present invention to process and transmit or receive data for each bearer or for the bearer for which the DAPS handover method is indicated (reference numeral 1j-02). That is, when the terminal receives a handover command message and the handover command message indicates the DAPS handover method proposed in the present invention, or when the DAPS handover method is indicated for a specific bearer, it converts from the structure or functions (1j-10) of the first SDAP layer device used for each bearer to the structure or functions (1j-20) of the second SDAP layer device proposed in the present invention for each bearer or for the bearer for which the DAPS handover method is indicated.

[0189] As another method, when the terminal satisfies the first condition proposed in the present invention, it converts from the structure or function (1j-10) of the first SDAP layer device used for each bearer to the structure or function (1j-20) of the second SDAP layer device proposed in the present invention for each bearer or for the bearer for which the DAPS handover method is instructed (reference numeral 1j-02). And, as described above, when the terminal receives a handover command message and the DAPS handover method proposed in the present invention is instructed in the handover command message, or when the DAPS handover method is instructed for a specific bearer, or when the "QoS flow" and bearer mapping information are newly set, the terminal converts from the structure or function (1j-10) of the first SDAP layer device to the structure or function (1j-20) of the second SDAP layer device proposed in the present invention for each bearer or for the bearer for which the DAPS handover method is instructed.

[0190] And, in the structure of the second SDAP layer device, it is characterized in that the "first QoS flow" and bearer mapping information for the source base station are maintained, and the uplink data transmitted to the source base station and the downlink data received from the source base station are processed, and the "second QoS flow" and bearer mapping information newly set in the handover command message are set for the target base station and are used to process the uplink data transmitted to the target base station and the downlink data received from the target base station. That is, in the structure of the second SDAP layer device proposed in the present invention, the "first QoS flow" and bearer mapping information, or the "second QoS flow" and bearer mapping information for the source base station are maintained, and the data for the source base station and the data for the target base station are separately processed.

[0191] In the structure of the second SDAP layer device, the SDAP layer device distinguishes whether the data received from the lower layer is data received from the source base station or data received from the target base station through a 1-bit indicator in the SDAP header, a 1-bit indicator in the PDCP header, or information indicated by the PDCP layer device. And in the above-mentioned situation, if the base station indicates the DAPS handover method for each bearer to the terminal in the handover command message, for the default bearer (default DRB), always indicate the DAPS handover method. During the DAPS handover procedure, if new "QoS flow" data that does not correspond to the "QoS flow" and bearer mapping information occurs, the default bearer shall always transmit the uplink data. If the DAPS handover method is not set for the default bearer, since uplink data transmission for the new "QoS flow" generated during handover is impossible, a data interruption time will occur.

[0192] And if the terminal is executing the DAPS handover method and meets the above-mentioned second condition, for each bearer or for the bearer for which the DAPS handover method is indicated, the structure and function (1j-20) of the second SDAP layer device that was applied shall release the first bearer for the source base station and further convert and apply it to the structure and function (1j-10) of the first SDAP layer device. When the terminal satisfies the second condition, it converts from the structure or function of the second SDAP layer device (1j-20) to the structure or function of the first SDAP layer device (1j-10) proposed in the present invention, for each bearer or for the bearer for which the DAPS handover method is indicated. For the second bearer or "second QoS flow" for the target base station, the mapping information of the bearer is maintained, and before releasing the mapping information of the bearer with the first bearer or "first QoS flow" for the source base station, the data received from the source base station (for example, all data received from the source base station) is applied with the "first QoS flow" and the mapping information of the bearer. After completing the data processing, the "first QoS flow" and the mapping information of the bearer or the first bearer are released.

[0193] Then, the processed data is transmitted to the upper layer in ascending order. That is, when the terminal satisfies the second condition, the "first QoS flow" for the source base station and the mapping information of the bearer are applied to the data stored in the buffer (for example, the data received from the source base station), and the data is processed (for example, reading the SDAP header information, updating the mapping information, or constructing the SDAP header, or routing or transmitting to the appropriate upper layer device or lower layer device based on the "first QoS flow" and the mapping information of the bearer), and the "first QoS flow" for the source base station and the mapping information of the bearer are discarded.

[0194] The SDAP layer device defines and applies a 1-bit indicator of the new SDAP header, a 1-bit indicator of the PDCP header, SDAP control data (for example, downlink "End marker"), or information indicated by the PDCP layer device, and based on this, it is confirmed what kind of data is the last data received from the source base station. Therefore, after applying the "first QoS flow" for the source base station and the mapping information of the bearer to the last data received from the source base station and performing data processing, the "first QoS flow" for the source base station and the mapping information of the bearer are discarded. Then, the SDAP layer device continues to maintain the "second QoS flow" and the mapping information of the bearer, and based on this, processes the uplink data or downlink data to the target base station.

[0195] In the aforementioned FIG. 1F, when the terminal receives a handover command message and applies the bearer configuration information included in the handover command message, the bearer configuration information is applied in different ways according to the handover type indicated in the handover command message. · If the terminal, when receiving a handover command message, indicates a first handover method (for example, the first embodiment of the present invention or a general handover method) in the "ReconfigWithSync" information, · When the default bearer is set in the SDAP layer device configuration information set in the handover command message, set the default bearer for the source base station as the default bearer for the target base station indicated by the configuration information. · When the "second QoS flow" and the bearer mapping information are set in the SDAP layer device configuration information set in the handover command message, release the "first QoS flow" and the bearer mapping information applied for the source base station, and apply the "second QoS flow" and the bearer mapping information. As another method, the "first QoS flow" and the bearer mapping information applied for the source base station can be replaced with the "second QoS flow" and the bearer mapping information.

[0196] · In the PDCP layer device configuration information set in the handover command message, when a data discard timer value is set, immediately apply the discard timer value to the PDCP layer device corresponding to the bearer identifier in the configuration information. · In the PDCP layer device configuration information set in the handover command message, when the "drb-ContinueROHC" indicator is set to "False", initialize the context of the header compression protocol or the header decompression protocol in the PDCP layer device corresponding to the bearer identifier in the configuration information. If the "drb-ContinueROHC" indicator is set to "True", do not initialize the context of the header compression protocol or the header decompression protocol in the PDCP layer device corresponding to the bearer identifier in the configuration information.

[0197] · In the PDCP layer device configuration information set in the handover command message, when a reordering timer value is set, immediately apply the reordering timer value to the PDCP layer device corresponding to the bearer identifier in the configuration information. · In the security configuration information set in the handover command message, when security key-related configuration information or a security algorithm is set, use the configuration information to derive a new security key or security configuration information, and release the existing security key or security configuration information, or replace the existing security key or security configuration information with the new security key or security configuration information.

[0198] · In the RLC layer device configuration information set in the handover command message, when a new logical channel identifier is set, use the new logical channel identifier to release the existing logical channel identifier corresponding to the bearer identifier indicated in the RLC layer device configuration information, or replace the existing logical channel identifier with the new logical channel identifier. · When RLC re-establishment procedure is set in the RLC layer device configuration information set in the handover command message, execute the RLC re-establishment procedure on the RLC layer device corresponding to the bearer identifier indicated in the RLC layer device configuration information. · When the RLC layer device configuration information set in the handover command message is newly set, execute the RLC re-establishment procedure on the RLC layer device corresponding to the bearer identifier indicated in the RLC layer device configuration information.

[0199] · In the MAC layer device configuration information set in the handover command message, when the second priority related to the logical channel is newly set, cancel the first priority corresponding to the logical channel identifier indicated in the configuration information, or replace and set the first priority corresponding to the logical channel identifier with the second priority newly set as described above. · In the MAC layer device configuration information set in the handover command message, when the second prioritised bit rate (PBR) related to the logical channel is newly set, cancel the first prioritised bit rate (PBR) corresponding to the logical channel identifier indicated in the configuration information, or replace and set the first prioritised bit rate (PBR) corresponding to the logical channel identifier with the second prioritised bit rate (PBR) newly set as described above. The prioritised bit rate as described above is a value that increases for each logical channel at regular intervals (e.g., for each TTI). When receiving uplink transmission resources, execute the LCP (logical channel prioritization) procedure, consider the priority and the prioritised bit rate, and be able to transmit data related to the logical channel. The higher the priority or the larger the value of the prioritised bit rate, the more data can be transmitted.

[0200] · In the MAC layer device configuration information set in the handover command message, when a second bucket size duration related to the logical channel is newly set, the first bucket size duration corresponding to the logical channel identifier indicated in the configuration information is released, or the first bucket size duration corresponding to the logical channel identifier is set by replacing it with the second bucket size duration newly set as described above. In the above, the bucket size indicates the maximum value that the priority bit rate can have when the priority bit rate is accumulated. · In the MAC layer device configuration information set in the handover command message, if the second allowed SCell information, allowed subcarrier spacing information, maximum PUSCH duration, or logical channel group configuration information is set, the previously set first allowed SCell information, allowed subcarrier spacing information, maximum PUSCH duration, or logical channel group configuration information is released, or the previously set first allowed SCell information, allowed subcarrier spacing information, maximum PUSCH duration, or logical channel group configuration information is set by replacing it with the second allowed SCell information, allowed subcarrier spacing information, maximum PUSCH duration, or logical channel group configuration information newly set as described above.

[0201] · If the terminal receives a handover command message and in the "ReconfigWithSync" information, a second handover method (for example, the second embodiment of the present invention or the DAPS handover method) is indicated, or the DAPS handover method is indicated for each bearer identifier, · In the SDAP layer device configuration information set in the handover command message, when the default bearer is set, execute the DAPS handover method proposed above in the present invention, apply the second SDAP layer device structure, maintain the default bearer for the existing source base station, and set the default bearer information indicated in the configuration information as the default bearer for the target base station. As another method, when the first condition proposed in the present invention is satisfied, the default bearer for the existing source base station can be switched to the default bearer for the target base station indicated in the configuration information.

[0202] · In the SDAP layer device configuration information set in the handover command message, when the "second QoS flow" and bearer mapping information are set, execute the DAPS handover method proposed above in the present invention, apply the second SDAP layer device structure, maintain the "first QoS flow" and bearer mapping information applied for the source base station, and apply the "second QoS flow" and bearer mapping information to the data for the target base station. · In the PDCP layer device configuration information set in the handover command message, when the data discard timer value is set, execute the DAPS handover method proposed above in the present invention, apply the second PDCP layer device structure, and immediately apply the discard timer value to the PDCP layer device corresponding to the bearer identifier in the configuration information.

[0203] · In the PDCP layer device configuration information set in the handover command message, when the "drb-ContinueROHC" indicator is set to "False", execute the DAPS handover method proposed above in the present invention, apply the second PDCP layer device structure, and in the PDCP layer device corresponding to the bearer identifier of the configuration information, the context of the header compression protocol or the header decompression protocol for the source base station is used as it is, and the context of the header compression protocol or the header decompression protocol for the target base station is initialized and starts in the initial state (for example, the IR state). If the "drb-ContinueROHC" indicator is set to "True", execute the DAPS handover method proposed above in the present invention, apply the second PDCP layer device structure, and in the PDCP layer device corresponding to the bearer identifier of the configuration information, the context of the header compression protocol or the header decompression protocol for the source base station is used as it is, and the context of the header compression protocol or the header decompression protocol for the target base station is applied to be the same as the context of the header compression protocol or the header decompression protocol for the source base station. For example, copy the context of the header compression protocol or the header decompression protocol for the source base station to the context of the header compression protocol or the header decompression protocol for the target base station and apply it as it is. As another method, the same header compression protocol or header decompression protocol context can also be applied to the target base station or the source base station.

[0204] · In the PDCP layer device configuration information set in the handover command message, when the reordering timer value is set, execute the DAPS handover method proposed above in the present invention, apply the second PDCP layer device structure, and immediately apply the reordering timer value to the PDCP layer device corresponding to the bearer identifier of the configuration information. · In the security setting information set in the handover command message, when security key related setting information or a security algorithm is set, or in the PDCP layer device setting information, when there is an indicator instructing a new procedure, use the setting information to derive a new security key or security setting information, execute the DAPS handover method proposed above in the present invention, apply the second PDCP layer device structure, maintain the existing security key or security setting information for the source base station, and set the security key or security setting information for the target base station with the aforementioned new security key or security setting information.

[0205] · In the RLC layer device setting information set in the handover command message, when a new logical channel identifier is set, execute the DAPS handover method proposed above in the present invention, apply the second PDCP layer device structure, for the RLC layer device or MAC layer device of the first bearer for the source base station corresponding to the bearer identifier indicated by the RLC layer device setting information, maintain the existing logical channel identifier, and for the RLC layer device or MAC layer device of the second bearer for the target base station, set it with the new logical channel identifier indicated by the setting information. · In the RLC layer device setting information set in the handover command message, when an RLC re - establishment procedure is set, execute the DAPS handover method proposed above in the present invention, apply the second PDCP layer device structure, and for the RLC layer device of the first bearer for the source base station corresponding to the bearer identifier indicated by the RLC layer device setting information, execute the RLC re - establishment procedure. · When the RLC layer device configuration information set in the handover command message is newly set, execute the DAPS handover method proposed above in the present invention, apply the second PDCP layer device structure, and for the RLC layer device of the first bearer for the source base station corresponding to the bearer identifier indicated by the RLC layer device configuration information, maintain the existing RLC configuration information, and for the RLC layer device of the second bearer for the target base station, set it with the new RLC layer device configuration information indicated by the configuration information.

[0206] · In the MAC layer device configuration information set in the handover command message, when the second priority related to the logical channel is newly set, execute the DAPS handover method proposed above in the present invention, apply the second PDCP layer device structure, and for the MAC layer device of the first bearer for the source base station corresponding to the bearer identifier indicated above, maintain the existing configuration information, and for the MAC layer device of the second bearer for the target base station, set the new logical channel identifier indicated by the configuration information, and set the newly set second priority corresponding to the logical channel identifier indicated by the configuration information. As another method, when the first condition proposed in the present invention is satisfied, the priority can also be applied to the MAC layer device of the second bearer for the target base station according to the logical channel identifier. · In the MAC layer device configuration information set in the handover command message, when the second priority bit rate (PBR) related to the logical channel is newly set, execute the DAPS handover method proposed above in the present invention, apply the second PDCP layer device structure, and for the MAC layer device of the first bearer for the source base station corresponding to the bearer identifier indicated above, maintain the existing configuration information, and for the MAC layer device of the second bearer for the target base station, set the new logical channel identifier indicated by the configuration information, and set the newly set second priority bit rate corresponding to the logical channel identifier indicated by the configuration information. As another method, the second priority bit rate can start being applied to the logical channel identifier in the MAC layer device of the second bearer for the target base station when the first condition proposed in the present invention is satisfied (by doing so, when different handover methods are indicated for each bearer, the uplink transmission resources are fairly allocated). The priority bit rate as described above is a value that increases for each logical channel every certain period of time (for example, every TTI) when it starts being applied to each logical channel identifier. When receiving uplink transmission resources, an LCP (logical channel prioritization) procedure is executed, considering the priority and the priority bit rate, and data related to the logical channel is transmitted. The higher the priority or the larger the value of the priority bit rate, the more data can be transmitted.

[0207] · Also, in the above situation, when applying the DAPS handover method, if the first condition proposed in the present invention is not yet satisfied and the terminal has to transmit uplink data via the first bearer for the source base station, the MAC layer device of the first bearer, when executing the LCP procedure, selects only the bearer or logical channel identifier for which the DAPS handover method (or a handover method that can continue to transmit data to the source base station even after receiving a handover command message) is indicated as the target of the LCP procedure and executes the LCP procedure. This is because for a bearer or logical channel identifier for which the DAPS handover method is not applied, if a handover command message is received, uplink data cannot be transmitted to the source base station, so it should not be selected as the target of the LCP procedure.

[0208] · In the MAC layer device configuration information set in the handover command message, when the second bucket size (bucket size duration) related to the logical channel is newly set, execute the DAPS handover method proposed above in the present invention, apply the second PDCP layer device structure, and for the MAC layer device of the first bearer for the source base station corresponding to the bearer identifier indicated above, maintain the existing configuration information, and for the MAC layer device of the second bearer for the target base station, set the new logical channel identifier indicated in the configuration information, and set the newly set second bucket size corresponding to the logical channel identifier indicated in the configuration information. As another method, the second bucket size can start to be applied to the logical channel identifier in the MAC layer device of the second bearer for the target base station since the first condition proposed in the present invention is satisfied (by doing so, when different handover methods are indicated for each bearer, the uplink transmission resources are fairly allocated). As described above, the bucket size indicates the maximum value that the priority bit rate can have when the priority bit rate is accumulated.

[0209] · In the MAC layer device configuration information set in the handover command message, if the second allowed SCell information, allowed subcarrier spacing information, maximum PUSCH period, or logical channel group configuration information is set, execute the DAPS handover method proposed above in the present invention, apply the second PDCP layer device structure, and for the MAC layer device of the first bearer for the source base station corresponding to the bearer identifier indicated above, maintain the existing configuration information, and for the MAC layer device of the second bearer for the target base station, set the second allowed SCell information, allowed subcarrier spacing information, maximum PUSCH period, or logical channel group configuration information indicated in the configuration information.

[0210] When a terminal executes the second embodiment of an efficient handover method (DAPS handover method) according to an embodiment of the present invention, if the terminal fails in handover, a method is provided to quickly fallback to a source base station and re-establish a connection with the source base station by utilizing the features of the DAPS handover method proposed above. The DAPS handover method according to an embodiment of the present invention specifically means that, even when executing a handover procedure, it maintains a connection with the source base station and performs data transmission or data reception, and in case of a handover failure, it can fallback by using the radio connection already established with the existing source base station.

[0211] As described in FIG. 1H above, in the second embodiment of an efficient handover method (DAPS handover method), even when receiving a handover command message from a source base station, as proposed in (reference numeral 1h-02), it executes a handover procedure to a target base station while maintaining data transmission or data reception with the source base station. Also, in the present invention, when a handover procedure to a target base station fails, it falls back to the source base station. If a terminal fails in a handover procedure to a target base station, there must be a method to check whether the radio connection between the terminal and the source base station is valid in order to fallback to the source base station. This is because if the radio connection between the terminal and the source base station is not valid, and the terminal fails in handover and executes a fallback to the source base station, the fallback procedure to the source base station will also fail, ultimately resulting in a long data interruption time and a significant occurrence of data cut-off phenomenon. Also, when the radio connection between the terminal and the source base station is valid, the SRB set between the terminal and the source base station must be maintained.

[0212] First, in the present invention, a new timer applicable to the handover method and the specific operations of each timer are proposed. Also, the specific operations of the timer execute different operations depending on the type of handover method indicated as a handover command message from the base station. Also, a method for releasing or maintaining the connection with the source base station or the setting of the SRB is proposed according to the handover method.

[0213] In the present invention, in order to efficiently execute the handover procedure, a first timer (e.g., T304), a second timer (e.g., T310), a third timer (e.g., T312), or a fourth timer (e.g., a timer for fallback) is introduced, and it is proposed to drive and apply at least one of the first to fourth timers in the handover procedure. It is proposed that the first timer (e.g., T304), the second timer (e.g., T310), the third timer (e.g., T312), or the fourth timer (e.g., a timer for fallback) proposed in the present invention execute different operations as follows depending on the type of handover method indicated in the handover command message.

[0214] The first timer (e.g., T304) is a timer for determining whether the handover has been successfully executed, the second timer (e.g., T310) is a timer for determining whether the radio connection is valid, the third timer (e.g., T312) is an auxiliary timer for determining whether the radio connection is valid, and is a timer for triggering the frequency measurement procedure and reporting the frequency measurement result. And the fourth timer (e.g., a timer for fallback) is executing a second embodiment of the handover method (DAPS handover method) according to an embodiment of the present invention. When the handover fails, it executes a fallback procedure to the source base station, and after transmitting a message indicating that the handover failure has been executed to the source base station, it is a timer for determining whether the fallback procedure has been successfully executed or has failed.

[0215] In the present invention, the specific operations related to the first timer (e.g., T304), the second timer (e.g., T310), the third timer (e.g., T312), or the fourth timer (e.g., a timer for fallback) proposed to assist an efficient handover method are proposed as follows according to the indicated handover method.

[0216] 1) If the terminal receives an out-of-sync indication that the wireless connection signal is not synchronized from a lower layer device (e.g., a MAC layer device or a PHY layer device) a predetermined number of times (e.g., that can be set by the base station), and detects that there is a problem with the physical layer device, if the first timer is not running, it starts the second timer (e.g., T310). And when the terminal receives an in-sync indication that the wireless connection signal is well synchronized from the lower layer device a predetermined number of times (e.g., that can be set by the base station), when the handover procedure is triggered (starts), or when the RRC connection re-establishment procedure starts, it stops the second timer. If the second timer expires, the terminal triggers or starts the RRC connection re-establishment procedure. Or, it transitions to the RRC idle mode and triggers or starts the RRC connection re-establishment procedure.

[0217] 1) When the second timer is running, the terminal starts the third timer when a frequency measurement procedure is triggered for the frequency measurement identifier set by the third timer. Then, when the terminal receives from the lower layer device an in-sync indication that the synchronization of the radio connection signal is good a predetermined number of times (for example, which can be set by the base station), when a handover procedure is triggered (starts), or when an RRC connection re-establishment procedure starts, the terminal stops the third timer. If the third timer expires, the terminal triggers or starts an RRC connection re-establishment procedure. Or, it transitions to the RRC idle mode and triggers or starts an RRC connection re-establishment procedure.

[0218] 1) If in the handover command message received by the terminal from the base station (a message included in the "RRCReocnfiguartion" message with a mobility indication ("MobilityControl info" or "ReconfigurationWithSync") or a handover indication), the first handover method (for example, the first embodiment, or a general handover method) is indicated, 2) In the present invention, when the terminal receives the handover command message described above (a message included in the "RRCReocnfiguartion" message with a mobility indication ("MobilityControl info" or "ReconfigurationWithSync") or a handover indication), it triggers a handover procedure and starts the first timer. 2) When triggering the handover procedure described above, the terminal releases the SRB (for example, SRB1) set for the source base station and sets the SRB (for example, SRB1) for the target base station based on the setting information set in the handover command message. 2) When triggering the handover procedure as described above, if the second timer is running, the terminal shall stop it. When the first timer is running, even if the above condition for starting the second timer (when the asynchronous indicator of the radio connection signal is received from the lower layer a predetermined number of times) is satisfied, the second timer shall not be started. That is, when the first timer is running, the second timer shall not be used. 2) When triggering the handover procedure as described above, if the third timer is running, the terminal shall stop it. And only when the second timer is running, when the above condition for starting the third timer (when a frequency measurement procedure is triggered for the frequency measurement identifier set for the third timer) is satisfied, the third timer shall be started. That is, when the first timer is running, since the second timer is not used, the third timer shall not be used either. 2) If the terminal as described above successfully completes the handover procedure to the target base station or successfully completes the random access procedure, the first timer shall be stopped. 2) If the first timer as described above expires (for example, if the handover procedure to the target base station fails), the terminal shall execute the RRC connection re-establishment procedure (release the connection with the base station, execute the RRC connection procedure from the beginning again, that is, execute the cell selection or cell reselection procedure, execute the random access procedure, and transmit the RRC connection re-establishment request message).

[0219] 1) If, in the handover command message received by the terminal from the base station (a message including a mobility instruction ("MobilityControl info" or "ReconfigurationWithSync") or a handover instruction in the "RRCReconfiguartion" message), a second handover method (for example, the second embodiment or the DAPS handover method) is instructed (or can also be extended and applied when both conditional handover methods are instructed), 2) When the terminal in the present invention receives the handover command message described above (a message including a mobility instruction ("MobilityControl info" or "ReconfigurationWithSync") or a handover instruction in the "RRCReocnfiguartion" message), it triggers a handover procedure and starts a first timer. If both conditional handover methods are instructed, the terminal selects one cell from among a plurality of target cells and starts the first timer when starting the handover procedure or when executing a random access procedure.

[0220] 2) When triggering the handover procedure, if the terminal starts the DAPS handover method, it maintains or terminates the SRB (e.g., SRB1) configured for the source base station, and configures the SRB (e.g., SRB1) for the target base station based on the configuration information set in the handover command message. In other methods, when triggering the handover procedure, if the terminal starts the DAPS handover method, it maintains or terminates the SRB (e.g., SRB1) configured for the source base station, re-establishes the PDCP layer device or re-establishes the RLC layer device for the SRB of the source base station, initializes the window state variable to stop the timer, and instructs to discard the stored data (PDCP SDU or PDCP PDU) (which can be executed when the fallback procedure proposed in the present invention is triggered), and configures the SRB (e.g., SRB1) for the target base station based on the configuration information set in the handover command message. As yet another method, the second PDCP layer device structure proposed in the present invention can be applied to the SRB, the first bearer for the source base station can be configured, and the second bearer for the target base station can be configured. As yet another method, when applying the second PDCP layer device structure to the SRB, it is also possible to re-establish the PDCP layer device for the first bearer or re-establish the RLC layer device, initialize the window state variable to stop the timer, and instruct to discard the stored data (PDCP SDU or PDCP PDU) (which can be executed when the fallback procedure proposed in the present invention is triggered).

[0221] 2) When the terminal triggers a handover procedure, if it triggers the DAPS handover method, it is characterized in that it does not stop even if the second timer for the source base station is running. And when the first timer is running (or not running), if the above condition for starting the second timer (when the asynchronous indicator of the radio connection signal is received from the lower layer a predetermined number of times) is satisfied, the second timer is started. The second timer is used for the radio connection between the terminal and the source base station. As another method, two second timers can be used. One second timer is used for the radio connection between the terminal and the source base station, and the other second timer can be used for the radio connection between the terminal and the target base station. That is, when the first timer is running, the second timer is used for the radio connection with the source base station or the target base station. However, even if the second timer expires and the first timer has not expired and is running, the terminal is characterized in that it does not trigger the RRC connection re-establishment procedure. Specifically, even if the second timer for the source base station expires or a radio link failure (RLF) occurs, if the first timer has not expired and is running, or a random access procedure is being executed at the target base station, or a handover procedure to the target base station is being executed, the terminal does not trigger the RRC connection re-establishment procedure, can release the radio connection with the source base station, does not release the RRC configuration information (such as bearer configuration information, etc.) set by the source base station, and can reuse the RRC configuration information if the RRC connection re-establishment procedure is triggered subsequently. Also, even if the second timer expires and the first timer has not expired and is running, the terminal does not trigger the RRC connection re-establishment procedure, and the source base station can also report that the source connection has failed at the target base station, and can also release the connection with the source base station (for example, can release the first bearer for the source base station), or can also suspend the first bearer for the source base station.However, when the second timer expires, if the first timer has expired, or has been stopped, or has not been started and is not in operation, it is characterized by triggering an RRC connection re-establishment procedure. Also, when performing the handover procedure as described above, the reason for operating the second timer is to monitor the radio connection between the terminal and the source base station, and when a handover failure occurs, if the radio connection with the source base station is valid, to perform a fallback procedure. Further, when the second timer for the target base station expires, when the radio connection with the target base station fails, if the first timer expires, or is stopped, or has not been started and is not in operation, or if the random access procedure to the target base station is successfully executed, an RRC connection re-establishment procedure may be triggered.

[0222] 2) When the terminal triggers the handover procedure, if it triggers the DAPS handover method, it is characterized in that it does not stop even if the third timer for the source base station is running. And only when the second timer is running, when satisfying the above condition (when the frequency measurement procedure is triggered for the frequency measurement identifier set for the third timer) to start the third timer, it is characterized in that the third timer is started. That is, even when the first timer is running, in order to use the second timer, it is characterized in that the third timer is also used. The third timer is operated for the radio connection between the terminal and the source base station. As another method, two third timers can be operated, one third timer is operated for the radio connection between the terminal and the source base station, and the other third timer can be operated for the radio connection between the terminal and the target base station. That is, even when the first timer is running, it is characterized in that the third timer is used for the radio connection with the source base station or the target base station. However, even if the third timer expires, if the first timer has not expired and is running, the terminal is characterized in that it does not trigger the RRC connection re-establishment procedure. Also, even if the third timer expires, if the first timer has not expired and is running, the terminal does not trigger the RRC connection re-establishment procedure, and the source base station can report that the source connection has failed at the target base station, or can also release the connection with the source base station (for example, can release the first bearer for the source base station), or can also stop the first bearer for the source base station. However, when the third timer expires, if the first timer has expired, or has been stopped, or has not been started and is not running, it is characterized in that the RRC connection re-establishment procedure is triggered. When executing the handover procedure as described above, the reason for operating the third timer is to monitor the radio connection between the terminal and the source base station, and when a handover failure occurs, if the radio connection with the source base station is valid, to execute the fallback procedure, and to report the frequency measurement result in the fallback procedure.

[0223] 2) If the terminal as described above successfully completes the handover procedure to the target base station, stop the first timer.

[0224] 2) If the first timer as described above expires (for example, if the handover procedure to the target base station fails), or at the target base station, the maximum retransmission count is exceeded in the RLC layer device, or the handover command message as described above is received, but the setting information of the handover command message exceeds the capabilities of the terminal, or an error occurs in the application of the above setting information, resulting in a handover failure, or a random access problem occurs at the target base station, and the random access procedure is continuously attempted, but the first timer expires and the handover procedure fails, or in the above situation, when driving the second timer or the third timer for the target base station, if the second timer or the third timer expires before the handover procedure is completed, stop or expire the T304 timer, and if it is determined that the handover procedure has failed, 3) If the second timer or the third timer for the radio connection between the terminal and the source base station as described above has not expired (or in the above situation, the second timer or the third timer for the radio connection between the terminal and the source base station has not been started or is being driven), or if the radio connection between the terminal and the source base station is effective, 4) The terminal determines that the radio connection between the terminal and the source base station is effective and executes the fallback procedure proposed in the present invention. 4) When the terminal described above starts the fallback procedure, if the SRB (for example, SRB1, or the MAC layer device, RLC layer device, or PDCP layer device of SRB1) configured for the source base station is suspended, it resumes or is newly configured, and the fallback procedure is executed for the SRB (for example, SRB1). As another method, if the second PDCP layer device structure proposed in the present invention is applied to the SRB, the fallback procedure can be executed via the first bearer for the source base station, and the second bearer for the target base station can be released. For example, the uplink data transmission is switched to the first bearer for the source base station, and it is indicated that there is data to be transmitted to the RLC layer device or MAC layer device of the first bearer, and the handover failure report message for the fallback procedure can be transmitted via the first bearer.

[0225] 4) The fallback procedure described above is such that when the handover fails, the terminal constructs a report message indicating that the handover has failed via the source base station and the configured SRB (e.g., SRB1), and reports the handover failure to the source base station. In the above situation, when the terminal transmits the report message indicating that the handover has failed to the source base station, the terminal also reports the frequency measurement results it has measured, which helps to quickly recover the connection with the source base station. As another method, the terminal can define and transmit MAC control information (e.g., new MAC control information or indicating that there is data to be transmitted in the buffer status report, or defining a special value to indicate that the handover has failed), RLC control information, or PDCP control information to indicate to the source base station that the handover has failed. As yet another method, the terminal can also transmit an RRC connection re-establishment request message to the SRB (e.g., SRB0 or SRB1) for the source base station as described above. As yet another method, the fallback procedure described above is also a procedure in which when the handover fails, for each bearer or in the second PDCP layer device structure of the bearer with the DAPS handover method configured, the second bearer for the target base station is released or converted to the first PDCP layer device structure, and the terminal resumes data transmission or reception via the first bearer for the source base station, and indicates that there is data to be transmitted to the MAC layer device of the first bearer. The terminal can report to the source base station that there is a scheduling request or data to be transmitted (e.g., buffer status report), or transmit new "MAC CE", or RLC control data or PDCP control data, indicating to the source base station that it has fallen back to the source base station and is about to start further data transmission. Then, a new SRB for the source base station can be set or resumed. Also, the fallback procedure is executed for each bearer when the handover fails.In addition, since a bearer for which the DAPS handover method is not set does not have a second PDCP layer device structure, the PDCP layer device, RLC layer device, or bearer configuration information or logical channel identifier information that has been previously set and reconfigured with the configuration information of the handover command message is released from the MAC layer device for the target base station, or after switching and connecting to the MAC layer device for the source base station and performing the setting, data transmission or reception for each bearer to the source base station can be resumed. This is because when the terminal receives a handover command message, for a bearer for which the DAPS handover method is not indicated, the bearer configuration information set in the handover command message can be applied to the MAC layer device for the target base station, and the connection of the PDCP layer device or RLC layer device corresponding to the bearer for which the DAPS handover method is not indicated can be switched from the MAC layer device for the source base station to the MAC layer device for the target base station and connected. For example, when the terminal receives a handover command message, the upper layer device (e.g., RRC layer device) of the terminal can instruct the MAC layer device for the source base station to perform MAC reconfiguration with the configuration information excluding the configuration information related to the bearer for which the DAPS handover method is not indicated in the handover command message in the current MAC layer device configuration information, or the upper layer device (e.g., RRC layer device) of the terminal can also instruct the MAC layer device for the source base station to perform MAC reconfiguration with the configuration information including only the configuration information related to the bearer for which the DAPS handover method is indicated in the handover command message in the current MAC layer device configuration information.That is, when the terminal receives a handover command message, the setting information of the PDCP layer device, RLC layer device, or MAC layer device of the bearer for which the DAPS handover method is not indicated is released by the MAC layer device for the source base station, and the MAC layer device for the target base station can perform application or connection so as to match the bearer setting for the target base station. Therefore, if the fallback procedure is executed, the bearer for which the DAPS handover method is not set must be reset to the MAC layer device for the source base station again. For example, when the fallback procedure is executed, the upper layer device (e.g., RRC layer device) of the terminal includes, in the current setting information of the MAC layer device, the setting information related to the bearer for which the DAPS handover method is not indicated in the handover command message, and instructs the MAC layer device for the source base station to perform reconfiguration of the setting information together with the bearer setting information for which the DAPS handover method is indicated. Or, if the fallback procedure is executed, the terminal further sets or restores the bearer setting (e.g., PDCP layer device setting information, RLC layer device setting information, MAC layer device setting information, or PHY layer device setting information) before the handover command message is received, and applies it to the bearer (SRB, PDCP layer device setting information, RLC layer device setting information, MAC layer device setting information, or PHY layer device of "AM DRB" or "UM DRB") for the source base station.

[0226] 4) In the fallback procedure, if the terminal transmits a report message indicating that the handover has failed (e.g., the RRC message, "MAC CE", or RLC control data or PDCP control data proposed above) to the source base station, it starts the fourth timer. As described above, if the terminal receives an instruction or message from the source base station as a response to the report message indicating that the handover has failed, the terminal stops the fourth timer. However, if the fourth timer expires or a response message cannot be received until it expires, the terminal executes the RRC connection reestablishment procedure (disconnects from the base station, and further executes the RRC connection procedure from the beginning, i.e., executes the cell selection or cell reselection procedure, executes the random access procedure, and transmits an RRC connection reestablishment request message). And if the expiration of the fourth timer triggers the RRC connection reestablishment procedure, if the second timer or the third timer is running, it stops.

[0227] u3) If in the above situation, the second timer or the third timer for the radio connection between the terminal and the source base station or the target base station expires, or the radio connection between the terminal and the source base station or the target base station is not valid, l4) The terminal executes the RRC connection reestablishment procedure (disconnects from the base station, and further executes the RRC connection procedure from the beginning, i.e., executes the cell selection or cell reselection procedure, executes the random access procedure, and transmits an RRC connection reestablishment request message).

[0228] n2) If, in the foregoing, when the terminal executes the DAPS handover procedure and satisfies the second condition proposed in the present invention, it shall release the connection with the source base station or release the SRB for the source base station. If the second timer or the third timer for the source base station is running, it shall be stopped and initialized. In the foregoing, only by stopping the second timer or the third timer can the unnecessary RRC connection re-establishment procedure due to the expiration of the second timer or the third timer be prevented. Because when the second condition is satisfied, it may mean that the handover procedure has been successfully executed, so the first timer is stopped, and the expiration of the second timer or the third timer can also trigger the unnecessary RRC connection re-establishment procedure. As another method, when the first condition proposed in the present invention is satisfied or when the handover procedure is successfully completed, the SRB for the source base station shall be released, or if the second timer or the third timer for the source base station is running, it can be stopped and initialized. In the foregoing, only by stopping the second timer or the third timer can the unnecessary RRC connection re-establishment procedure due to the expiration of the second timer or the third timer be prevented. Because when the first condition is satisfied, it may mean that the handover procedure has been successfully executed, so the first timer is stopped, and the expiration of the second timer or the third timer can also trigger the unnecessary RRC connection re-establishment procedure.

[0229] According to the method proposed in the present invention, when the terminal determines that a handover failure has occurred, satisfies the proposed conditions, and executes the fallback procedure, the terminal shall include information that a handover failure has occurred in an RRC message (for example, "ULInformationTransferMRDC" message or "FailureInformation" message), transmit it to SRB1 or SRB1 that applies the second PDCP layer device structure, and cause the source base station to confirm the handover failure of the terminal. If the source base station detects a handover failure of the terminal, as a response thereto, it configures an RRC message (for example, an "RRCReconfiguration" message or an "RRCRelease" message) and transmits it to the terminal. When the terminal receives an "RRCReconfiguration" message (SRB1 applying the second PDCP layer device structure, or an RRC message received via SRB1) as an RRC message in response to the handover failure report, it completes the application of the related configuration information, and as a response thereto, further transmits an "RRC Reconfiguration Complete" message to the source base station via SRB1 applying the second PDCP layer device structure, or SRB1. If in "RRCReconfiguration", a handover is instructed or connection to another cell is further instructed, the terminal completes the random access procedure to the cell and transmits the "RRC Reconfiguration Complete" message via SRB1. However, if the terminal receives an "RRCRelease" message as an RRC message in response to the handover failure report, the terminal transitions to the RRC idle mode or the RRC inactive mode according to the configuration information instructed in the "RRCRelease" message, and does not transmit any further RRC messages as a response to the RRC message to the base station.

[0230] FIG. 1K is a flowchart for explaining the operation of a terminal according to an embodiment of the present invention. In FIG. 1K, the terminal (1k-01) performs data transmission or data reception with the source base station via the first PDCP layer device structure for each bearer. However, when the terminal (1k-01) receives a handover command message and the DAPS handover method of the second embodiment proposed in the present invention is indicated in the handover command message, or when the DAPS handover method is indicated for each bearer, the terminal converts to the structure of the second PDCP layer device for the target base station indicated in the handover command message for each bearer or for the bearer for which the DAPS handover method is indicated, sets up and establishes the protocol layer device of the second bearer, and when performing a random access procedure with the target base station via the established protocol layer device (steps 1k-10, 1k-15), the terminal (1k-01) continues data transmission or data reception (uplink data transmission and downlink data reception) with the source base station via the protocol layer device of the first bearer (step 1k-20).

[0231] If the terminal (1k-01) satisfies the first condition (step 1k-25), it interrupts the transmission of uplink data to the source base station via the protocol layer device of the first bearer, switches the uplink data transmission, transmits the uplink data to the target base station via the protocol layer device of the second bearer, and continues to receive downlink data from the source base station and the target base station via the protocol layer devices of the first bearer and the second bearer (step 1k-30). Also, the PDCP layer device of the second bearer uses information such as the transmission data or reception data, sequence number information, or the context of header compression and decompression stored in the PDCP layer device of the first bearer to continuously perform data transmission or data reception without interruption with the target base station. If the terminal (1k-01) does not satisfy the aforementioned first condition, it continuously checks the first condition while continuing to execute the previously executed procedure (step 1k-35).

[0232] Also, in the above-mentioned situation, if the terminal (1k-01) satisfies the second condition, it interrupts the reception of downlink data from the source base station via the protocol layer device of the first bearer (step 1k-45). In addition, the PDCP layer device of the second bearer uses information such as the transmission data or reception data stored in the PDCP layer device of the first bearer, the sequence number information, or the context of header compression and header decompression, and continues to execute uninterrupted data transmission or data reception with the target base station. If the terminal (1k-01) does not satisfy the aforementioned second condition, it continues to execute the existing procedures while continuously checking the second condition (step 1k-50).

[0233] Specific embodiments of the PDCP layer device proposed in the present invention can execute different procedures according to the type of handover indicated by the handover command message received by the terminal, as follows. · If the type of handover indicated by the handover command message received by the terminal from the source base station indicates the handover of the first embodiment (for example, the general handover procedure), The terminal executes a PDCP layer device re-establishment procedure (「PDCP re-establishment」) for the PDCP layer device for each bearer.

[0234] · If the type of handover indicated by the handover command message received by the terminal from the source base station indicates the handover of the second embodiment (or when indicated for each bearer), The terminal executes the proposed procedure for each bearer (or for the bearer indicated by the second embodiment) when the first condition of the present invention is satisfied.

[0235] In addition, in the foregoing of the present invention, when the source base station instructs the terminal to perform a handover applying the embodiment proposed in the present invention, if the source base station satisfies the following third condition, it starts data forwarding to the target base station. The third condition may mean satisfying one or more of the following conditions. · When receiving an instruction from the target base station that the terminal has successfully completed the handover. · When transmitting a handover command message to the terminal. · When transmitting a handover command message to the terminal and confirming a successful transmission (「HARQ ACK」 or NACK, or 「RLC ACK」 or NACK) for the handover command message. · When the source base station receives an instruction (e.g., an RRC message (e.g., 「RRCReconfiguration」 message), or 「MAC CE」, or 「RLC control PDU」 or 「PDCP control PDU」) to release the connection with the terminal from the terminal. · When transmitting a handover command message to the terminal, driving a predetermined timer, and the timer expires. · When confirmation (HARQ ACK or NACK, or 「RLC ACK」 or NACK) information related to a successful transmission for downlink data is not received from the terminal for a predetermined period of time.

[0236] Figure 1L is a flowchart for explaining the operation of a terminal that executes a fallback procedure when a handover fails in a DAPS handover method according to an embodiment of the present invention. In Figure 1L, the terminal (1l-05) performs data transmission or reception with the source base station via the first PDCP layer device structure for each bearer. However, when the terminal (1l-05) receives a handover command message and the handover command message indicates the DAPS handover method of the second embodiment proposed in the present invention, or when the DAPS handover method is indicated for each bearer, the terminal switches to the structure of the second PDCP layer device for each bearer or for the bearer for which the DAPS handover method is indicated with respect to the target base station indicated in the message, and sets and establishes the protocol layer device of the second bearer.

[0237] Also, when the terminal (1l-05) executes a random access procedure to the target base station via an established protocol layer device (steps 1l-10, 1l-15), it can also continue data transmission or reception (uplink data transmission and downlink data reception) with the source base station via the protocol layer device of the first bearer (step 1l-20). If the terminal (1l-05) successfully completes the handover procedure (step 1l-25), the handover procedure is completed by the second embodiment of the handover method proposed in the present invention (DAPS handover method).

[0238] However, if the terminal (1l-05) fails in the handover procedure (step 1l-25) (for example, if in the above, the first timer expires (for example, if the handover procedure to the target base station fails), or exceeds the maximum retransmission count in the RLC layer device, or in the above, a handover command message is received, but the setting information of the handover command message exceeds the capabilities of the terminal, or an error occurs in the application of the setting information and the handover fails, or a random access problem occurs at the target base station and the handover procedure fails, or in the above, when driving the second timer or the third timer for the target base station, if the second timer or the third timer expires before completing the handover procedure, stop or expire the T304 timer and determine that the handover procedure has failed), if the second timer or the third timer for the radio connection between the terminal and the source base station has not expired (or, in the above, the second timer or the third timer for the radio connection between the terminal and the source base station has not been started or is being driven) (step 1l-40), or if the radio connection between the terminal and the source base station is valid, the terminal can determine that the radio connection between the terminal and the source base station is valid and execute the fallback procedure proposed in the present invention (step 1l-45).

[0239] If the second timer or the third timer for the radio connection between the terminal and the source base station expires, or if the radio connection between the terminal and the source base station is not valid (step 1l-30), the terminal executes an RRC connection re-establishment procedure (releases the connection with the base station, starts the RRC connection procedure from the beginning again, that is, executes a cell selection or cell reselection procedure, executes a random access procedure, and transmits an RRC connection re-establishment request message) (step 1l-45).

[0240] FIG. 1M is a block diagram showing the structure of a terminal according to an embodiment of the present invention. Referring to FIG. 1M, the terminal includes an RF (radio frequency) processing unit (1m-10), a baseband processing unit (1m-20), a storage unit (1m-30), and a control unit (1m-40).

[0241] The RF processing unit (1m-10) performs functions for transmitting and receiving signals via a radio channel, such as band conversion and amplification of signals. That is, the RF processing unit (1m-10) upwardly converts the baseband signal provided from the baseband processing unit (1m-20) into an RF band signal, then transmits it via an antenna, and downwardly converts the RF band signal received via the antenna into a baseband signal. For example, the RF processing unit (1m-10) may also include a transmission filter, a reception filter, an amplifier, a mixer, an oscillator, a DAC (digital to analog convertor), an ADC (analog to digital convertor), and the like.

[0242] In the above-mentioned figure, only one antenna is shown in the figure, but the terminal can be equipped with a plurality of antennas. Also, the RF processing unit (1m-10) includes a plurality of RF chains. Furthermore, the RF processing unit (1m-10) performs beamforming. For beamforming, the RF processing unit (1m-10) adjusts the phase and magnitude of each signal transmitted and received via a plurality of antennas or antenna elements. Also, the RF processing unit executes MIMO and receives various layers during MIMO operation. The RF processing unit (1m-10) appropriately sets a plurality of antennas or antenna elements under the control of the control unit, performs reception beam sweeping, or adjusts the direction and beam width of the reception beam so that the reception beam is coordinated with the transmission beam.

[0243] The baseband processing unit (1m-20) executes the conversion function between the baseband signal and the bit sequence according to the physical layer standard of the system. For example, during data transmission, the baseband processing unit (1m-20) generates complex symbols by encoding and modulating the transmission bit sequence. Also, during data reception, the baseband processing unit (1m-20) restores the received bit sequence from the baseband signal provided by the RF processing unit (1m-10) through demodulation and decoding.

[0244] For example, in the case of the OFDM (orthogonal frequency division multiplexing) method, during data transmission, the baseband processing unit (1m-20) generates complex symbols by encoding and modulating the transmission bit sequence, maps the complex symbols to subcarriers, and then constructs OFDM symbols through IFFT (inverse fast Fourier transform) operation and CP (cyclic prefix) insertion. Also, during data reception, the baseband processing unit (1m-20) divides the baseband signal provided by the RF processing unit (1m-10) into OFDM symbol units, restores the signal mapped to the subcarriers through FFT (fast Fourier transform) operation, and then restores the received bit sequence through demodulation and decoding.

[0245] The baseband processing unit (1m-20) and the RF processing unit (1m-10) transmit and receive signals as described above. Thereby, the baseband processing unit (1m-20) and the RF processing unit (1m-10) are also referred to as a transmitting unit, a receiving unit, a transceiver unit, or a communication unit. Furthermore, at least one of the baseband processing unit (1m-20) and the RF processing unit (1m-10) includes a plurality of communication modules to support a plurality of different wireless connection technologies.

[0246] Also, at least one of the baseband processing unit (1m-20) and the RF processing unit (1m-10) includes different communication modules to process signals in different frequency bands. For example, the aforementioned different wireless connection technologies may include an LTE network, an NR network, and the like. Also, the aforementioned different frequency bands may include a super high frequency (SHF) (e.g., 2.5 GHz, 5 GHz) band, a millimeter wave (mmWave) (e.g., 60 GHz) band.

[0247] The storage unit (1m-30) stores data such as basic programs, applications, and setting information for the operation of the terminal. The storage unit (1m-30) provides the data stored upon request by the control unit (1m-40).

[0248] The control unit (1m-40) controls the overall operation of the terminal. For example, the control unit (1m-40) transmits and receives signals via the baseband processing unit (1m-20) and the RF processing unit (1m-10). Also, the control unit (1m-40) records and reads data in the storage unit (1m-40). Therefore, the control unit (1m-40) includes at least one processor. For example, the control unit (1m - 40) includes a CP (communication processor) that controls communication and an AP (application processor) that controls upper layers such as application programs.

[0249] Figure 1N is a block diagram showing the configuration of a network entity according to an embodiment of the present invention. Specifically, Figure 1N shows the block configuration of a TRP (Tx / Rx point) in a wireless communication system to which the embodiment of the present invention is applied. As shown in Figure 1N, the TRP includes an RF processing unit (1n - 10), a baseband processing unit (1n - 20), a backhaul communication unit (1n - 30), a storage unit (1n - 40), and a control unit (1n - 50).

[0250] The RF processing unit (1n - 10) executes functions for transmitting and receiving signals via a wireless channel, such as signal band conversion and amplification. That is, the RF processing unit (1n - 10) upwardly converts the baseband signal provided from the baseband processing unit (1n - 20) into an RF band signal and then transmits it via an antenna, and downwardly converts the RF band signal received via the antenna into a baseband signal. For example, the RF processing unit (1n - 10) includes a transmission filter, a reception filter, an amplifier, a mixer, an oscillator, a DAC, an ADC, and the like.

[0251] In this figure, only one antenna is shown in the figure, but the first connection node can be equipped with a plurality of antennas. Also, the RF processing unit (1n - 10) includes a plurality of RF chains. Furthermore, the RF processing unit (1n - 10) performs beamforming. For beamforming, the RF processing unit (1n - 10) adjusts the phase and magnitude of each signal transmitted and received via a plurality of antennas or antenna elements. The RF processing unit executes a downlink MIMO operation by transmitting one or more layers.

[0252] The baseband processing unit (1n-20) executes a conversion function between a baseband signal and a bit sequence according to the physical layer standard of the first radio connection technology. For example, during data transmission, the baseband processing unit (1n-20) generates complex symbols by encoding and modulating the transmission bit sequence. Also, during data reception, the baseband processing unit (1n-20) restores the received bit sequence from the baseband signal provided by the RF processing unit (1n-10) through demodulation and decoding.

[0253] For example, in the case of the OFDM method, during data transmission, the baseband processing unit (1n-20) generates complex symbols by encoding and modulating the transmission bit sequence, maps the complex symbols to subcarriers, and then constructs OFDM symbols through IFFT operation and CP insertion. Also, during data reception, the baseband processing unit (1n-20) divides the baseband signal provided by the RF processing unit (1n-10) into OFDM symbol units, restores the signal mapped to subcarriers through FFT operation, and then restores the received bit sequence through demodulation and decoding. The baseband processing unit (1n-20) and the RF processing unit (1n-10) transmit and receive signals as described above. Thereby, the baseband processing unit (1n-20) and the RF processing unit (1n-10) are also referred to as a transmission unit, a reception unit, a transceiver unit, a communication unit, or a wireless communication unit.

[0254] The communication unit (1n-30) provides an interface for communicating with other nodes in the network. The storage unit (1n-40) stores data such as a basic program, an application, and setting information for the operation of the TRP. In particular, the storage unit (1n-40) stores information related to the bearer assigned to the connected terminal, measurement results reported from the connected terminal, and the like. Also, the storage unit (1n-40) stores information that serves as a criterion for determining whether to provide or interrupt multi-connection to the terminal. Then, the storage unit (1n-40) provides the stored data upon request from the control unit (1n-50).

[0255] The control unit (1n-50) controls the overall operation of the TRP. For example, the control unit (1n-50) transmits and receives signals via the baseband processing unit (1n-20) and the RF processing unit (1n-10), or via the backhaul communication unit (1n-30). Also, the control unit (1n-50) records and reads data in the storage unit (1n-40). Therefore, the control unit (1n-50) includes at least one processor.

[0256] The method according to the embodiments described in the claims or the specification of the present invention can be implemented in the form of hardware, software, or a combination of hardware and software. When implemented by software, a computer-readable recording medium storing one or more programs (software modules) can be provided. One or more programs stored in a computer-readable recording medium are configured for execution by one or more processors in an electronic device. The one or more programs include instructions that cause the electronic device to execute the method according to the embodiments described in the claims or the specification of the present invention.

[0257] Such a program (software module, software) can also be stored in a non-volatile memory including a RAM (random access memory) and a flash memory, a ROM (read-only memory), an EEPROM (electrically erasable programmable read-only memory), a magnetic disc storage device, a CD-ROM (compact disc read-only memory), a digital versatile disc (DVD), or other forms of optical storage devices, or a magnetic cassette. Or it can also be stored in a memory constituted by a part or all of them in combination. Moreover, a plurality of each constituent memory may be included.

[0258] Also, the program can be accessed via a communication network constituted by the Internet, an intranet, a LAN (local area network), a WLAN (wide LAN), or a SAN (storage area network), or a combination thereof, and stored in an attachable storage device. Such a storage device can be connected to a device that executes an embodiment of the present invention via an external port. Also, a separate storage device on the communication network can be connected to a device that executes an embodiment of the present invention.

[0259] In the foregoing specific embodiments of the present invention, the components included in the invention are represented as singular or plural according to the presented specific embodiments. However, the singular or plural expressions are selected as appropriate for the situations presented for the convenience of explanation, and the present invention is not limited to singular or plural components. Even if the components are expressed in the plural, they may be constituted by a single component or may be expressed in the singular, and they may also be constituted by a plurality of components. Note that the embodiments of the present invention disclosed in this specification and the drawings are only presented as specific examples for easily explaining the description content of the present invention and assisting in the understanding of the present invention, and do not limit the scope of the present invention. That is, for those skilled in the art in the technical field to which the present invention pertains, it is obvious that other modifications based on the technical idea of the present invention are feasible. In addition, each of the above-described embodiments can be combined and operated with each other as necessary. For example, a part of an embodiment different from an embodiment of the present invention can be combined with each other, and a base station and a terminal can be operated. In addition, although the embodiments are presented based on the "FDD LTE" system, other modifications based on the technical idea of the above-described embodiments are also feasible for other systems such as the "TDD LTE" system, 5G, or NR system.

Explanation of Reference Numerals

[0260] 1a-05, 1a-10, 1a-15, 1a-20 Next-generation base station (ENB) 1a-25, 1c-25 MME 1a-30 S-GW 1a-35 User terminal (UE) 1c-05 NR CN 1c-10 NR gNB 1c-15 NR UE 1c-30 eNB 1m-10, 1n-10 RF processing unit 1m-20, 1n-20 Baseband processing unit 1m-30, 1n-40 Storage unit 1m-40, 1n-50 Control unit 1n-30 Communication unit

Claims

1. In a method performed by a terminal in a wireless communication system, receiving, from a source base station, a message including "Reconfiguration with sync"; starting a first timer; when a DAPS (dual active protocol stack) bearer is configured based on the message, establishing an RLC (radio link control) entity related to a target base station and stopping an SRB (signaling radio bearer) related to the source base station; when the first timer expires and the radio connection between the terminal and the source base station is valid, releasing the RLC entity related to the target base station, restarting the stopped SRB related to the source base station, and transmitting a DAPS handover failure report to the source base station; when the first timer expires and the radio connection between the terminal and the source base station is not valid, performing an RRC (radio resource control) re-establishment procedure, a method characterized by comprising.

2. The method according to claim 1, further comprising starting a third timer while a second timer is running when a condition for starting the third timer is satisfied.

3. The method according to claim 1, further comprising starting a second timer related to the source base station when the DAPS bearer is configured, an "out-of-sync" indicator is received from a lower layer entity, and the first timer is running.

4. The method according to claim 3, further comprising stopping the second timer when an "in-sync" indicator is received from the lower layer entity while the second timer is running, and stopping the third timer if the third timer is running.

5. The method according to claim 1, further comprising starting the RRC re-establishment procedure when the third timer expires.

6. The method according to claim 2 or claim 4, wherein the first timer is a T304 timer, the second timer is a T310 timer, and the third timer is a T312 timer.

7. In a terminal in a wireless communication system, a transceiver, It has at least one processor connected to the transceiver unit, The at least one processor receives a message including "Reconfiguration with sync" from the source base station, Starts a first timer, When a DAPS (dual active protocol stack) bearer is configured based on the message, it establishes an RLC (radio link control) entity related to the target base station, stops the SRB (signaling radio bearer) related to the source base station, When the first timer expires and the radio connection between the terminal and the source base station is valid, it releases the RLC entity related to the target base station, resumes the stopped SRB related to the source base station, and transmits a DAPS handover failure report to the source base station, When the first timer expires and the radio connection between the terminal and the source base station is not valid, the terminal is characterized by performing an RRC (radio resource control) re-establishment procedure.

8. The terminal according to claim 7, wherein the at least one processor starts a third timer when a condition for starting the third timer is satisfied while the second timer is running.

9. The terminal according to claim 7, wherein the at least one processor starts a second timer related to the source base station when the DAPS bearer is configured, an "out-of-sync" indicator is received from a lower layer entity, and the first timer is running.

10. The terminal according to claim 9, wherein the at least one processor aborts the second timer when an "in-sync" indicator is received from the lower layer entity while the second timer is running, and aborts it if the third timer is running.

11. The terminal according to claim 7, wherein the RRC re-establishment procedure is performed when the third timer expires.

12. The terminal according to claim 8 or claim 10, wherein the first timer is a T304 timer, the second timer is a T310 timer, and the third timer is a T312 timer.

13. In a method performed by a source base station in a wireless communication system, comprising transmitting, to a terminal, a message including "Reconfiguration with sync", starting, at the terminal, a first timer, when a DAPS (dual active protocol stack) bearer is configured based on the message, establishing, at the terminal, an RLC (radio link control) entity related to a target base station and stopping, at the terminal, an SRB (signaling radio bearer) related to the source base station, when the first timer expires and the radio connection between the terminal and the source base station is valid, releasing, at the terminal, the RLC entity related to the target base station, restarting, at the terminal, the stopped SRB related to the source base station, and transmitting, at the terminal, a DAPS handover failure report to the source base station, when the first timer expires and the radio connection between the terminal and the source base station is not valid, performing, at the terminal, an RRC (radio resource control) re-establishment procedure.

14. In a source base station in a wireless communication system, a transceiver, and at least one processor connected to the transceiver, wherein the at least one processor transmits, to a terminal, a message including "Reconfiguration with sync", starting, at the terminal, a first timer, when a DAPS (dual active protocol stack) bearer is configured based on the message, establishing, at the terminal, an RLC (radio link control) entity related to a target base station and stopping, at the terminal, an SRB (signaling radio bearer) related to the source base station, when the first timer expires and the radio connection between the terminal and the source base station is valid, releasing, at the terminal, the RLC entity related to the target base station, restarting, at the terminal, the stopped SRB related to the source base station, and transmitting, at the terminal, a DAPS handover failure report to the source base station, When the first timer expires and the wireless connection between the terminal and the source base station is not valid, the source base station is characterized in that an RRC (radio resource control) re-establishment procedure is executed in the terminal.

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