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

Abstract

According to an embodiment of the present disclosure, an operation method of a user equipment (UE) in a wireless communication system may include: receiving configuration information from a source base station (BS); determining whether a dual active protocol stack (DAPS) switching is configured for at least one bearer based on the configuration information; based on determining that the DAPS switching is configured, maintaining the link to the source BS, performing DAPS switching on the target BS, suspending the signaling radio bearer SRB configured for the source BS, and establishing the SRB for the target BS; when it is determined that the DAPS switching fails, determining whether a radio link failure (RLF) occurs in the link to the source BS; and when the RLF does not occur in the link to the source BS, restoring the SRB configured for the source BS and reporting the failure of the DAPS switching.

Description

Technical Field

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

[0002] In order to meet the significantly increased demand for wireless data services caused by the commercialization of the fourth generation (4G) communication system and the increase in multimedia services, an evolved fifth generation (5G) system or a pre-5G communication system has been developed. For this reason, the 5G or pre-5G communication system is referred to as a "super 4G network" communication system or a "post-long term evolution (post-LTE)" system.

[0003] In order to increase the data rate, the implementation of 5G communication systems in ultra-high frequency or millimeter wave (mmwave) bands (e.g., 60 GHz bands) is being considered. In order to reduce the path loss of radio waves and increase the transmission distance of radio waves in the ultra-high frequency bands of 5G communication systems, various technologies such as beamforming, massive multiple-input multiple-output (massive MIMO), full-dimensional MIMO (FD-MIMO), array antennas, analog beamforming, and massive antennas are being studied.

[0004] In addition, in order to improve network functions for 5G communication systems, various technologies have been developed, such as evolved small cells, advanced small cells, cloud radio access networks (Cloud-RAN), ultra-dense networks, device-to-device communication (D2D), wireless backhaul, mobile networks, cooperative communications, coordinated multi-point (CoMP), interference cancellation, etc. In addition, for 5G communication systems, advanced coding and modulation (ACM) technologies such as hybrid frequency shift keying (FSK) and quadrature amplitude modulation (QAM) (FQAM) and sliding window superposition coding (SWSC) have been developed, as well as advanced access technologies such as filter bank multi-carrier (FBMC), non-orthogonal multiple access (NOMA), sparse coded multiple access (SCMA), etc.

[0005] The Internet has evolved from a human-based connection network where humans create and consume information to the Internet of Things (IoT), where distributed elements such as objects exchange information with each other to process information. The Internet of Everything (IoE) technology has emerged, which is a combination of IoT technology and technology for processing big data, such as through connection with a cloud server. In order to realize the IoT, various technical elements such as sensing technology, wired / wireless communication and network infrastructure, service interface technology and security technology are required, so that in recent years, technologies related to sensor networks for connecting objects, machine-to-machine (M2M) communication and machine type communication (MTC) have been studied. In the IoT environment, smart Internet technology (IT) services can be provided to collect and analyze data obtained from connected objects, thereby creating new value in human life. Since existing information technology (IT) and various industries are integrated and combined with each other, IT can be applied to various fields such as smart homes, smart buildings, smart cities, smart cars or connected cars, smart grids, health care, smart home appliances and advanced medical services.

[0006] Various attempts are being made to apply 5G communication systems to IoT networks. For example, 5G communication technologies such as sensor networks, M2M communications, MTC, etc. are implemented by using technologies including beamforming, MIMO, array antennas, etc. Cloud-RAN as an application of the above-mentioned big data processing technology can be an example of the fusion of 5G communication technology and IoT technology.

[0007] Due to the above-mentioned technical features and the development of wireless communication systems, various services may be provided, and in particular, a method for seamlessly supporting services related to handover of a user equipment (UE) is required. Summary of the invention

[0008] Technical issues

[0009] Various embodiments provide an apparatus and method for efficiently providing services in a mobile communication system.

[0010] Solution

[0011] According to an embodiment of the present disclosure, an operation method of a user equipment (UE) in a wireless communication system may include: receiving configuration information from a source base station (BS); determining whether a dual active protocol stack (DAPS) switching is configured for at least one bearer based on the configuration information; based on determining that the DAPS switching is configured, maintaining the link to the source BS, performing DAPS switching on the target BS, suspending the signaling radio bearer SRB configured for the source BS, and establishing the SRB for the target BS; when it is determined that the DAPS switching fails, determining whether a radio link failure (RLF) occurs in the link to the source BS; and when the RLF does not occur in the link to the source BS, restoring the SRB configured for the source BS and reporting the failure of the DAPS switching.

[0012] The operation method may further include: starting a T304 timer in the DAPS switching; and starting a T310 timer when an out-of-sync signal is received.

[0013] The operating method may further include, when the DAPS switching is completed, stopping the T304 timer and the T310 timer.

[0014] The operating method may further include, when the T304 timer expires, determining that the DAPS handover fails and no RLF is detected.

[0015] The operation method may further include, for an SRB configured with DAPS handover, releasing the SRB for the target BS and reconfiguring the SRB for which DAPS handover is released, and for an SRB not configured with DAPS handover, restoring the configuration used by the source BS before DAPS handover.

[0016] When the DAPS handover is released from the SRB in which the DAPS handover is configured, the operating method may further include maintaining the security key and the header compression context.

[0017] A user equipment (UE) in a wireless communication system may include: a transceiver; and at least one processor connected to the transceiver, wherein the at least one processor is configured to receive configuration information from a source base station (BS), determine whether a dual active protocol stack (DAPS) switching is configured for at least one bearer based on the configuration information, maintain a link to the source BS based on the determination that the DAPS switching is configured, perform a DAPS switching on a target BS, suspend a signaling radio bearer (SRB) configured for the source BS, and establish an SRB for the target BS, when it is determined that the DAPS switching fails, determine whether a radio link failure (RLF) occurs in the link to the source BS, and when the RLF does not occur in the link to the source BS, restore the SRB configured for the source BS and report the DAPS switching failure.

[0018] The at least one processor may be configured to start a T304 timer in a DAPS switch and start a T310 timer when an out-of-sync signal is received.

[0019] The at least one processor may be configured to stop the T304 timer and the T310 timer when the DAPS switch is completed.

[0020] The at least one processor may be configured to determine that the DAPS handover has failed and no RLF has been detected when the T304 timer expires.

[0021] The at least one processor may be configured to: release the SRB for the target BS and reconfigure the SRB for which the DAPS handover is released, with respect to the SRB for which the DAPS handover is configured, and restore the configuration used by the source BS before the DAPS handover, with respect to the SRB for which the DAPS handover is not configured.

[0022] The at least one processor may be configured to maintain the security key and the header compression context when releasing the DAPS handover from the SRB in which the DAPS handover was configured.

[0023] According to an embodiment of the present disclosure, an operation method of a source base station (BS) in a wireless communication system may include: sending configuration information to a user equipment (UE), the configuration information including information about whether a dual active protocol stack (DAPS) switching is configured for at least one bearer; based on the configuration information, the UE maintains a link to the source BS, and when the UE performs a DAPS switching to a target BS, the UE suspends a signaling radio bearer SRB configured for the source BS, and the UE establishes an SRB for the target BS; and when it is determined that the DAPS switching has failed and no radio link failure (RLF) has occurred in the link to the source BS, the UE restores the SRB configured for the source BS.

[0024] The operation method may further include: when it is determined that the DAPS handover fails, receiving a report on the DAPS handover failure from the UE.

[0025] A source base station (BS) in a wireless communication system may include: a transceiver; and at least one processor connected to the transceiver, wherein the at least one processor is configured to send configuration information to a user equipment (UE), the configuration information including information about whether a dual active protocol stack (DAPS) switching is configured for at least one bearer, based on the configuration information, the UE maintains a link to the source BS, and when the UE performs DAPS switching to a target BS, the UE suspends a signaling radio bearer (SRB) configured for the source BS, and the UE establishes an SRB for the target BS, and when it is determined that the DAPS switching fails and no radio link failure (RLF) occurs in the link to the source BS, the UE restores the SRB configured for the source BS.

[0026] Beneficial Effects

[0027] Various embodiments provide an apparatus and method for efficiently providing services in a mobile communication system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1A is a diagram showing a configuration of a Long Term Evolution (LTE) system according to an embodiment of the present disclosure.

[0029] Figure 1B is a diagram showing a wireless protocol architecture of an LTE system according to an embodiment of the present disclosure.

[0030] Figure 1C is a diagram showing the architecture of a next generation mobile communication system according to an embodiment of the present disclosure.

[0031] Figure 1D is a diagram illustrating a wireless protocol architecture of a next generation mobile communication system according to an embodiment of the present disclosure.

[0032] Figure 1E is a diagram for describing a process of establishing a connection between a user equipment (UE) and a network according to an embodiment of the present disclosure.

[0033] Figure 1F is a diagram illustrating a signaling process for a UE to perform handover in a next generation mobile communication system according to an embodiment of the present disclosure.

[0034] Figure 1G is a diagram for describing Embodiment 1 of a handover method for minimizing a data interruption time due to handover according to an embodiment of the present invention.

[0035] Figure 1H 2 is a diagram for describing Embodiment 2 of a switching method for minimizing a data interruption time due to switching according to an embodiment of the present disclosure.

[0036] Fig. 1I 1 is a diagram for describing an architecture of an effective data convergence protocol (PDCP) layer entity to be applied to a dual active protocol stack (DAPS) switching method and an embodiment 2 of a switching method using the architecture according to an embodiment of the present disclosure.

[0037] Figure 1J 1 is a diagram for describing an architecture of an effective Service Data Adaptation Protocol (SDAP) layer entity to be applied to a DAPS switching method and an embodiment 2 of a switching method using the architecture according to an embodiment of the present disclosure.

[0038] Figure 1K is a flow chart illustrating the operation of a UE according to an embodiment of the present invention.

[0039] Figure 1L is a flowchart illustrating an operation of a UE performing a fallback procedure when a handover fails in a DAPS handover method according to an embodiment of the present disclosure.

[0040] Figure 1M is a block diagram showing the configuration of a UE according to an embodiment of the present disclosure.

[0041] Figure 1N is a block diagram showing the configuration of a network entity according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0042] Hereinafter, the operating principle of the present disclosure will be described in detail with reference to the accompanying drawings. In the following description of the present disclosure, well-known functions or configurations are not described in detail because they will obscure the present disclosure with unnecessary details. The terms used in this specification are defined in consideration of the functions used in the present disclosure and may be changed according to the intention of the user or operator or the method commonly used. Therefore, the definition of the terms is understood based on the overall description of this specification.

[0043] In the following description of the present disclosure, when it is considered that well-known functions or configurations may unnecessarily obscure the essence of the present disclosure, they will not be described in detail. Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings.

[0044] Hereinafter, for the convenience of explanation, the terms used in the following description to identify access nodes, the terms indicating network entities, the terms indicating messages, the terms indicating interfaces between network entities, and the terms indicating various identification information are exemplified. Therefore, the present disclosure is not limited to the terms to be described below, and other terms indicating objects having the same technical meaning may be used.

[0045] For ease of description, the present disclosure uses terms and names defined in the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard. However, the present disclosure is not limited to these terms and names and can be equally applied to communication systems that conform to other standards. In the present disclosure, for ease of description, an evolved Node B (eNB) may be used interchangeably with a next-generation Node B (gNB). That is, a base station (BS) described by an eNB may represent a gNB.

[0046] The present disclosure relates to a handover method and apparatus for minimizing a data transceiving stop time due to handover or making the data transceiving stop time 0 ms in a next generation mobile communication system.

[0047] Specifically, the effective switching method provided in the present disclosure may have one or more of the features described below.

[0048] -When a user equipment (UE) receives a handover command message (e.g., a handover command message or a radio resource control (RRC) reconfiguration message) from a source BS, the UE, which performs sending or receiving data to or from the source BS (uplink (UL) or downlink (DL) data transmission and reception) via each of the protocol layer entities for multiple first bearers (i.e., a physical (PHY) layer entity, a medium access control (MAC) layer entity, a radio link control (RLC) layer entity, and a packet data convergence protocol (PDCP) layer entity), can configure protocol layer entities for multiple new second bearers (e.g., having the same bearer identifier) ​​corresponding to the protocol layer entities for the multiple first bearers and can perform data sending or receiving (UL or DL ​​data transmission and reception) without stopping, but maintains sending or receiving data to or from the source BS (UL or DL ​​data transmission and reception) via the multiple first bearers.

[0049] -The protocol entities (PHY layer entity, MAC layer entity, RLC layer entity and PDCP layer entity) for multiple second bearers newly configured after the UE receives the switching command message can be configured to send and receive data to and from the target BS based on the bearer configuration information or multiple protocol layer entity information included in the switching command message.

[0050] - While the UE performs data transmission to or reception from the source BS (UL or DL ​​data transmission and reception) via the protocol layer entity for the plurality of first bearers, the UE may perform a random access procedure to the target BS via the protocol layer entity for the plurality of second bearers (e.g., MAC layer entity). The random access procedure may include transmission of a preamble, reception of a random access response, transmission of message 3, reception of message 4 (e.g., reception of a contention resolution MAC control element (CE) or UL transmission resource), etc.

[0051] -While the UE is sending data to or receiving data from the source BS via a protocol layer entity for multiple first bearers, the UE can complete a random access procedure with respect to the target BS via a protocol layer entity for multiple second bearers (e.g., a MAC layer entity), and can send a switching completion message to the target BS via the protocol layer entity for multiple second bearers.

[0052] -While the UE is sending data to or receiving data from the source BS via a protocol layer entity for multiple first bearers, the UE can complete a random access procedure to the target BS via a protocol layer entity for multiple second bearers (e.g., a MAC layer entity), can send a switching completion message to the target BS via the protocol layer entity for multiple second bearers, and can perform data sending and receiving (UL or DL).

[0053] -When the UE successfully completes the random access procedure with respect to the target BS and then initially receives UL transmission resources from the target BS, the UE may stop sending data to the source BS via the protocol layer entity for multiple first bearers, may switch UL transmission, and may send data to the target BS via multiple second bearers.

[0054] -When the UE receives the handover command message, the UE may continue to send data to the source BS or receive data from the source BS (UL or DL ​​data transmission and reception) via the protocol layer entity for the multiple first bearers, and may perform a random access procedure to the target BS via the protocol layer entity for the multiple second bearers. In addition, when the UE successfully completes the random access procedure and then initially receives UL transmission resources from the target BS, the UE may stop sending data to the source BS via the protocol layer entity for the multiple first bearers, and may perform UL data transmission to the target BS only via the protocol layer entity for the multiple second bearers, and at this point, the UE may continuously receive DL data from the source BS via the protocol layer entity for the multiple first bearers, and may continuously receive DL data from the target BS via the protocol layer entity for the multiple second bearers.

[0055] - A first bearer and a second bearer may be configured for a second PDCP layer entity architecture, and in the second PDCP layer entity architecture, a first bearer (e.g., an RLC layer entity, a MAC layer entity, or a PHY layer entity) for a source BS and a second bearer (e.g., an RLC layer entity, a MAC layer entity, or a PHY layer entity) for a target BS are both connected to one PDCP layer entity. UL data may be sent via one bearer from the first bearer or the second bearer via the PDCP layer entity. That is, before the UE performs a random access procedure on the target BS, successfully completes the random access procedure, and initially receives UL transmission resources from the target BS, the UE sends UL data via the first bearer, and when the UE performs a random access procedure on the target BS, successfully completes the random access procedure, and initially receives UL transmission resources from the target BS, the UE may stop data transmission via the first bearer, may switch data transmission, and then may send UL data to the target BS via the second bearer. However, a UE having a second PDCP layer entity architecture may receive DL data from the source BS or the target BS via the first bearer or the second bearer.

[0056] The present disclosure provides a method and apparatus for performing an effective handover process without a data transceiving stop time based on the above features. In addition, the present disclosure provides a method by which, when a UE performs an effective handover method without a data transceiving stop time, if the UE handover fails, the UE can fall back to the source BS and reconfigure the connection to the source BS. According to an embodiment of the present disclosure, when the UE performs a handover process, the UE can maintain a link to the source BS, and even when the UE handover fails, the UE can fall back by using a previous wireless link to the source BS.

[0057] Figure 1A is a diagram showing a configuration of an LTE system according to an embodiment of the present disclosure.

[0058] Reference Figure 1A The radio access network (RAN) of the LTE system includes multiple eNBs (or Node Bs or BSs) 1a-05, 1a-10, 1a-15, and 1a-20, a mobility management entity (MME) 1a-25, and a serving gateway (S-GW) 1a-30. The UE (or terminal) 1a-35 accesses the external network through the eNBs 1a-05, 1a-10, 1a-15, or 1a-20 and the S-GW 1a-30.

[0059] exist Figure 1A In the LTE system, eNB 1a-05, 1a-10, 1a-15 or 1a-20 may correspond to a conventional Node B of a Universal Mobile Telecommunications System (UMTS). The eNB may be connected to UE 1a-35 via a wireless channel and may perform functions that are complex compared to conventional Node Bs. All user service data including real-time services such as Voice over Internet Protocol (VoIP) may be served via a shared channel in the LTE system, and therefore an entity for checking status information (e.g., buffer status information of the UE, available transmit power status information, and channel status information) and performing scheduling may be required, and eNB 1a-05, 1a-10, 1a-15 or 1a-20 may operate as such an entity. One eNB typically controls multiple cells. For example, an LTE system may use a wireless access technology such as orthogonal frequency division multiplexing (OFDM) in a bandwidth of 20 MHz to achieve a data rate of 100 Mbps. In addition, the eNB may also use adaptive modulation and coding (AMC) to determine a modulation scheme and a channel coding rate according to the channel state of the UE. The S-GW 1a-30 is an entity for providing a data bearer, and can establish and release a data bearer under the control of the MME 1a-25. The MME 1a-25 is an entity for performing a mobility management function and various control functions on a UE, and is connected to a plurality of eNBs.

[0060] Figure 1Bis a diagram showing a wireless protocol architecture of an LTE system according to an embodiment of the present disclosure.

[0061] Reference Figure 1B , the radio protocol of the LTE system may include the Packet Data Convergence Protocol (PDCP) layers 1b-05 and 1b-40, the RLC layers 1b-10 and 1b-35, and the Medium Access Control (MAC) layers 1b-15 and 1b-30 in the UE and the eNB, respectively. The PDCP layer 1b-05 or 1b-40 may perform, for example, IP header compression / decompression. The main functions of the PDCP layer 1b-05 or 1b-40 are summarized as follows.

[0062] -Header compression and decompression: Robust Header Compression (ROHC) only

[0063] -Transfer user data

[0064] - In-sequence delivery of upper layer packet data units (PDUs) during PDCP re-establishment procedure for RLC Acknowledged Mode (AM)

[0065] - For split bearers in DC (supports RLC AM only): PDCP PDU routing for transmission and PDCP PDU reordering for reception

[0066] - Duplicate detection of lower layer service data units (SDUs) during PDCP re-establishment for RLC AM

[0067] - Retransmission of PDCP SDUs at handover for split bearers in DC and retransmission of PDCP PDUs during PDCP data recovery for RLC AM

[0068] -Encryption and decryption

[0069] - Timer-based SDU discard in uplink

[0070] The RLC layer 1b-10 or 1b-35 may perform an automatic repeat request (ARQ) operation by reconfiguring a packet data convergence protocol packet data unit (PDCP PDU) to an appropriate size. The main functions of the RLC layer may be summarized as follows.

[0071] -Transmission of upper layer PDU

[0072] - Error correction via ARQ (for AM data transmission only)

[0073] - Concatenation, segmentation and reassembly of RLC SDUs (only for Unacknowledged Mode (UM) and AM data transmission)

[0074] - Re-segmentation of RLC data PDU (for AM data transmission only)

[0075] - Reordering of RLC data PDUs (only for UM and AM data transmission)

[0076] - Duplicate detection (for UM and AM data transmission only)

[0077] -Protocol error detection (for AM data transmission only)

[0078] -RLC SDU discard (only for UM and AM data transmission)

[0079] -RLC reconstruction

[0080] The MAC layer 1b-15 or 1b-30 may be connected to a plurality of RLC layer entities configured for one UE, and may multiplex RLC PDUs into MAC PDUs, and may demultiplex RLC PDUs from MAC PDUs. The main functions of the MAC layer may be summarized as follows.

[0081] - Mapping between logical channels and transport channels

[0082] - Multiplexing / demultiplexing MAC SDUs belonging to one or different logical channels into / from a transport block (TB) on a transport channel delivered to the physical layer

[0083] -Dispatch information report

[0084] - Error correction through Hybrid ARQ (HARQ).

[0085] - Priority handling between logical channels of a UE

[0086] - Prioritization between UEs through dynamic scheduling

[0087] -Multimedia Broadcast / Multicast Service (MBMS) service identifier

[0088] -Transmission format selection

[0089] -filling

[0090] The physical (PHY) layer 1b-20 or 1b-25 can channel-code and modulate the upper layer data into OFDM symbols and send the OFDM symbols through the wireless channel, or can demodulate and channel-decode the OFDM symbols received through the wireless channel and deliver the OFDM symbols to the upper layer.

[0091] Figure 1C is a diagram showing the architecture of a next generation mobile communication system according to an embodiment of the present disclosure.

[0092] Reference Figure 1C As shown in the figure, the wireless access network of the next generation mobile communication system (hereinafter referred to as NR or 5G communication system) includes a new wireless node B (NR gNB or NR BS) 1c-10 and a new wireless core network (NR CN) 1c-05. NRUE (or terminal) 1c-15 can access the external network via NR NB 1c-10 and NR CN 1c-05.

[0093] exist Figure 1C In the NR gNB 1c-10, the NR gNB 1c-10 may correspond to the eNB of the conventional LTE system. The NR gNB may be connected to the NR UE 1c-15 through a wireless channel, and may provide services superior to those of the conventional Node B. All user service data may be served through a shared channel in the NR or 5G mobile communication system, and therefore, an entity for checking the buffer status information, available transmission power status information, and channel status information of the UE and performing scheduling may be required, and the NR gNB1c-10 may operate as such an entity. One NR gNB typically controls multiple cells. Compared with the conventional LTE system, an ultra-high data rate may be achieved using a bandwidth greater than the maximum bandwidth of the conventional LTE system, and OFDM may be used as a radio access technology, and beamforming technology may be additionally applied thereto. In addition, AMC may be applied to determine the modulation scheme and channel coding rate according to the channel state of the UE. The NR CN 1c-05 performs functions such as mobility support, bearer configuration, and quality of service (QoS) configuration. The NR CN is an entity for performing mobility management functions and various control functions on the NR UE, and is connected to multiple BSs. In addition, the NR or 5G communication system can cooperate with the legacy LTE system, and the NR CN can be connected to the MME 1c-25 through a network interface. The MME is connected to the eNB 1c-30 which is a legacy BS.

[0094] Figure 1D is a diagram showing a wireless protocol architecture of an NR or 5G communication system according to an embodiment of the present disclosure.

[0095] Reference Figure 1D , the radio protocol architecture of the NR or 5G communication system may include NR Service Data Adaptation Protocol (SDAP) layers 1d-01 and 1d-45, NR PDCP layers 1d-05 and 1d-40, NR RLC layers 1d-10 and 1d-35, and NR MAC layers 1d-15 and 1d-30 for UE and NR gNB, respectively.

[0096] The main functions of NR SDAP layer 1d-01 or 1d-45 may include some of the following functions.

[0097] -Transmission of user plane data

[0098] - Mapping between QoS flows and DRBs for both DL and UL

[0099] - Marking QoS flow identifier (ID) in DL and UL packets

[0100] - Reflective QoS flow to DRB mapping for UL SDAP PDU

[0101] For the SDAP layer entity, the UE may be configured with information about whether to use the header of the SDAP layer entity or the function of using the SDAP layer entity through a radio resource control (RRC) message for each PDCP layer entity, each bearer, or each logical channel. In addition, when configuring the SDAP header of the SDAP layer entity, a 1-bit non-access stratum (NAS) reflective QoS indicator and a 1-bit access stratum (AS) reflective QoS indicator of the SDAP header may instruct the UE to update or reconfigure the UL and DL QoS flows and data bearer mapping information. The SDAP header may include QoS flow ID information indicating QoS. The QoS information may be used as data processing priority information or scheduling information to appropriately support services.

[0102] The main functions of NR PDCP layer 1d-05 or 1d-40 may include some of the following functions.

[0103] -Header compression and decompression: ROHC only

[0104] -Transfer user data

[0105] - In-sequence delivery of upper layer PDUs

[0106] - Out-of-order delivery of upper layer PDUs

[0107] - Reordering of received PDCP PDUs

[0108] - Duplicate detection of lower layer SDU

[0109] -Retransmission of PDCP SDU

[0110] -Encryption and decryption

[0111] - Timer-based SDU discard in uplink

[0112] The reordering function of the NR PDCP layer entity may include a function of reordering the PDCP PDUs received from the lower layer based on the PDCP sequence number (SN), and a function of delivering the reordered data to the upper layer in sequence. Alternatively, the reordering function of the NR PDCP layer entity may include a function of delivering the reordered data to the upper layer out of sequence, a function of recording the lost PDCP PDUs by reordering the received PDCP PDUs, a function of reporting the status information of the lost PDCP PDUs to the transmitter, and a function of requesting retransmission of the lost PDCP PDUs.

[0113] The main functions of the NR RLC layer 1d-10 or 1d-35 may include some of the following functions.

[0114] -Transmission of upper layer PDU

[0115] - In-sequence delivery of upper layer PDUs

[0116] - Out-of-order delivery of upper layer PDUs

[0117] - Error correction via ARQ

[0118] - Concatenation, segmentation and reassembly of RLC SDU

[0119] - Re-segmentation of RLC data PDUs

[0120] - Reordering of RLC data PDUs

[0121] - Duplicate detection

[0122] -Protocol error detection

[0123] -RLC SDU discarded

[0124] -RLC reconstruction

[0125] The in-sequence delivery function of the NR RLC layer entity may indicate a function of delivering RLC SDUs received from a lower layer to an upper layer in sequence, and may include a function of reassembling the RLC SDU and delivering the reassembled RLC SDU when a plurality of RLC SDUs segmented from one RLC SDU are received, and includes a function of reordering the received RLC PDU based on the RLC SN or the PDCP SN. In addition, the in-sequence delivery function of the NR RLC layer entity may include a function of recording lost RLC PDUs by reordering received RLC PDUs, a function of reporting status information of lost RLC PDUs to a transmitter, a function of requesting retransmission of lost RLC PDUs, a function of delivering in sequence to an upper layer an RLC SDU only before a lost RLC SDU when a lost RLC SDU exists, a function of delivering in sequence to an upper layer all RLC SDUs received before a timer is started in spite of the presence of lost RLC SDUs when a certain timer expires, or a function of delivering in sequence to an upper layer all RLC SDUs received so far in spite of the presence of lost RLC SDUs when a certain timer expires. In addition, the NR RLC layer entity can process the RLC PDU in the order of reception (regardless of SN and in the order of arrival), and can deliver the RLC PDU to the PDCP layer entity in an out-of-order delivery manner, and when the RLC PDU is segmented, the NR RLC layer entity can reassemble the segment with other segments stored in the buffer or received subsequently into a complete RLC PDU, and can send the RLC PDU to the DCP layer entity. The NR RLC layer may not have a cascading function, and the cascading function may be performed by the NR MAC layer, or may be replaced by a multiplexing function of the NR MAC layer.

[0126] The out-of-order delivery function of the NR RLC layer entity may include a function of directly delivering the RLC SDU received from the lower layer to the upper layer out of order, a function of reassembling multiple RLC SDUs segmented from one RLC SDU and delivering the reassembled RLC SDU when the segmented RLC SDU is received, and a function of recording lost RLC PDUs by storing the RLC SN or PDCP SN of the received RLC PDU and reordering the received RLC PDUs.

[0127] The NR MAC layer 1d-15 or 1d-30 can be connected to multiple NR RLC layers configured for one UE, and the main functions of the NR MAC layer may include some of the following functions.

[0128] - Mapping between logical channels and transport channels

[0129] -Multiplexing / demultiplexing of MAC SDU

[0130] -Dispatch information report

[0131] - Error correction through HARQ

[0132] - Priority handling between logical channels of a UE

[0133] - Prioritization between UEs through dynamic scheduling

[0134] -MBMS service logo

[0135] -Transmission format selection

[0136] -filling

[0137] The NR PHY layer 1d-20 or 1d-25 can channel-code and modulate the upper layer data into OFDM symbols, and can send the OFDM symbols through the wireless channel, or can demodulate and channel-decode the OFDM symbols received through the wireless channel, and can deliver the OFDM symbols to the upper layer. Figure 1E is a diagram for describing a process of establishing a connection between a UE and a network according to an embodiment of the present disclosure.

[0138] More specifically, Figure 1E is a diagram for describing a process in which a UE transitions from an RRC idle mode to an RRC connected mode and establishes a connection with a network according to the present disclosure.

[0139] Reference Figure 1E , when a UE configured to send and receive data in RRC connected mode does not send or receive data due to a predetermined reason or within a predetermined time, the gNB may send an RRC connection release message to the UE to allow the UE to transition to RRC idle mode (1e-01). Then, when a UE that is not currently configured for connection (hereinafter also referred to as an idle mode UE) has data to send, the UE may perform an RRC connection establishment procedure on the gNB. The UE establishes reverse transmission synchronization with the gNB through a random access procedure and sends an RRCConnectionRequest message to the gNB (1e-05). The RRCConnectionRequest message may include an identifier of the UE, an establishment cause, etc. The gNB sends an RRCConnectionSetup message to allow the UE to establish an RRC connection (1e-10).

[0140] The message may include configuration information for each service / bearer / RLC layer entity or each logical channel or each bearer, and whether to use ROHC for each bearer / logical channel, ROHC configuration information (e.g., ROHC version, initial information, etc.), statusReportRequired information (information for the BS to indicate the PDCP status report to the UE), and drb-ContinueROHC information (configuration information indicating to continue and unchanged use of ROHC configuration information) may be included in the PDCP layer entity configuration information (pdcp-config) and sent. Moreover, RRCConnectionSetup may include RRC connection configuration information. The bearer used for the RRC connection is called a signaling radio bearer (SRB) and is used for transmission and reception of RRC messages as control messages between the UE and the gNB.

[0141] The UE establishing the RRC connection sends an RRCConnectionSetupComplete message to the gNB (1e-15). The RRCConnectionSetupComplete message may include a control message such as a SERVICE REQUEST message for the UE to request the MME or the Access and Mobility Management Function (AMF) to configure a bearer for a specific service. The gNB sends the SERVICE REQUEST message included in the RRCConnectionSetupComplete message to the MME or the AMF (1e-20), and the MME or the AMF may determine whether to provide the service requested by the UE. As a result of the determination, when the service requested by the UE is to be provided, the MME or the AMF sends an INITIAL CONTEXT SETUP REQUEST message to the gNB (1e-25). The INITIAL CONTEXT SETUP REQUEST message may include QoS information to be applied when configuring a data radio bearer (DRB), security information to be applied to the DRB (e.g., security keys, security algorithms, etc.), etc.

[0142] When the gNB does not receive the UE capability information from the MME or AMF, the gNB may send a UE capability information request message to the UE to identify the UE capability information (1e-26). When the UE receives the UE capability information request message, the UE may configure, generate, and report the UE capability information message to the gNB (1e-27). The UE capability information message may include information about which types of handover methods the UE supports. For example, the UE may report the UE capability to the gNB via an indicator indicating whether the UE supports the effective handover method proposed in the present disclosure (i.e., the dual active protocol stack (DAPS) handover method). When the gNB identifies the UE capability information, the gNB may indicate the handover to the UE by defining an indicator indicating which handover type is indicated in the handover command message according to each of the handover methods. For example, the gNB may indicate the effective handover method (DAPS handover method) proposed in the present disclosure to the UE, or may configure the DAPS handover method for the UE according to each bearer (DRB or SRB). When the gNB configures the DAPS handover method for the UE, the gNB may also indicate other handover methods that prevent data loss or data delay that may occur in the handover. For example, the BS may configure the UE's handover method with the DAPS handover method, which may include a conditional handover method or a handover method without a random access procedure. The conditional handover method may be a method in which multiple target cells and multiple conditions are configured for the UE, and when the UE meets the configured conditions during cell selection or reselection, the UE performs a handover procedure on one target cell. The UE may perform a handover procedure on the target gNB according to the handover method indicated in the handover command message.

[0143] To configure security with the UE, the gNB exchanges SecurityModeCommand messages (1e-30) and SecurityModeComplete messages (1e-35). When security configuration is complete, the gNB sends an RRCConnectionReconfiguration message (1e-40) to the UE.

[0144] The message may include configuration information for each service / bearer / RLC layer entity or each logical channel or each bearer, and whether to use ROHC for each bearer / logical channel, ROHC configuration information (e.g., ROHC version, initial information, etc.), status report required information (information for BS to indicate PDCP status report to UE), and drb-ContinueROHC information (configuration information indicating configuration information to continue and unchanged use of ROHC) may be included in PDCP layer entity configuration information (pdcp-config) and sent. In addition, the message may include RRC connection configuration information. The bearer used for RRC connection is called SRB and is used for transmission and reception of RRC messages as control messages between UE and gNB.

[0145] This message includes configuration information of the DRB in which user data is to be processed, and the UE uses the information to configure the DRB and sends an RRCConnectionReconfigurationComplete message to the gNB (1e-45). When the configuration of the DRB for the UE is completed, the gNB sends an INITIAL CONTEXT SETUP COMPLETE message to the MME or AMF (1e-50), and upon reception, the MME or AMF may exchange S1 BEARER SETUP messages and S1 BEARER SETUP RESPONSE messages with the S-GW to configure the S1 bearer (1e-055 and 1e-60). The S1 bearer refers to a link for data transmission that is configured between the S-GW and the gNB and corresponds one-to-one to the DRB. After completing the above process, the UE sends data to or receives data from the gNB through the S-GW (1e-65 and 1e-70). In this way, the general data transmission process mainly includes three steps: RRC connection establishment, security setup and DRB configuration. In addition, the gNB can send an RRCConnectionReconfiguration message to the UE to update, add or change the configuration for some reason (1e-75).

[0146] In the present disclosure, a bearer may include an SRB and a DRB, where an SRB stands for a signaling radio bearer and a DRB stands for a data radio bearer. An SRB is mainly used to send and receive RRC messages of an RRC layer entity, and a DRB is mainly used to send and receive multiple user plane data. An unacknowledged mode (UM) DRB indicates a DRB configured to use an RLC layer entity operating in an unacknowledged mode (UM), and an acknowledged mode (AM) DRB indicates a DRB configured to use an RLC layer entity operating in an acknowledged mode (AM).

[0147] Figure 1F It is a diagram showing the signaling process of a UE performing a handover in a next generation mobile communication system according to an embodiment of the present disclosure. A UE 1f-01 in an RRC connected (RRC_connected) mode state performs a cell measurement report (1f-05) to the current source BS (source gNB) 1f-02 in a periodic manner or when a specific event is met. The source BS determines whether the UE is to perform a handover to a neighboring cell based on the cell measurement report. Handover refers to switching the source BS to another BS (or another cell in the same BS), and the source BS provides services to the UE in a connected mode state. When the source BS determines the handover, the source BS requests the handover by sending a handover request message (e.g., a handover preparation information message) to the target BS (target gNB) 1f-03 as the new BS to provide services to the UE (1f-10). When the target BS accepts the handover request, the target BS sends a handover request confirmation (Ack) message (e.g., a handover command message) to the source BS (1f-15). Upon receiving the handover request confirm message, the source BS sends a handover command message (RRCReconfiguration message included in the dedicated control channel (DCCH) of the handover request confirm message) to the UE (1f-20). The source BS extracts the handover command message from the message received from the target and sends the handover command message to the UE by using an RRC connection reconfiguration message (1f-20).

[0148] The present disclosure provides an embodiment which determines a DAPS handover method by using two messages when a source BS transmits a handover preparation information message (1f-10), and in response thereto, a target BS transmits a handover command message (1f-15) to the source BS.

[0149] Embodiment 1 of performing a DAPS switching process according to an embodiment of the present disclosure will now be described.

[0150] In Embodiment 1, the entity for determining the DAPS switching method may be the source BS. In addition, in Embodiment 1, when the source BS requests the target BS to perform DAPS switching, the target BS may instruct or perform DAPS switching.

[0151] -The source BS may define a new indicator in the handover preparation information message, and the indicator may indicate that the source BS is to perform a DAPS handover procedure and may request a DAPS handover procedure. The handover preparation information message may include the current bearer configuration information of the UE, security key information, cell group configuration information, UE capability information, etc. The source BS may pre-share the capabilities of the target BS, and may therefore pre-identify whether the target BS supports the DAPS handover method, and may indicate that the source BS will perform the DAPS handover method on the target BS. Therefore, the source BS may indicate to the target BS that the source BS may perform early data forwarding quickly or in advance, and may instruct the target BS to prepare to receive data forwarding and perform processing. The source BS may request a DAPS handover method request for each bearer (DRB or SRB).

[0152] - In the case where the target BS receives the handover preparation information message and recognizes that the indicator requesting the DAPS handover method is included in the handover preparation information message, when the target BS configures the RRCReconfiguration message to instruct the UE to perform the handover, the target BS may add the indicator indicating the DAPS handover method to the RRCReconfiguration message, and may configure the RRCReconfiguration message including the bearer configuration information or security key information or cell group configuration information or system information required for the UE to perform the DAPS handover method. The target BS may add the configured RRCReconfiguration message to the DL-DCCH message of the handover command message, and may send the handover command message to the source BS. The target BS may perform the indication of the DAPS handover method for each bearer (DRB or SRB).

[0153] -When the source BS receives the handover command message, the source BS may extract the RRCReconfiguration message included in the handover command message, or may send the RRCReconfiguration message to the UE, thereby indicating the handover. The source BS may identify the indicated DAPS handover method for each bearer, and may perform the DAPS handover method for each bearer (DRB or SRB).

[0154] Embodiment 2 of performing a DAPS switching process according to an embodiment of the present disclosure will now be described.

[0155] In Embodiment 2, the entity for determining the DAPS switching method may be the target BS. In addition, in Embodiment 2, in the case where the source BS requests the target BS for the DAPS switching method through an indicator, the target BS may reject or accept the request, or may indicate another switching method via a switching command message indicating another switching method for the source BS.

[0156] -The source BS may define a new indicator in the handover preparation information message, and the indicator may indicate that the source BS is to perform a DAPS handover procedure and may request a DAPS handover procedure. The handover preparation information message may include the current bearer configuration information of the UE, security key information, cell group configuration information, UE capability information, etc. The source BS may pre-share the capabilities of the target BS, and may therefore pre-identify whether the target BS supports the DAPS handover method, and may indicate that the source BS will perform the DAPS handover method on the target BS. Therefore, the source BS may indicate to the target BS that the source BS may quickly perform early data forwarding, and may indicate that the target BS is ready to receive data forwarding and perform processing. The source BS may request a DAPS handover method request for each bearer (DRB or SRB).

[0157] -In the case where the target BS receives the handover preparation information message and identifies that the indicator requesting the DAPS handover method is included in the handover preparation information message, the target BS may reject or accept the request for the DAPS handover method or may indicate another handover method based on whether the target BS can support the DAPS handover method, the amount of current transmission resources, or scheduling. The target BS may add an indicator for rejecting or accepting the request for the DAPS handover method or add an indicator indicating another handover method to the handover command message, and may send the handover command message. When the target BS configures the RRCReconfiguration message to indicate handover to the UE, if the target BS accepts the DAPS handover request, the target BS may add an indicator to indicate the DAPS handover method. If the target BS rejects the DAPS handover request, when the target BS configures the RRCReconfiguration message to indicate handover to the UE, the target BS may configure the RRCReconfiguration message by including an indicator indicating another handover method and including bearer configuration information or security key information or cell group configuration information or system information required for the UE to perform the DAPS handover method or another handover method. The target BS may add the configured RRCReconfiguration message to the DL-DCCH message of the handover command message and may deliver the message to the source BS. The target BS may perform indication of the DAPS handover method for each bearer (DRB or SRB).

[0158] When the source BS receives the handover command message, the source BS can identify the indicator included in the handover command message, and thus can identify whether the request for the DAPS handover method is accepted, and when the request for the DAPS handover method is accepted, the source BS can perform the DAPS handover process, and can extract the RRCReconfiguration message included in the handover command message, or can send the RRCReconfiguration message to the UE, thereby indicating the handover. However, when the source BS identifies the indicator included in the handover command message, if the request for the DAPS handover method is rejected or another handover method is indicated, the source BS can perform another handover method indicated by the target BS. Then, the source BS can extract the RRCReconfiguration message included in the handover command message, or can send the RRCReconfiguration message to the UE, thereby indicating the handover. According to another embodiment of the present disclosure, even when the handover command message does not include a separate indicator, the source BS can check the RRCReconfiguration message included in the handover command message to identify which handover method the target BS indicates, and identify whether the request for the DAPS handover method is accepted, and can perform the handover method indicated by the RRCReconfiguration message (e.g., DAPS handover method or other handover method). The source BS can identify the indicated DAPS handover method for each bearer, and can perform the DAPS handover method for each bearer (DRB or SRB).

[0159] Embodiment 3 of performing a DAPS switching process according to an embodiment of the present disclosure will now be described.

[0160] In Embodiment 3, the entity for determining the DAPS switching method may be the target BS. In addition, in Embodiment 3, the target BS may identify the capability of the UE and may determine the switching method (e.g., the DAPS switching method) according to whether the target BS can support the DAPS switching method or the amount or scheduling of current transmission resources.

[0161] The source BS may include the current bearer configuration information or security key information or cell group configuration information or UE capability information of the UE in the handover preparation information message, and may send the handover preparation information message to request the target BS to perform handover. The source BS may pre-share the capabilities of the target BS, so that it may pre-identify whether the target BS supports the DAPS handover method, and when the target BS indicates that the target BS is to perform the DAPS handover procedure, the source BS may quickly or pre-perform early data forwarding.

[0162] The target BS may receive the handover preparation information message, and may determine the handover method (e.g., DAPS handover) according to the capability information of the UE or whether the target BS can support the DAPS handover method or the amount of current transmission resources or scheduling. When the target BS determines the DAPS handover method, the target BS may include an indicator indicating the DAPS handover method in the handover command message, and may send the message. When the target BS configures the RRCReconfiguration message to indicate the handover to the UE, if the target BS determines the DAPS handover, the RRCReconfiguration message may include an indicator indicating the DAPS handover method, and if the target BS determines another handover method other than the DAPS handover, the RRCReconfiguration message may include an indicator indicating another handover method. In addition, the target BS may configure the RRCReconfiguration message by including the bearer configuration information or security key information or cell group configuration information or system information required for the UE to perform the DAPS handover method or other handover methods. The target BS may add the configured RRCReconfiguration message to the DL-DCCH message of the handover command message, and may deliver the message to the source BS. The target BS may perform indication of the DAPS switching method for each bearer (DRB or SRB).

[0163] -When the source BS receives the handover command message, the source BS may identify the indicator included in the handover command message, and thus may identify whether the DAPS handover is determined, and when the DAPS handover is determined, the source BS may perform the DAPS handover method, and may extract the RRCReconfiguration message included in the handover command message, or may send the RRCReconfiguration message to the UE, thereby indicating the handover. However, when the source BS identifies the indicator included in the handover command message, if the DAPS handover method is not determined or another handover method is indicated, the source BS may perform another handover method indicated by the target BS. Then, the source BS may extract the RRCReconfiguration message included in the handover command message, or may send the RRCReconfiguration message to the UE, thereby indicating the handover. In another method, even when the handover command message does not include a separate indicator, the source BS may check the RRCReconfiguration message included in the handover command message to identify which handover method the target BS indicates, and identify whether the determination of the DAPS handover method is performed, and when another handover method is indicated, the indicated handover method may be performed. The source BS may identify the indicated DAPS switching method for each bearer and may perform the DAPS switching method for each bearer (DRB or SRB).

[0164] The new embodiment may be implemented by combining the method of embodiment 1, embodiment 2, or embodiment 3 for performing the DAPS switching process according to the embodiments of the present disclosure.

[0165] The BS may indicate the switching method (DAPS switching method) according to an embodiment of the present disclosure to the UE through an RRCReconfiguration message, or in another method, the BS may configure the DAPS switching method for each bearer (DRB or SRB) of the UE. For example, a new indicator indicating a valid switching method (DAPS switching method) may be defined in the bearer configuration information or PDCP configuration information or RLC configuration information for each bearer identifier or each logical channel identifier in the RRC message, and the BS may indicate a valid switching message for each bearer or logical channel identifier to the UE by using the identifier. When the BS configures the DAPS switching method for the UE, the BS may also indicate other switching methods that prevent data loss or data delay that may occur in the switching. For example, the switching method that the BS may configure the UE with the DAPS switching method may include a conditional switching method or a switching method without a random access process. The conditional switching method may be a method in which multiple target cells and multiple conditions are configured for the UE, and when the UE meets the configured conditions during the cell selection or reselection process, the UE performs a switching process on one target cell. When the UE receives the message, the UE stops sending and receiving data to and from the source BS by using the configured handover method, or continues sending and receiving data to and from the source BS, and starts the T304 timer. When the UE cannot successfully switch to the target BS within a predetermined time (for example, when the T304 timer expires), the UE returns to its default configuration and transitions to the RRC idle state. In addition, the RRC connection reestablishment process can be triggered, and in another method, when a valid handover method is configured and the link to the source BS is active, the UE can fall back and report its handover failure to the source BS. The source BS provides a sequence number (SN) status of UL / DL data for each bearer (for example, for each RLC UM bearer or each RLC AM bearer), and when DL data or UL data exists, the source BS sends DL data or UL data to the target BS (1f-30 and 1f-35). The UE attempts to randomly access the target cell indicated by the source BS (1f-40). Random access is performed to notify the UE to switch to the target cell via handover, and at the same time match UL synchronization. For random access, the UE sends a preamble corresponding to the preamble ID provided by the source BS or to a randomly selected preamble to the target cell. After sending the preamble and after a time period corresponding to a specific number of subframes, the UE monitors whether a random access response (RAR) message is sent from the target cell. The time interval for monitoring is called the RAR window. When the RAR is received during a specific time (1f-45), the UE sends a handover complete message (1f-55) to the target BS in the RRC reconfiguration complete message.When the UE successfully receives the RAR from the target BS, the UE stops or ends the T304 timer (1f-50). In order to switch the path of the bearer configured for the source BS, the target BS requests a path switch (1f-60 and 1f-65) and instructs the source BS to discard the UE context of the UE (1f-70). The target BS may send an RRC message (e.g., an RRCReconfiguration message 1f-71) to the UE, and may instruct the UE to release the link to the source BS by using an indicator in the RRC message. In another method, the target BS may send MAC control information or RLC control information or PDCP control information to the UE, thereby instructing the UE to release the link to the source BS. Therefore, the UE attempts to receive data from the target BS at the starting point of the RAR window, and after receiving the RAR message, the UE sends an RRC reconfiguration completion message and receives DL transmission resources or UL transmission resources, thereby starting to send and receive data to and from the target BS.

[0166] The UE may configure multiple first bearers with the source BS, and may perform data transmission and reception (UL or DL ​​data transmission and reception) for each of the multiple first bearers via a protocol layer entity (PHY layer entity, MAC layer entity, RLC layer entity, PDCP layer entity, etc.), however, for ease of description, it is assumed that the UE has one bearer. That is, it is obvious that the embodiments of the present disclosure are applicable to the case where the UE has multiple bearers.

[0167] Figure 1G is a diagram for describing Embodiment 1 of a handover method for minimizing a data interruption time due to handover according to an embodiment of the present invention.

[0168] Reference Figure 1G In operation 1g-01, when UE 1g-20 sends data to or receives data from source BS 1g-05 and then receives a handover command message from source BS 1g-05, UE 1g-20 may release the link to source BS 1g-05 based on the handover method indicated by the handover command message (e.g., RRCReconfiguration message). A random access procedure may be performed on target BS 1g-10, and a handover procedure may be performed. In another method, in order to minimize a data interruption time occurring in a handover based on the indicated handover method, the UE may continuously send data to and receive data from source BS 1g-05.

[0169] In operation 1g-02, when UE 1g-20 performs a random access procedure on the target BS 1g-10 according to the handover method indicated by the handover command message, or sends a preamble code or initially transmits data in a UL transmission resource by using a PUCCH or PUSCH transmission resource, UE1g-20 may stop sending and receiving data to and from the source BS 1g-05 (UL data transmission and DL data reception).

[0170] In operation 1g-03, the UE 1g-20 may complete a random access procedure with respect to the target BS 1g-10, may send a handover complete message, and may start transmitting and receiving data (UL data transmission and DL data reception) to the target BS 1g-10.

[0171] Figure 1H 2 is a diagram for describing Embodiment 2 of a switching method for minimizing a data interruption time due to switching according to an embodiment of the present disclosure.

[0172] In operation 1h-01, in the case where UE 1h-20 receives a handover command message from the source BS 1h-05 when UE 1h-20 transmits or receives data with the source BS 1h-05, if the handover command message indicates a handover method (e.g., a DAPS handover method) according to the above-mentioned embodiment 2 or indicates handover of each bearer, even if UE 1h-20 has received the handover command message, UE 1h-20 may continuously send and receive data to and from the source BS 1h-05 via the protocol layer entity 1h-22 for the first bearer, so as to minimize the data interruption time occurring in the handover. In addition, when the RRC layer entity identifies an indication of a handover method (e.g., a DAPS handover method) according to embodiment 2 of the present disclosure in the received handover command message, or identifies an identifier of a DAPS handover method for each bearer, the RRC layer entity may send the indicator to a PDCP layer entity corresponding to each bearer or a bearer indicating the DAPS handover method. When the PDCP layer entity receives the indicator, the PDCP layer entity can switch from the first PDCP layer entity architecture 1l-11 or 1l-12 to the second PDCP layer entity architecture 1l-20. To this end, the UE 1h-20 can receive a switching command message (RRCReconfiguration message) from the BS. In addition, when the UE 1h-20 switches to the second PDCP layer entity architecture according to the configuration included in the received switching command message, the UE1h-20 can pre-configure or pre-set the protocol layer entity (PHY layer entity or MAC layer entity or RLC layer entity or PDCP layer entity) 1h-21 of the second bearer for the target BS 1h-10. The security key for the target BS 1h-10 can be derived and updated, and the header (or data) compression context for the target BS1h-10 can be configured. In addition, when the UE 1h-20 receives a handover command message and the handover command message indicates a DAPS handover method according to an embodiment of the present disclosure or indicates a DAPS handover method for a specific bearer, or when the UE 1h-20 newly sets a PDCP reordering timer value when switching the first PDCP layer entity architecture or function 1i-11 or 1i-12 for each bearer or a bearer indicating a DAPS handover method to a second PDCP layer entity architecture or function 1i-20, the UE 1h-20 can update the variable used for reordering to the PDCPSN or the count (COUNT) value predicted to be received next time, can stop the reordering timer, and can restart the reordering timer. Then, when a handover command message (e.g., an RRCReconfiguration message) is received, the RRC layer entity of the UE can start a first timer (e.g., T304).When the UE performs a random access procedure to the target BS in order to perform a handover, the first timer may be stopped, and then the random access procedure is successfully completed (e.g., when the first condition proposed in the present disclosure is met), and if the handover fails and thus the first timer expires, when the link to the source BS is active, the UE may fall back and report its handover failure to the source BS, and may attempt link recovery. And when the link to the source BS is inactive, the UE may perform an RRC connection reestablishment procedure.

[0173] According to the handover command message, the second bearer may be configured and established to have the same identifier as the first bearer, so that no data interruption time occurs in each bearer. In addition, in embodiment 2, the PDCP layer entity for the first bearer and the PDCP layer entity for the second bearer may logically operate as one PDCP layer entity, and the reference Fig. 1I A detailed description about the operation is provided. In embodiment 2, when UE 1h-20 is configured to send UL data to both the source BS 1h-05 and the target BS 1h-10, in order to avoid the problem of reduced coverage due to insufficient transmission power of UE 1h-20, or to prevent link selection, by which when UE 1h-20 sends UL data, UE 1h-20 must determine which BS UE1h-20 must request transmission resources from and send UL data. UE 1h-20 may send UL data to only one of the source BS 1h-05 and the target BS 1h-10. Specifically, in embodiment 2, when the UE does not have the ability to simultaneously send UL data to different BSs on different frequencies or the same frequency (dual uplink transmission), the UE may send UL data to only one of the source BS and the target BS within one time unit. Therefore, UE 1h-20 can perform a scheduling request for only one BS of the source BS 1h-05 or the target BS 1h-10, can send a report (e.g., a buffer status report) about the size of multiple data items to be sent by the PDCP layer entity to only one BS of the source BS 1h-05 or the target BS 1h-10, can receive UL transmission resources, and can therefore send UL data to only one BS. In addition, even when UE 1h-20 receives a switching command message from the source BS 1h-05, the UE can prevent data loss due to HARQ retransmission by continuing data transmission and reception. In addition, in the above description, the MAC layer entity for the first bearer may not be reset to continue HARQ retransmission. In addition, the RLC layer entity in AM mode can continuously perform data transmission for RLC retransmission.

[0174] In another method, when the handover command message indicates embodiment 2 (DAPS handover method) of the effective handover method proposed in the present disclosure for each bearer, the UE may continuously send data to the source BS or continuously receive data from the source BS only for the PDCP layer entity or RLC layer entity or MAC layer entity corresponding to the bearer or logical channel identifier of which embodiment 2 (DAPS handover method) is indicated in the handover command message, or only for the data corresponding to the bearer or logical channel identifier. In addition, when the first condition proposed in the present disclosure is met (for example, when UL data transmission is handed over to the target BS), the UE may continuously send or receive RLC control data (RLC status report) or PDCP control data (ROHC feedback or PDCP status report) or HARQ retransmission to the source BS only for the PDCP layer entity or RLC layer entity or MAC layer entity corresponding to the bearer or logical channel identifier of which embodiment 2 (DAPS handover method) is indicated in the handover command message. In addition, when the handover command message indicates embodiment 2 (DAPS handover method) of the effective handover method proposed in the present disclosure for each bearer, the UE can stop sending data to or receiving data from the source BS relative to the PDCP layer entity or the RLC layer entity or the MAC layer entity, and the PDCP layer entity or the RLC layer entity or the MAC layer entity corresponds to the bearer or logical channel identifier for which embodiment 2 (DAPS handover method) is not indicated in the handover command message.

[0175] Then, the UE 1h-20 may receive a handover command message, and when the handover command message indicates a DAPS handover method or indicates a DAPS handover method for a specific bearer or newly configured QoS flow and bearer mapping information, the UE 1h-20 may switch the first SDAP layer entity architecture or function 1j-10 to the second SDAP layer entity architecture or function 1j-20 with respect to each bearer or the bearer indicated by the DAPS handover method according to the embodiment. In addition, the second SDAP layer entity architecture may be configured in such a manner that the existing first QoS flow and the existing bearer mapping information of the source BS 1h-05 are maintained to process the UL data to be sent to the source BS 1h-05 and the DL data to be received from the source BS 1h-05, and the second QoS flow and bearer mapping information newly configured in the handover command message may be configured for the target BS 1h-10, and the second QoS flow and bearer mapping information may be used to process the UL data to be sent to the target BS 1h-10 and the DL data to be received from the target BS 1h-10. That is, in the second SDAP layer entity architecture according to the embodiment of the present disclosure, the first QoS flow and bearer mapping information for the source BS 1h-05 or the second QoS flow and bearer mapping information for the target BS 1h-10 is maintained, so that the data for the source BS 1h-05 and the data for the target BS 1h-10 can be processed separately. The SDAP layer entity in the second SDAP layer entity architecture can identify whether the data received from the lower layer is the data received from the source BS 1h-05 or the data received from the target BS 1h-10 via information indicated by a 1-bit indicator of the SDAP header or a 1-bit indicator of the PDCP header or the PDCP layer entity. When the source BS 1h-05 or the target BS 1h-10 indicates the UE with the DAPS switching method with respect to each bearer by using the switching command message, the source BS 1h-05 or the target BS 1h-10 can always indicate the DAPS switching method with respect to the default bearer (default DRB), and by doing so, when data occurs in a new QoS flow that does not correspond to the QoS flow and bearer mapping information when performing the DAPS switching procedure, the source BS 1h-05 or the target BS 1h-10 can instruct the UE to always send UL data via the default bearer. When the DAPS switching method is not configured for the default bearer, UL data transmission with respect to the new QoS flow occurring in the switching is unavailable, so that a data interruption time may occur.

[0176] In another method, when a handover command message (e.g., an RRCReconfiguration message) is received, embodiment 2 (DAPS handover method) is indicated, and the SDAP layer entity configuration information or the second QoS flow and bearer mapping information for the target BS is configured in the RRC message, and when the first condition proposed in the present disclosure is met, the UE can apply the SDAP layer entity configuration information or the second QoS flow and bearer mapping information. In addition, when the handover command message indicates embodiment 2 (DAPS handover method) for each bearer, the UE can only maintain and apply the first QoS flow and bearer mapping information corresponding to the bearer indicating embodiment 2, and when the UE maintains the first QoS flow and bearer mapping information for the source BS, the UE can release or not apply the first QoS flow and bearer mapping information corresponding to the bearer not indicating embodiment 2. In addition, when the SDAP layer entity configuration information or the second QoS flow and bearer mapping information for the target BS is configured in the RRC message, the UE can apply the SDAP layer entity configuration information or the second QoS flow and bearer mapping information to send data to the target BS or receive data from the target BS when the first condition proposed in the present disclosure is met.

[0177] In operation 1h-02, when UE 1h-20 performs a random access procedure on the target BS 1h-10 indicated by the handover command message via the protocol layer entity for the second bearer, UE1h-20 can continue to send data to the source BS 1h-05 or continue to receive data (UL data transmission or DL ​​data reception) from the source BS 1h-05 via the protocol layer entity for the first bearer. In operation 1h-02, UE 1h-20 can perform a cell selection procedure or a cell reselection procedure, and can perform a random access procedure on the target cell indicated by the handover command message (RRC reconfiguration message) received from the source BS 1h-05.

[0178] In operation 1h-03, when the first condition is met, UE 1h-20 can stop sending UL data to the source BS 1h-05 via the protocol layer entity 1h-22 for the first bearer, and can send UL data to the target BS 1h-10 via the protocol layer entity 1h-21 for the second bearer, and can continuously receive DL data from the source BS 1h-05 and the target BS 1h-10 via the protocol layer entities for the first and second bearers. In operation 1h-03, UE 1h-20 may satisfy the first condition, so that UL transmission may be switched from source BS 1h-05 to target BS 1h-10, and specifically, UE 1h-20 may send UL data to source BS 1h-05 via the first bearer until UE 1h-20 satisfies the first condition, and when UE 1h-20 satisfies the first condition, UE1h-20 may stop sending UL data to source BS 1h-05 via the first bearer, and may start sending UL data to target BS 1h-10 via the second bearer. Specifically, in the second PDCP layer entity architecture proposed in the present disclosure, when the PDCP layer entity sends UL data via the first bearer and satisfies the first condition and thus receives an indicator from the lower layer (when the MAC layer entity succeeds in the random access process to the target BS) or the upper layer (when the first timer expires in the RRC layer entity), the PDCP layer entity may stop the transmission of UL data via the first bearer, may switch to the second bearer, and may start the transmission of UL data via the second bearer. In addition, as described in reference Fig. 1I In the proposed PDCP layer entity structure, the receiving PDCP layer entity 1h-21 for the second bearer and the receiving PDCP layer entity 1h-22 for the first bearer can operate as one entity, and the receiving PDCP layer entity 1h-21 can continuously and uninterruptedly receive data from the source BS 1h-05 or the target BS 1h-10 by using the stored transceiver data or SN information or information such as header compression and decompression context. The first condition can be one of the following conditions. The following first condition proposes a UL data transmission switching time point at which the transmission resources can be maximized and effectively used, and the data interruption time can be minimized.

[0179] -When the UE successfully completes a random access procedure to the target BS via a layer entity (e.g., a MAC layer entity) for a second bearer, or when the UE successfully completes a random access procedure on the target BS 1h-10 via a layer entity (e.g., a MAC layer entity) for a second bearer and receives an allocation of a first UL transmission resource from the target BS 1h-10, or when the UL transmission resource is first indicated to the UE 1h-20, it can be determined that the UE 1h-20 satisfies the first condition.

[0180] - For example, when the UE 1h-20 receives a handover command message from the source BS 1h-05 and receives an indication of random access to the target BS 1h-10, if the indicated random access is a contention-free random access (CFRA) (for example, if a predefined preamble or UE cell identifier (for example, a cell radio network temporary identifier (C-RNTI)) is allocated), then

[0181] -It can be determined that the random access procedure is successfully completed when the UE 1h-20 sends a predefined preamble code to the cell of the target BS and receives the RAR message, and therefore, when the UE 1h-20 receives the first UL transmission resource allocated, included or indicated in the RAR message, it can be determined that the first condition is satisfied. In another method, when the UE first receives the UL transmission resource after receiving the RAR message, it can be determined that the first condition is satisfied.

[0182] -When the UE 1h-20 receives the handover command message from the source BS 1h-05 and receives an indication of random access to the target BS 1h-10, if the indicated random access is contention-based random access (CBRA) (e.g., if no predefined preamble or UE cell identifier (e.g., C-RNTI) is allocated), then

[0183] -UE 1h-20 may send a preamble (e.g., a random preamble) to the cell of the target BS 1h-10, and may receive a RAR message. Here, when UE 1h-20 sends message 3 (e.g., a handover completion message) by using the UL transmission resources allocated, included, or indicated in the RAR message, and receives a contention resolution MAC CE indicating a solution to the contention from the target BS 1h-10 via message 4, or receives UL transmission resources via a PDCCH corresponding to the UE's C-RNTI, UE 1h-20 sends message 3 (e.g., a handover completion message) by using the UL transmission resources allocated, included, or indicated in the RAR message. UE 1h-20 may determine that the random access procedure to the target BS 1h-10 is successfully completed, and therefore, when the UE monitors the PDCCH and first receives the UL transmission resources or first indicates the UL transmission resources via the PDCCH corresponding to the UE's C-RNTI, it may be determined that the first condition is satisfied. In another method, when the size of the UL transmission resource allocated in the RAR message is sufficient and thus the UE 1h-20 can send message 3 and additionally send UL data, the UE 1h-20 can determine that the UE 1h-20 first receives the UL transmission resource and thus can determine that the first condition is satisfied. That is, when the UE 1h-20 receives the RAR message, the UE 1h-20 can determine that the UE 1h-20 first receives the UL transmission resource and thus can determine that the first condition is satisfied.

[0184] - If the handover command message received by the UE 1h-20 also indicates a handover method that does not request a random access procedure (no RACH handover), then

[0185] - If the handover command message includes UL transmission resources about the target BS 1h-10, then

[0186] -When UE 1h-20 sends message 3 (e.g., a handover complete message or an RRCReconfigurationComplete message) using the UL transmission resources of the target BS 1h-10, and receives a UE identity confirmation MACCE from the target BS 1h-10 via message 4, or receives UL transmission resources via a PDCCH corresponding to the C-RNTI of the UE, the UE 1h-20 can determine that the random access procedure is successfully completed and the first condition is satisfied. In another method, after successfully completing the random access procedure, when UE 1h-20 monitors the PDCCH and receives the first UL transmission resource via the PDCCH corresponding to the C-RNTI of the UE 1h-20, the UE 1h-20 can determine that the first condition is satisfied.

[0187] - If the handover command message does not include UL transmission resources for the target BS 1h-10, then

[0188] -When UE 1h-20 performs PDCCH monitoring on the target BS 1h-10 (or cell) and receives UL transmission resources through the PDCCH corresponding to the C-RNTI of UE 1h-20 or sends message 3 (for example, a handover completion message or an RRCReconfiguration Complete message) using the UL transmission resources, and receives a UE identity confirmation MAC CE from the target BS or receives UL transmission resources through the PDCCH, the CCH corresponding to the C-RNTI of the UE, UE 1h-20 can determine that the random access procedure is successfully completed and the first condition is met. In another method, after the random access procedure is successfully completed, when UE 1h-20 performs PDCCH monitoring and receives the first UL transmission resource through the PDCCH corresponding to the C-RNTI of UE 1h-20, UE 1h-20 can determine that the first condition is met.

[0189] According to an embodiment of the present disclosure, a method for switching UL data from a source BS to a target BS is provided, and the method is performed when a DAPS switching method is performed. A MAC layer entity or an RRC layer entity for a target BS corresponding to a second bearer can identify or detect whether a first condition is satisfied by one of the following methods, and a new method can be applied by combining the following methods.

[0190] -First method: For example, when the RRCReconfiguration message received by the UE indicates DAPS switching, the UE may configure a MAC layer entity for the target BS corresponding to the second bearer, and the MAC layer entity may perform a random access procedure and may identify whether the first condition is satisfied. When the first condition is satisfied, in the DAPS switching method, the MAC layer entity may instruct an upper layer (e.g., a PDCP layer entity) to switch UL data transmission from the source BS to the target BS via the first bearer by using an indicator.

[0191] -Second method: For example, when the RRCReconfiguration message received by the UE indicates DAPS switching, the UE may configure a MAC layer entity for the target BS corresponding to the second bearer, and the MAC layer entity may perform a random access procedure and may identify whether the first condition is met. When the first condition is met, the MAC layer entity may indicate to the upper layer (e.g., the RRC layer entity) that the first condition is met. In addition, in the DAPS switching method, the upper layer (e.g., the RRC layer entity) may instruct the lower layer (e.g., the PDCP layer entity) to switch UL data transmission from the source BS to the target BS via the first bearer by using an indicator. When the first condition proposed in the present disclosure is met or the random access procedure on the target BS is successfully performed, the upper layer (e.g., the RRC layer entity) stops the first timer, and therefore, when the first timer stops, the RRC layer entity may instruct the PDCP layer entity to switch UL data transmission by using an indicator.

[0192] -Third method: When the RRCReconfiguration message received by the UE indicates DAPS switching, the UE may configure a MAC layer entity for the target BS corresponding to the second bearer. Here, in the case where an indicator indicating that the RRC layer entity of the UE performs DAPS switching is delivered to a lower layer (e.g., a MAC layer entity), the MAC layer entity may perform a random access procedure and may identify whether the first condition is satisfied. When the first condition is satisfied, in the DAPS switching method, the MAC layer entity may instruct an upper layer (e.g., a PDCP layer entity) to switch UL data transmission from the source BS to the target BS via the first bearer by using an indicator.

[0193] -Fourth method: When the RRCReconfiguration message received by the UE indicates DAPS switching, the UE may configure a MAC layer entity for the target BS corresponding to the second bearer. Here, in the case where an indicator indicating that the RRC layer entity of the UE performs DAPS switching is delivered to a lower layer (e.g., a MAC layer entity), the MAC layer entity may perform a random access procedure and may identify whether the first condition is met. When the first condition is met, the MAC layer entity may indicate to the upper layer (e.g., an RRC layer entity) that the first condition is met. In the case of an identification indicator, when the first condition proposed in the present disclosure is met or the random access procedure on the target BS is successfully performed, the upper layer (e.g., an RRC layer entity) may stop the first timer. In the DAPS switching method, the upper layer (e.g., an RRC layer entity) may instruct the lower layer (e.g., a PDCP layer entity) to switch UL data transmission from the source BS to the target BS via the first bearer by using an indicator.

[0194] When the PDCP layer entity receives (for example, when the DAPS switching method is indicated) an indicator indicating that the first condition is satisfied or an indicator indicating that UL data transmission is switched from the source BS to the target BS from an upper layer (for example, an RRC layer entity) or a lower layer (for example, a MAC layer entity) according to the first method or the second method or the third method or the fourth method, the PDCP layer entity may perform the following operations to effectively perform the switching of the UL data transmission, and may perform one or more of the following operations to prevent data loss due to the UL data transmission. The following operations may be applied to a PDCP layer entity (an RLC layer entity operating in an AM mode or an RLC layer entity operating in an UM mode) connected to an AM DRB or a UM DRB. If the PDCP layer entity has data to be sent to a buffer before the first condition is satisfied or before receiving an indicator indicating that the first condition is satisfied, the PDCP layer entity may indicate to the MAC layer entity of the source BS corresponding to the first bearer that there is data to be sent by indicating the size or amount of data to be sent (for example, the PDCP data amount). And UL data transmission to the source BS may be performed. Then, the MAC layer entity of the source BS corresponding to the first bearer may perform a scheduling request or buffer status reporting process to receive an allocation of UL transmission resources from the source BS. However, when the first condition is met or the PDCP layer entity receives an indicator indicating that the first condition is met, switching the UL data transmission to the target BS may be performed as follows.

[0195] In order to switch UL data transmission from a first bearer of a source BS to a second bearer of a target BS, the PDCP layer entity may indicate to the MAC layer entity of the first bearer that the size or amount of data to be transmitted is 0 (or none). That is, the PDCP layer entity may indicate to the MAC layer entity for the first bearer that the amount of data of the PDCP layer entity (PDCP data amount) is 0, thereby indicating that there is no more data to be sent (even when the buffer actually stores a plurality of data items to be sent, in order to switch UL data transmission, the PDCP layer entity may indicate to the MAC layer entity of the source BS corresponding to the first bearer that there is no more data to be sent). However, in the case of indicating the switching method (DAPS switching method) of embodiment 2 proposed in the present disclosure or the switching method (DAPS switching method) of embodiment 2 for a bearer indication, when RLC control data (RLC status report) or PDCP control data (PDCP status report or ROHC feedback) is generated, the amount of data corresponding to the RLC control data or the PDCP control data may be indicated to the MAC layer entity, and data transmission to the source BS may be performed.

[0196] -The PDCP layer entity connected to the AM DRB (i.e., the RLC layer entity operating in the AM mode) may discard all pre-stored PDCP PDUs (e.g., without discarding the PDCP SDUs to prevent the original data from being lost), and may perform the following process on a plurality of data items (buffered PDCP SDUs) in ascending order of a count (COUNT) value (or PDCPSN) assigned before a first condition is satisfied or before an indicator indicating that the first condition is satisfied is received, the ascending order starting from the first data (e.g., PDCP SDU) for which successful delivery is not confirmed by a lower layer (e.g., the RLC layer entity of the source BS corresponding to the first bearer). Specifically, the PDCP layer entity connected to the AM DRB (i.e., the RLC layer entity operating in the AM mode) may perform a new header compression process on a plurality of data items (buffered PDCP PDUs) based on a header context of the target BS, may re-perform an integrity process or a ciphering process by applying a security key of the target BS, may configure a PDCP header, and may transmit the PDCP header to its lower layer entity (the RLC layer entity of the target BS corresponding to the second bearer), thereby performing retransmission or transmission. That is, cumulative retransmission of data starting from the first data that has not been confirmed to be successfully delivered can be performed. In another method, when retransmission of data is performed, retransmission can be performed only on multiple data items that have not been confirmed to be successfully delivered by a lower layer (e.g., an RLC layer entity of a source BS corresponding to a first bearer). More specifically, a PDCP layer entity (or an RLC layer entity operating in an AM mode) connected to an AM DRB can discard all stored PDCP PDUs to be sent to the source BS via a first protocol layer entity previously connected to the PDCP layer entity (e.g., PDCP SDUs may not be discarded to prevent loss of original data), and can perform the following process (e.g., PDCP SDUs) only for multiple data items that have not been confirmed to be successfully delivered by a lower layer (e.g., an RLC layer entity), which is a first protocol layer entity of the source BS, based on a count (COUNT) value (or PDCP SN) assigned before the first condition is met or before an indicator indicating that the first condition is met is received. Specifically, the PDCP layer entity (or the RLC layer entity working in the AM mode) connected to the AM DRB may perform a new header or data compression process for multiple data items (e.g., PDCP SDU) for which successful delivery is not confirmed by applying a header compression (or data compression) protocol context or a security key corresponding to the target BS, may re-execute an integrity process or a ciphering process, may configure a PDCP header, and may configure and transmit the PDCP header to a lower layer entity as a second protocol layer entity, thereby performing retransmission or transmission to the target BS. That is, in order to prevent waste of transmission resources, multiple data items for which successful delivery is not confirmed are only selectively retransmitted.The above-mentioned transmission or retransmission operation may be performed after a lower layer (e.g., a transmitting or receiving RLC layer entity or a MAC layer entity) as a first protocol layer entity for transmitting data to a source BS is released. When the transmission or retransmission process is extended to a UM DRB, a PDCP layer entity connected to an RLC layer entity operating in a UM mode may treat data that has not been transmitted to a lower layer entity, data to which a PDCP discard timer has not expired, or a plurality of data items to which a PDCP SN (or a count value) has been assigned as a plurality of data items received or newly received from an upper layer entity. The PDCP discard timer for each data may not be restarted, and header (or data) compression may be performed on a plurality of data items by using a header (or data) compression context or a security key for a target BS, or a ciphering or integrity protection process may be performed, a PDCP header may be generated and connected to a plurality of data items, and then transmission or retransmission may be performed, and data may be processed in ascending order of a count value assigned before the process is triggered, and then transmission or retransmission may be performed. The window state variable of the PDCP layer entity connected to a UM DRB or an AM DRB may not be reset, and may be constantly maintained and used.

[0197] When the PDCP layer entity has data to be sent to the buffer, the PDCP layer entity may indicate to the MAC layer entity of the target BS corresponding to the second bearer that there is data to be sent by indicating the size or amount of the data to be sent (e.g., the PDCP data amount), and may perform switching of UL data transmission to the target BS. Then, the MAC layer entity of the target BS corresponding to the second bearer may perform a scheduling request or buffer status report procedure to receive allocation of UL transmission resources from the target BS.

[0198] In embodiment 2 of the handover method (e.g., DAPS handover method) according to an embodiment of the present disclosure, even after the UE receives a handover command message (e.g., RRCReconfiguration message), the UE may continuously receive DL data from the source BS or the target BS via a protocol layer entity of the source BS corresponding to the first bearer or the target BS corresponding to the second bearer. In order to allow the UE to seamlessly receive DL data from the source BS (or target BS) or allow the source BS (or target BS) to seamlessly send DL data, for an AM bearer, the UE may be allowed to continuously perform UL transmission of an RLC status report on the source BS (or target BS) instead of UL transmission of data via the protocol layer entity used for the first bearer (or the second bearer). That is, even in the case where the first condition is met and the UE thus switches UL data transmission to the target BS, when the UE must send an RLC status report, HARQ ACK or NACK, or PDCP control data (PDCP ROHC feedback or PDCP status report) to the source BS, the UE may be allowed to send data via the first bearer of the source BS. In the case of an AM bearer, when data is transmitted to the transmitting end and then successful delivery is not indicated by using an RLC status report (ie, when no RLC status report is received), data cannot be continuously transmitted thereafter. Figure 1H , in operation 1h-03 of embodiment 2, even when UE 1h-20 satisfies the first condition and stops transmitting UL data to the source BS 1h-05 through the protocol layer entity 1h-22 for the first bearer, switching to the target BS 1h-10 is performed, and then transmission of UL data to the target BS 1h-10 through the protocol layer entity 1h-21 for the second bearer is started. UE1h-20 can continuously send HARQ ACK or HARQ NACK information, RLC status report (ACK or NACK information), or PDCP control data (for example, PDCP status report or PDCP PROHC feedback information) via the protocol layer entity for the first bearer (or the second bearer) so as to seamlessly receive DL data from the source BS 1h-05 (or target BS 1h-10) or allow the source BS 1h-05 (or target BS 1h-10) to smoothly send DL data. Similarly, referring to Figure 1H, in operation 1h-03 of embodiment 2, even when UE 1h-20 satisfies the first condition and thus stops transmitting UL data to the source BS 1h-05 via the protocol layer entity 1h-22 for the first bearer, switching to the target BS 1h-10 is performed, and then UL data transmission to the target BS 1h-10 via the protocol layer entity 1h-21 for the second bearer is started. UE 1h-20 can continuously perform data transmission due to HARQ retransmission of the MAC layer entity or continuously perform data transmission due to retransmission of the RLC layer entity in the AM mode to prevent data loss to the source BS 1h-05. When UE 1h-20 satisfies the first condition and stops transmitting UL data to the source BS 1h-05 through the protocol layer entity 1h-22 for the first bearer, switches to the target BS 1h-10, and then starts transmitting UL data to the target BS 1h-10 through the protocol layer entity 1h-21 for the second bearer, the source BS 1h-05 or the target BS 1h-10 can allocate transmission resources to UE 1h-20 at different time points to prevent conflict between the UL transmission resources to the target BS 1h-10 and the UL transmission resources to the source BS1h-05. When the UL transmission resources to the target BS 1h-10 conflict with the UL transmission resources to the source BS 1h-05 and thus overlap with the UL transmission resources to the source BS 1h-05, the UE 1h-20 may perform data transmission to the source BS 1h-05 by giving priority to the UL transmission resources to the source BS 1h-05 so as to maintain the transmission of DL data or continuously receive DL data from the source BS 1h-05 without problem. In another method, when the UL transmission resources to the target BS 1h-10 conflict with the UL transmission resources to the source BS 1h-05 and thus overlap with the UL transmission resources to the source BS 1h-05, the UE 1h-20 may perform data transmission to the target BS 1h-10 by giving priority to the UL transmission resources to the target BS 1h-10 so as to maintain the transmission of DL data from the target BS 1h-10.

[0199] Specifically, when UE 1h-20 receives a switching command message indicating or indicating a switching corresponding to Embodiment 2 (DAPS switching method) for each bearer, until the first condition is met, UE 1h-20 or the bearer indicating DAPS switching can perform a scheduling request via a first protocol layer entity, can receive UL transmission resources by sending a buffer status report to the source BS 1h-05, can send UL data, and can receive DL data from the source BS 1h-05. However, when the first condition is met, UE 1h-20 no longer sends data to the source BS 1h-05, can perform a scheduling request via a second protocol layer entity by switching the UL to the target BS 1h-10, can receive UL transmission resources by sending a buffer status report to the target BS 1h-10, and can send UL data to the target BS 1h-10. However, UE 1h-20 can continuously receive DL data from the source BS 1h-05, and even after UL transmission is switched, UE 1h-20 can continuously send HARQ ACK or HARQ NACK corresponding to DL data, RLC status report or PDCP control data (e.g., PDCP status report or ROHC feedback information). Moreover, even when the first condition is met, UE 1h-20 can continuously receive DL data from the source BS 1h-05 or the target BS 1h-10.

[0200] When the second condition is met in operation 1h-04, UE 1h-20 may stop receiving DL data from the source BS 1h-05 via the protocol layer entity 1h-22 for the first bearer, or may release the link to the source BS 1h-05. The second condition may be one of the following conditions. In addition, the PDCP layer entity 1h-21 for the second bearer may continuously and uninterruptedly perform sending data to or receiving data from the target BS 1h-10 by using the transceived data for the first bearer or SN information or information such as header compression and decompression context stored in the PDCP layer entity 1h-22.

[0201] - When the UE 1h-20 performs a random access procedure on the target BS 1h-10 via the layer entity 1h-21 for the second bearer and receives the RAR, the UE 1h-20 may determine that the second condition is satisfied.

[0202] - When the UE 1h-20 performs a random access procedure on the target BS 1h-10 via the layer entity for the second bearer, receives the RAR, and configures and sends a handover complete message to the target BS 1h-10, the UE 1h-20 may determine that the second condition is satisfied.

[0203] -When the UE 1h-20 performs a random access procedure on the target BS 1h-10 via the layer entity for the second bearer and first sends data on a PUCCH or PUSCH UL transmission resource or first receives a PUCCH or PUSCH UL transmission resource, the UE 1h-20 may determine that the second condition is met.

[0204] - When the source BS 1h-05 configures a separate timer for the UE 1h-20 through an RRC message and the separate timer expires, the UE 1h-20 may determine that the second condition is satisfied.

[0205] -The timer may be started when UE 1h-20 receives a handover command message from the source BS 1h-05, or when UE 1h-20 starts random access (sending a preamble) to the target BS 1h-10, or when UE 1h-20 receives a RAR from the target BS 1h-10, or when UE 1h-20 sends a handover completion message to the target BS 1h-10, or when UE 1h-20 sends data for the first time on PUCCH or PUSCH UL transmission resources.

[0206] -When UE 1h-20 performs a random access procedure on the target BS 1h-10 via a protocol layer entity for a second bearer, receives an RAR, configures and sends a handover completion message to the target BS 1h-10, and then identifies a confirmation of successful delivery of the handover completion message through a MAC layer entity (HARQ ACK) or an RLC layer entity (RLC ACK), UE 1h-20 can determine that the second condition is met.

[0207] -When UE 1h-20 performs a random access procedure on the target BS 1h-10 via a protocol layer entity for the second bearer, receives an RAR or configuration and sends a switching completion message to the target BS 1h-10, and then first receives an allocation of UL transmission resources from the target BS 1h-10 or first receives an indication of UL transmission resources, UE 1h-20 can determine that the second condition is met.

[0208] -When the source BS 1h-05 performs a handover process, the source BS 1h-05 may determine when to stop sending DL data to the UE 1h-20 or when to release the link to the UE 1h-20. For example, when to stop sending DL data to the UE 1h-20 or when to release the link to the UE 1h-20 may be determined as follows: when a certain timer expires (the timer may be started after the handover is indicated) or when the source BS 1h-05 receives an indication from the target BS 1h-10 indicating that the UE 1h-20 has successfully performed a handover to the target BS 1h-10. When the UE 1h-20 does not receive DL data from the source BS 1h-05 within a certain period of time, the UE 1h-20 may determine that the second condition is met, and may determine that the link to the source BS 1h-05 is released, and thus the link may be released.

[0209] -UE 1h-20 may receive an indication from the target BS 1h-10 indicating the release of the link to the source BS 1h-05. For example, when UE 1h-20 receives an RRC message (e.g., an RRCReconfiguration message) or a MAC CE or an RLC control PDU or a PDCP control PDU, UE 1h-20 may determine that the second condition is met.

[0210] -When UE 1h-20 receives an indication (e.g., an RRC message (e.g., an RRCReconfiguration message)) or a MAC CE or an RLC control PDU or a PDCP control PDU from the source BS 1h-05 indicating the release of the link to the source BS 1h-05, UE 1h-20 may determine that the second condition is met.

[0211] -When the UE 1h-20 does not receive DL data from the source BS 1h-05 for a certain period of time, the UE 1h-20 may determine that the second condition is satisfied.

[0212] -When the UE successfully completes a random access procedure to the target BS via a layer entity (e.g., a MAC layer entity) for the second bearer or when the UE successfully completes a random access procedure on the target BS 1h-10 via a layer entity for the second bearer and then receives an allocation of a first UL transmission resource from the target BS 1h-10 or when the UE 1h-20 first receives an indication of a UL transmission resource, the UE 1h-20 can determine that the second condition is met.

[0213] - For example, when the UE 1h-20 receives a handover command message from the source BS 1h-05 and receives an indication of random access to the target BS 1h-10, if the indicated random access is CFRA (for example, if a predefined preamble or UE cell identifier (for example, C-RNTI) is allocated), then

[0214] -It can be determined that when the UE 1h-20 sends a predefined preamble code to the cell of the target BS 1h-10 and receives the RAR message, the random access procedure is successfully completed, and therefore, when the UE 1h-20 receives the first UL transmission resource allocated, included or indicated in the RAR message, the UE 1h-20 can determine that the second condition is satisfied. In another method, when the UE 1h-20 first receives the UL transmission resource after receiving the RAR, the UE 1h-20 can determine that the second condition is satisfied.

[0215] - When the UE 1h-20 receives the handover command message from the source BS 1h-05 and receives an indication of random access to the target BS 1h-10, if the indicated random access is CBRA (e.g., if no predetermined preamble or UE cell identifier (e.g., C-RNTI) is allocated), then

[0216] -The UE may send a preamble (e.g., a random preamble) to the cell of the target BS, may receive a RAR message, and may send a message 3 (e.g., a handover completion message) by using the UL transmission resources allocated or included or indicated in the RAR message. In addition, when the UE receives a contention resolution MAC CE indicating a solution to the contention from the target BS via message 4 or receives UL transmission resources via a PDCCH corresponding to the C-RNTI of the UE, the UE may determine that the random access procedure on the target BS is successfully completed, and therefore, when the UE monitors the PDCCH and first receives UL transmission resources via a PDCCH corresponding to the C-RNTI of the UE or first indicates UL transmission resources, the UE may determine that the second condition is satisfied. In another method, when the size of the UL transmission resources allocated in the RAR message is sufficient and therefore the UE may send message 3 and additionally send UL data, the UE may determine that the UE first receives the UL transmission resources and therefore may determine that the second condition is satisfied. That is, when the UE receives the RAR, the UE may determine that the UE first receives the UL transmission resources and therefore may determine that the second condition is satisfied.

[0217] - If the handover command message received by the UE also indicates a handover method that does not request a random access procedure (no RACH handover), then

[0218] - If the handover command message includes the UL transmission resources about the target BS, then

[0219] -When the UE sends message 3 (e.g., a handover complete message or an RRCReconfigurationComplete message) by using the UL transmission resources of the target BS, the UE may receive a UE identity confirmation MAC CE from the BS via message 4, or receive the UL transmission resources via a PDCCH corresponding to the C-RNTI of the UE, the UE may determine that the random access procedure is successfully completed and may determine that the second condition is satisfied. In another method, after successfully completing the random access procedure, when the UE performs PDCCH monitoring and thus receives a first UL transmission resource via a PDCCH corresponding to the C-RNTI of the UE, the UE may determine that the second condition is satisfied.

[0220] - If the handover command message does not include UL transmission resources for the target BS, then

[0221] -When the UE performs PDCCH monitoring on the target BS (or cell) and receives UL transmission resources through the PDCCH corresponding to the UE's C-RNTI, or sends message 3 (e.g., a handover complete message or an RRCReconfigurationComplete message) by using the UL transmission resources and receives a UE identity confirmation MAC CE from the BS or receives UL transmission resources through the PDCCH corresponding to the UE's C-RNTI, the UE can determine that the random access procedure is successfully completed and can determine that the second condition is satisfied. In another method, after successfully completing the random access procedure, when the UE performs PDCCH monitoring and thus receives a first UL transmission resource via the PDCCH corresponding to the UE's C-RNTI, the UE can determine that the second condition is satisfied.

[0222] When the UE performs embodiment 2 of the handover method (e.g., DAPS handover method) according to an embodiment of the present disclosure, if it is identified that the RRC layer entity, MAC layer entity, or RLC layer entity of the UE for the source BS corresponding to the first bearer, and the RRC layer entity, MAC layer entity, or RLC layer entity of the UE for the target BS corresponding to the second bearer satisfy the second condition, an indicator indicating that the second condition is satisfied may be indicated to the PDCP layer entity of the UE or the bearer performing the DAPS handover method. When the PDCP layer entity of the UE receives the indicator indicating that the second condition is satisfied from the lower layer entity or the upper layer entity, one or more of the following processes may be performed so that the handover process according to embodiment 2 of the present disclosure may be successfully completed.

[0223] -The UE may release the first bearer for the source BS and may release the link to the source BS. Then, before the UE releases the first bearer for the source BS, the UE may perform an RLC reestablishment procedure on an RLC layer entity corresponding to the first bearer for the source BS (for example, when the reordering timer is running, the UE may stop or reset the timer, and when received data is stored in the buffer, the UE may process and provide the stored data to the upper layer entity. In addition, when data to be sent exists in the buffer, the UE may discard the data, or may reset the MAC layer entity.

[0224] -When the UE releases the link to the source BS in order to report the reception status of multiple DL data items received from the source BS to the target BS, the UE may trigger a PDCP status reporting procedure, may configure a PDCP status report, and may send a PDCP status report to the target BS.

[0225] -When the second condition is met, the UE may switch from the second PDCP layer entity architecture or function 1i-20 to the first PDCP layer entity architecture or function 1i-11 or 1i-12 with respect to each bearer or bearer indicating the DAPS switching method, may reset the variable for reordering, may stop and reset the reordering timer, may perform a decryption process or header (or data) decompression by applying the security key or header decompression context of the source BS to a plurality of data items stored in a buffer for reordering (e.g., a plurality of data items received from the source BS), and then may discard the security key or header decompression context of the source BS. Then, the plurality of processed data items may be provided to an upper layer in ascending order. That is, when the second condition is met, the UE may perform a decryption process or header (or data) decompression by applying the security key or header decompression context of the source BS to a plurality of data items stored in a buffer for reordering (e.g., a plurality of data items received from the source BS), and then may discard the security key or header decompression context of the source BS. In another method, when the second condition is met, the UE may switch from the second PDCP layer entity architecture or function 1i-20 to the third PDCP layer entity architecture or function 1i-30 with respect to each bearer or bearer indicating the DAPS switching method, and may not stop or reset, but may continuously use the variables and timers for reordering. However, the UE may perform a decryption process or header (or data) decompression by applying the security key or header decompression context of the source BS to a plurality of data items stored in a buffer for reordering (e.g., a plurality of data items received from the source BS), and then may discard the security key or header decompression context of the source BS. Then, the plurality of processed data items may be provided to the upper layer in ascending order. That is, when the second condition is met, the UE may perform a decryption process or header (or data) decompression by applying the security key or header decompression context of the source BS to a plurality of data items stored in a buffer for reordering (e.g., a plurality of data items received from the source BS), and then may discard the security key or header decompression context of the source BS. UE 1h-20 may release the QoS mapping information of the SDAP layer entity or the security key information of the PDCP layer entity of the source BS, or the header (or data) compression context information of the source BS, or the RLC layer entity or MAC layer entity of the source BS.

[0226] -In the case where the second condition is satisfied when the UE performs the DAPS switching method, the UE may release the first bearer of the source BS relative to the second SDAP layer entity architecture and function 1j-20 that has been applied to each bearer or the bearer indicating the DAPS switching method, and may switch back to the first SDAP layer entity architecture or function 1j-10. In addition, when the second condition is satisfied, the UE may switch from the second SDAP layer entity architecture and function 1j-20 to the first SDAP layer entity architecture or function 1j-10 relative to each bearer or the bearer indicating the DAPS switching method, and may maintain the second bearer or the second QoS flow and the bearer mapping information for the target BS, and before the UE releases the first bearer or the first QoS flow and the bearer mapping information for the source BS, the UE may complete data processing by applying the first QoS flow and the bearer mapping information to a plurality of data items received from the source BS (e.g., all data received from the source BS), and then may release the first QoS flow and the bearer mapping information or the first bearer. Then, the plurality of processed data items may be provided to the upper layer in ascending order. That is, when the second condition is met, the UE may process the data (e.g., a process of reading SDAP header information and updating mapping information or configuring an SDAP header, or a process of routing or providing the processed data to an appropriate upper layer entity or a lower layer entity based on the first QoS flow and the bearer mapping information) by applying the first QoS flow and the bearer mapping information of the source BS to a plurality of data items stored in a buffer (e.g., a plurality of data items received from the source BS), and then may discard the first QoS flow of the source BS and the bearer mapping information of the source BS. The SDAP layer entity may define information indicated by a 1-bit indicator of a new SDAP header or a new PDCP header or SDAP control data (e.g., a DL end marker) or a 1-bit indicator of a PDCP layer entity, and may identify which data has been recently received from the source BS based on the information. Therefore, the SDAP layer entity may perform data processing on the data, which has been recently received from the source BS, by applying the first QoS flow and the bearer mapping information of the source BS, and then may discard the first QoS flow and the bearer mapping information of the source BS. Then, the SDAP layer entity may continuously maintain the second QoS flow and the bearer mapping information, and may process UL data or DL ​​data with respect to the target BS based on the second QoS flow and the bearer mapping information.

[0227] When the above Figure 1FWhen the BS in sends a handover command message to the UE (1f-20), the BS may define indicators related to the above-mentioned embodiments in the handover command message (e.g., RRCReconfiguration message), and may indicate to the UE which handover process corresponding to which embodiment will be triggered, and the UE may perform the handover process according to the handover method indicated in the handover command message, and may perform the handover method (DAPS handover method) according to Embodiment 2 of the present disclosure, and may thus perform handover to the target BS while minimizing the data interruption time. In another method, the BS may define indicators for each bearer in the handover command message, which indicators are related to the above-mentioned embodiments, and may further specifically indicate which embodiment is applied to which bearer in the handover. For example, Embodiment 2 may be indicated as being applied only to an AM bearer in which an RLC layer entity working in AM mode is active, or may be applied to an UM bearer in which an RLC layer entity working in UM mode is active. In addition, it is assumed that an embodiment of the present disclosure is applied to a DRB. However, the aforementioned embodiments may be applied to SRB when necessary (eg, where the UE maintains the SRB for the source BS and fails to perform handover to the target BS, and thus the UE may report a handover failure message via the SRB for the source BS or may restore the link).

[0228] In an embodiment of the present disclosure, when the UE performs sending and receiving data to and from a source BS via a protocol layer entity for a first bearer and performs sending and receiving data to and from a source BS via a protocol layer entity for a second bearer, the MAC layer entity for the first bearer and the MAC layer entity for the second bearer can each operate a discontinuous reception (DRX) cycle, thereby reducing battery consumption in the UE. That is, even after the UE receives a switching command message, the UE can continuously apply the DRX cycle of the MAC layer entity applied when sending and receiving data via the protocol layer entity for the first bearer, and can stop the DRX cycle according to the first condition or the second condition. In addition, the UE can manage whether to apply the DRX cycle to the MAC layer entity of the second bearer separately in response to an indication from the target BS.

[0229] Similarly, the meaning of the UE stopping UL transmission to the source BS via the protocol layer entity for the first bearer and stopping receiving DL data from the source BS via the protocol layer entity for the first bearer may mean that the UE reestablishes or resets or releases the protocol layer entity (PHY layer entity or MAC layer entity or RLC layer entity or PDCP layer entity) for the first bearer.

[0230] In the embodiments of the present disclosure, for the convenience of description, it is described that the UE configures the first bearer for the source BS or the second bearer for the target BS, and the embodiments of the present disclosure can be equally applied to the case where the UE configures multiple first bearers for the source BS or multiple second bearers for the target BS. In addition, the embodiments of the present disclosure can be equally applied to the case where multiple bearers of multiple target BSs are configured. For example, the UE can configure the second bearer while performing a handover process on the first target BS, and when the handover fails, the UE configures the second bearer while performing a handover process on the second target BS, so that the UE can autonomously detect and determine a cell that meets a specific condition (for example, a signal with a strength equal to or greater than a specific value) from multiple target BSs. A cell can be determined, and then the handover process can be performed.

[0231] Fig. 1I 1 is a diagram for describing an architecture of an effective PDCP layer entity to be applied to a DAPS handover method and embodiment 2 of a handover method using the architecture according to an embodiment of the present disclosure.

[0232] Reference Fig. 1I , provides a specific architecture and function of an effective PDCP layer entity applied to a DAPS switching method according to Embodiment 2 of the present disclosure. Here, when performing a DAPS switching process, different PDCP layer entity architectures may be applied to corresponding bearers at different time points.

[0233] For example, before the UE receives a handover command message from the BS, the UE may process and send or receive data by applying the first PDCP layer entity architecture and function 1i-11 or 1i-12 to each bearer (1i-01).

[0234] However, when the UE receives a handover command message from the BS, and the handover command message indicates the DAPS handover method proposed in the present disclosure or indicates the DAPS handover method for a specific bearer, the UE can process and send or receive data by applying the second PDCP layer entity architecture and function 1i-20 to each bearer or the bearer indicating the DAPS handover method (1i-02). That is, when the UE receives a handover command message, and the handover command message indicates the DAPS handover method or indicates the DAPS handover method for a specific bearer, the UE can switch from the first PDCP layer entity architecture or function 1i-11 or 1i-12 for each bearer to the second PDCP layer entity architecture or function 1i-20 relative to each bearer or the bearer indicating the DAPS handover method. In another method, when the first condition is met, the UE can switch from the first PDCP layer entity architecture or function 1i-11 or 1i-12 for each bearer to the second PDCP layer entity architecture or function 1i-20 (1i-02) relative to each bearer or the bearer indicating the DAPS handover method. In addition, when the UE receives a switching command message and the switching command message indicates a DAPS switching method or indicates a DAPS switching method for a specific bearer, or when the UE switches from the first PDCP layer entity architecture or function 1i-11 or 1i-12 to the second PDCP layer entity architecture or function 1i-20 proposed in the present disclosure for each bearer or a bearer indicating a DAPS switching method, and a PDCP reordering timer value is newly set, the UE can update the variable used for reordering to the PDCP SN or count value predicted to be received next, and can stop and restart the reordering timer.

[0235] In the case where the second condition is satisfied when the UE performs the DAPS switching method proposed in the present disclosure, the UE may release the second PDCP layer entity architecture and function 1i-20 applied to each bearer or the bearer indicating the DAPS switching method from the first bearer of the source BS, and may switch back to the first PDCP layer entity architecture and function 1i-11 or 1i-12, and may apply the first PDCP layer entity architecture and function 1i-11 or 1l-12. When the second condition is satisfied and the UE switches from the second PDCP layer entity architecture or function 1i-20 to the first PDCP layer entity architecture or function 1i-11 or 1i-12 relative to each bearer or the bearer indicating the DAPS switching method, the UE may reset the variable for reordering, and may stop and reset the reordering timer. Then, the UE may perform a decryption process or header (or data) decompression by applying the security key or header decompression context of the source BS to a plurality of data items stored in a buffer for reordering (e.g., a plurality of data items received from the source BS), and may discard the security key or header decompression context of the source BS. The plurality of processed data items may be provided to an upper layer in ascending order. That is, when the second condition is satisfied, the UE may perform a decryption process or header (or data) decompression by applying the security key or header decompression context of the source BS to the plurality of data items stored in the buffer for reordering (e.g., the plurality of data items received from the source BS), and then may discard the security key or header decompression context of the source BS.

[0236] In another method, in the case where the second condition is satisfied when the UE performs the DAPS switching method, the UE may release the second PDCP layer entity architecture and function 1i-20 applied to each bearer or the bearer indicating the DAPS switching method from the bearer of the source BS, and may switch to the third PDCP layer entity architecture or function 1i-30, and may apply the third PDCP layer entity architecture or function 1i-30. When the second condition is satisfied and the UE is therefore switched from the second PDCP layer entity architecture or function 1i-20 to the third PDCP layer entity architecture or function 1i-30 relative to each bearer or the bearer indicating the DAPS switching method, the UE may not stop or reset, but may continuously use the variables and reordering timers for reordering. However, the UE may perform a decryption process or header (or data) decompression by applying the security key or header decompression context of the source BS to a plurality of data items (e.g., a plurality of data items received from the source BS) stored in a buffer for reordering, and then may discard the security key or header decompression context of the source BS. Then, the plurality of processed data items may be provided to the upper layer in ascending order. That is, when the second condition is met, the UE can perform a decryption process or header (or data) decompression by applying the security key or header decompression context of the source BS to multiple data items stored in a buffer for reordering (e.g., multiple data items received from the source BS), and then discard the security key or header decompression context of the source BS.

[0237] Reference Fig. 1I , when the UE performs switching by applying the first PDCP layer entity architecture and function 1i-11 or 1i-12, the second PDCP layer entity architecture and function 1i-20, or the third PDCP layer entity architecture and function 1i-30, which are different from each other, to respective bearers at different time points, data loss may not occur and data interruption time may be minimized.

[0238] Fig. 1I The first PDCP layer entity structure 1i-11 or 1i-12 may have a 1-1PDCP layer entity architecture or a 1-2PDCP layer entity architecture or a 1-3PDCP layer entity architecture or a 1-4PDCP layer entity architecture, and may have the following features.

[0239] 1> (When it is 1-1PDCP layer entity architecture) For example, when the UE applies the first PDCP layer entity architecture and function 1i-11 to a PDCP layer entity (e.g., E-UTRA PDCP layer entity or LTE PDCP layer entity) connected to an AM RLC layer entity (e.g., E-UTRA AM RLC layer entity), the UE may have the following features.

[0240] 2> The receiving PDCP layer entity may first perform detection of out-of-window data or duplicate data on multiple received data items. (Retransmission may occur in RLC AM, and the sizes of LTE RLC SN and PDCP SN may be different, so that duplicate data or out-of-window data may be received. In the above, the window indicates the range of PDCP SN or count value for receiving valid data.)

[0241] 3> Before the UE discards the out-of-window data or duplicate data, the UE performs a decryption process and a header decompression process, and then performs a discard operation. (Because the data may include useful information for the header decompression process (e.g., initialization and refresh (IR) packets or header compression information), the UE may check and then discard the data.)

[0242] 2> The UE can decrypt multiple data items immediately without sorting, the data is received without being discarded, and the header decompression process can be performed. This is because the E-UTRA AM RLC layer entity performs sorting on multiple data items and provides the multiple data items to the PDCP layer entity.

[0243] 2> Then, the plurality of data items are provided to the upper layer in ascending order of the count values.

[0244] 1> (when it is a 1-2PDCP layer entity architecture), for example, when the UE applies the first PDCP layer entity architecture and function 1i-11 to a PDCP layer entity (e.g., an E-UTRA PDCP layer entity or an LTE PDCP layer entity) connected to a UM RLC layer entity (e.g., an E-UTRA UM RLC layer entity), the UE may have the following features.

[0245] 2> The process of detecting data outside the window or duplicate data may not be performed. This is because the UM E-UTRA RLC layer entity does not perform the retransmission process.

[0246] 2>The UE may then immediately perform a decryption process on the multiple received data items, followed by a header decompression process.

[0247] 2> The UE may then perform a reordering procedure and may then provide the multiple data items (e.g., in ascending order) to its upper layer.

[0248] 1> (When it is a 1-3 PDCP layer entity architecture) For example, when the UE applies the first PDCP layer entity architecture and function 1i-11 to a PDCP layer entity (for example, an E-UTRA PDCP layer entity or an LTE PDCP layer entity) configured for a sliced ​​bearer or a packet copy bearer or an LTE WLAN aggregation (LWA) bearer, the UE can always apply the reordering procedure and the reordering timer, and can have the following characteristics.

[0249] 2> The UE may be configured to first perform out-of-window data or duplicate data detection on multiple received data items. (Retransmission may occur in RLC AM, or data may be received from different RLC layer entities at different time points, and the sizes of LTE RLC SN and PDCP SN may be different, so that out-of-window data or duplicate data may be received.)

[0250] 3> The UE may perform the decryption process. However, the UE may not perform the header decompression process. (This is because the E-UTRA PDCP layer cannot configure the header compression protocol for the sliced ​​bearer or LWA bearer).

[0251] 3> When integrity protection or verification procedures have been performed, if the integrity verification procedure fails, the received data may be discarded. If the integrity verification procedure fails, the data may be discarded and the failure may be reported to an upper layer entity.

[0252] 3>Discard data outside the window or duplicate data.

[0253] 2> When the data is not discarded, the UE can immediately perform the decryption process without reordering the multiple received data items. Then, when the integrity protection or verification process is configured, the integrity verification can be performed. When the integrity protection or verification process is performed, the data can be discarded after performing the integrity protection or verification process. When the integrity verification process fails, the data can be discarded and the failure can be reported to the upper layer entity.

[0254] 2>Afterwards, reordering may be performed on the plurality of received data items, and when the PDCP SNs or count values ​​are arranged sequentially in ascending order without gaps therebetween, a header compression process may be performed on the data (when the header compression process or the header decompression process is configured), and the data may be provided to an upper layer in ascending order.

[0255] 2> If the reordering timer is running, then

[0256] 3> When data is provided to an upper layer entity, the data corresponding to the count value has the same value as the value obtained by subtracting 1 from the value of the variable maintained for reordering, or when multiple data items are all provided to the upper layer entity without gaps between PDCP SNs (count values), then

[0257] 4>UE stops and resets the reordering timer.

[0258] 2> If the reorder timer is not running, then

[0259] 3> When the buffer stores data that is not provided to the upper layer entity, or when there is a gap between PDCP SNs (count values), then

[0260] 4>UE starts the reordering timer.

[0261] 4> The UE then updates the variable used for reordering to the PDCP SN or count value predicted to be received next time.

[0262] 2> If the reordering timer expires, then

[0263] 3> When the header decompression process is configured for the value of multiple stored data items, the header decompression process is performed and the data is provided to the upper layer entity, the value is less than the variable used for reordering, and the header decompression process is performed on the data in ascending order of the PDCP SN or count value, and the data is provided to the upper layer entity.

[0264] 3> When the header decompression process is configured for the value of multiple stored data items, the value is equal to or greater than the variable used for reordering, and the header decompression process is performed on the data in ascending order of PDCP SN or count value, and the data is provided to the upper layer entity.

[0265] 3> The UE then updates the variable value of the data most recently provided to the upper layer to the PDCP SN or count value of the data most recently provided to the upper layer.

[0266] 3> When the buffer stores data that is not provided to the upper layer entity, or when there is a gap between PDCP SNs (count values), then

[0267] 4>Reorder timer starts.

[0268] 4> The UE then updates the variable used for reordering to the PDCP SN or count value predicted to be received next time.

[0269] 1> (When it is 1-4PDCP layer entity architecture) For example, when the UE applies the first PDCP layer entity architecture and functions 1l-12 to the NR PDCP layer entity, the UE can always apply the reordering process and reordering timer, and can have the following characteristics.

[0270] 2> The UE may first perform a decryption process on the plurality of received data items.

[0271] 2> When an integrity protection or verification process is configured, the integrity protection or verification process may be performed on the received data, and when the integrity verification process fails, the data may be discarded and the failure may be reported to the upper layer entity.

[0272] 2> The UE performs detection of out-of-window data or duplicate data on the received data. (A decryption process may be performed, and then detection of out-of-window data or duplicate data may be performed. The UE may perform the decryption process only when the integrity protection or verification process is configured, and then detection of out-of-window data or duplicate data may be performed, or when the integrity protection or verification process is not configured, the UE may perform the decryption process only on a plurality of data items for which detection of out-of-window data or duplicate data is performed and which are not discarded.)

[0273] 3>UE discards data outside the window or duplicate data.

[0274] 2> When data is not discarded, the UE can perform reordering on multiple received data items, and when the PDCP SNs or count values ​​are arranged sequentially in ascending order without gaps therebetween, a header compression process can be performed (when the header compression process or the header decompression process is configured), and the data can be provided to the upper layer in ascending order.

[0275] 2>Then, the data is provided to the upper layer in ascending order of the count value.

[0276] 2> If the reorder timer is running, then

[0277] 3> When data is provided to an upper layer entity, the data corresponding to the count value has the same value as the value obtained by subtracting 1 from the value of the variable for reordering, or when a plurality of data items are all provided to an upper layer entity without a gap between PDCP SNs (count values), or when the value of the variable storing the PDCP SN or count value of data to be provided to an upper layer is equal to or greater than the value of the variable for reordering, then

[0278] 4>UE stops and resets the reordering timer.

[0279] 2> If the reorder timer is not running, then

[0280] 3> When the buffer stores data that is not provided to the upper layer entity, or when there is a gap between PDCP SNs (count values), or when the value of the variable storing the count value of the first data that is not provided to the upper layer is less than the value of the variable for reordering, then

[0281] 4> The UE updates the variable used for reordering to the PDCP SN or count value predicted to be received next time.

[0282] 4>Reorder timer starts.

[0283] 2> If the reordering timer expires, then

[0284] 3> When the header decompression process is configured for a value of multiple stored data items, the value is less than the variable used for reordering, and the header decompression process is performed on the data in ascending order of the PDCP SN or count value, and the data is provided to the upper layer entity.

[0285] 3> When the header decompression process is configured for the value of multiple stored data items, the value is equal to or greater than the variable used for reordering, and the header decompression process is performed on the data in ascending order of PDCP SN or count value, and the data is provided to the upper layer entity.

[0286] 3> Then, the UE updates the variable value of the first data not provided to the upper layer to the PDCP SN or count value of the first data not provided to the upper layer.

[0287] 3> When the buffer stores data that is not provided to the upper layer, or when there is a gap between PDCP SNs (count values), or when the value of the variable storing the count value of the first data that is not provided to the upper layer is less than the value of the variable for reordering, then

[0288] 4> The UE updates the variable used for reordering to the PDCP SN or count value predicted to be received next time.

[0289] 4>Reorder timer starts.

[0290] Fig. 1I The second PDCP layer entity architecture 1i-20 may have the 2-1PDCP layer entity architecture or the 2-2PDCP layer entity architecture proposed in the present disclosure, and may have the following features.

[0291] The present disclosure provides a second PDCP layer entity architecture 1i-20 that is effective in handover. The second PDCP layer entity architecture can be applied to Embodiment 2 of the effective handover method for minimizing data interruption time proposed in the present disclosure.

[0292] In the second PDCP layer entity architecture, the UE can send data to and receive data from the source BS 1i-21 via the protocol layer entity for the first bearer (for example, the SDAP layer entity or the PDCP layer entity or the RLC layer entity or the MAC layer entity), and can send data to and receive data from the target BS 1i-22 via the protocol layer entity for the second bearer (for example, the SDAP layer entity or the PDCP layer entity or the RLC layer entity or the MAC layer entity).

[0293] Each of the PDCP layer entity for the first bearer and the PDCP layer entity for the second bearer can be configured in the UE, but can be logically operated as one PDCP layer entity, as shown in 1i-20. Specifically, by distinguishing the functions of the PDCP layer entity, one PDCP layer entity can be implemented as the functions of an upper PDCP layer entity (such as an SN allocation function or a reordering function or an in-sequence delivery function or a duplicate detection function) and the functions of two lower PDCP layer entities for the source BS and the target BS, respectively (such as a decryption or encryption function, a header (or data) compression or decompression function, an integrity protection or verification function, or a duplicate detection function). In addition, as described above, when performing the DAPS switching method, the UE can be configured to send UL data transmission to the source BS, switch to the target BS when the first condition is met, and continuously receive DL data from the source BS and the target BS. Therefore, only one header (or data) compression protocol context of the source BS or the target BS can be maintained and applied to the UL, and two contexts of the source BS or the target BS can be maintained and applied to the DL.

[0294] The 2-1 PDCP layer entity architecture based on the second PDCP layer entity architecture (eg, an E-UTRA PDCP layer entity for a DAPS handover method) may have the following features.

[0295] The function of the upper layer sending PDCP layer entity can be used to assign a PDCP SN to multiple data items received from the upper layer entity. The functions of the two lower layer sending PDCP layer entities 1i-21 and 1i-22 for the source BS and the target BS, respectively, can be used to apply a header (or data) compression context or a security key configured for the source BS to the data to be sent to the source BS by using a separate security key configured for each of the source BS and the target BS, and apply the header (or data) compression context or the security key configured for the target BS to the data to be sent to the target BS, and the header (or data) compression process can be applied when the header (or data) compression process is configured. When integrity protection is configured, the functions of the lower layer sending PDCP layer entities 1i-21 and 1i-22 can be used to apply a ciphering process by applying an integrity protection process to the PDCP header and data (PDCP SDU), the data to be sent to the source BS can be provided to the sending RLC layer entity for the first bearer, and the data to be sent to the target BS can be provided to the sending RLC layer entity for the second bearer, thereby performing transmission. In order to speed up data processing, the functions of the two lower-layer transmission PDCP layer entities 1i-21 and 1i-22 can be used to perform parallel processing or encryption processes of header compression or integrity protection, and the functions of the two lower-layer transmission PDCP layer entities can be used to perform integrity protection or encryption processes by using different security keys. In addition, compression or integrity protection or encryption processes for multiple different data items can be performed by applying different compression contexts or different security keys or different security algorithms in the logically integrated transmission PDCP layer entity.

[0296] The function of the receiving PDCP layer entity as the function of the lower layer receiving PDCP layer entities 1i-21 and 1i-22 of the source BS or the target BS can each independently perform an out-of-window data detection or a duplicate detection process on multiple data items received from each lower layer entity (for example, multiple data items received from two RLC layer entities of the source BS and the target BS, respectively) based on the PDCP SN or the count value. In another method, for ease of implementation, the out-of-window data detection or the duplicate detection process can be performed on all received data based on the PDCP SN or the count value without distinguishing the RLC layer entities. In another method, for more accurate duplicate detection, out-of-window data detection can be performed on all received data based on the PDCP SN or the count value without distinguishing the RLC layer entities, and the duplicate detection process can be independently performed on multiple data items received from each RLC layer entity. In another method, when multiple data items received from different BSs overlap with each other, in order to prevent data loss of the header compression protocol, for multiple data items received from each RLC layer entity, out-of-window data detection can be performed on all received data based on the PDCP SN or count value without distinguishing the RLC layer entities, and a repeated detection process can be performed on all data after receiving the decryption process or the integrity protection process or the header (or data) decompression process.

[0297] The sub-function of the receiving PDCP layer entity can be used to immediately apply the decryption process to multiple received data items by using a separate header (or data) compression context or security key configured for each of the source BS and the target BS, and when integrity protection is configured, the integrity verification process can be applied to the PDCP header and data (PDCP SDU).

[0298] In a 2-1 PDCP layer entity architecture, a header (or data) decompression process may be performed immediately on a plurality of data items received from an RLC layer entity of a source BS corresponding to a first bearer without reordering, and a header (or data) decompression process may be performed immediately on a plurality of data items received from an RLC layer entity of a target BS corresponding to a second bearer without reordering. In addition, in order to distinguish between data received from an RLC layer entity of a source BS corresponding to the first bearer and data received from an RLC layer entity of a target BS corresponding to the second bearer, the 2-1 PDCP layer entity may define an indicator for each data so that the UE may identify whether the data is received from the source BS or the target BS. In another method, the 2-1 PDCP layer entity may define a 1-bit indicator in a PDCP header, an SDAP header, or an RLC header so that the UE may identify whether the data is received from the source BS or the target BS. In addition, the 2-1PDCP layer entity may perform a duplicate detection process (a process in which only one data (including pre-received data or data provided to an upper layer) is allocated to each PDCP SN or each count value and the other data are discarded) on all of the multiple data items received from the RLC layer entity of the source BS corresponding to the first bearer and the multiple data items received from the RLC layer entity of the target BS corresponding to the second bearer based on the PDCP SN or the count value, wherein the header (or data) compression process has been completed for the multiple data items. Then, the 2-1PDCP layer entity may perform a reordering process on all of the multiple data items received from the RLC layer entity of the source BS corresponding to the first bearer and the multiple data items received from the RLC layer entity of the target BS corresponding to the second bearer in ascending order based on the PDCP SN or the count value, and may provide the multiple data items to the upper layer entity in sequence. Because one PDCP layer entity may receive data out of order from different BSs (i.e., from the first bearer or the second bearer), the PDCP layer entity may always have to perform a reordering process.

[0299] In order to speed up data processing, the functions of the two lower-layer receiving PDCP layer entities can perform parallel processing or encryption processes of header compression or integrity protection based on each PDCP SN or each count value. In addition, the integrity protection or encryption process or header decompression process can be performed by using different header (or data) compression contexts or different security keys. The integrity protection or encryption process or decompression process can be performed on multiple different data items by applying different header (or data) compression contexts or different security keys or different security algorithms in a logically integrated sending PDCP layer entity. In addition, the function of the lower-layer receiving PDCP layer entity can perform an out-of-order decryption or integrity verification process on each of the multiple received data items, which is independent of the order of the PDCP SN or count value.

[0300] When a PDCP layer entity distinguishes a layer entity for a first bearer from a layer entity for a second bearer, considering that the layer entity for the first bearer and the layer entity for the second bearer are connected to different MAC layer entities, or have different logical channel identifiers, or are connected to different MAC layer entities or different RLC layer entities using different encryption keys, the PDCP layer entity can distinguish the layer entity for the first bearer (or the first RLC layer) from the layer entity for the second bearer (or the second RLC layer entity), and then, an encryption process or a decryption process can be performed on the UL data and the DL data by using different security keys, and the UL data and the DL data can be compressed or decompressed by using different compression protocol contexts.

[0301] The 2-2 PDCP layer entity architecture based on the second PDCP layer entity architecture (eg, an NR PDCP layer entity for a DAPS switching method) may have the following features.

[0302] The function of the sending PDCP layer entity can be used to assign a PDCP SN to multiple data items received from an upper layer entity. The functions of the two lower layer sending PDCP layer entities 1i-21 and 1i-22 for the source BS and the target BS, respectively, can be used to: apply the header (or data) compression context or the security key configured for the source BS to the data to be sent to the source BS by using a separate security key configured for each of the source BS and the target BS, and apply the header (or data) compression context or the security key configured for the target BS to the data to be sent to the target BS, and the header (or data) compression process can be applied when the header (or data) compression process is configured. When integrity protection is configured, the functions of the lower layer sending PDCP layer entities 1i-21 and 1i-22 can be used to apply a ciphering process by applying an integrity protection process to the PDCP header and data (PDCP SDU), and the data to be sent to the source BS can be provided to the sending RLC layer entity for the first bearer, and the data to be sent to the target BS can be provided to the sending RLC layer entity for the second bearer, thereby performing data transmission. In order to speed up data processing, the functions of the two lower-layer transmission PDCP layer entities 1i-21 and 1i-22 can be used to perform parallel processing or encryption processes of header compression or integrity protection, and the functions of the two lower-layer transmission PDCP layer entities can be used to perform integrity protection or encryption processes by using different security keys. In addition, compression or integrity protection or encryption processes for multiple different data items can be performed by applying different compression contexts or different security keys or different security algorithms in a logically integrated transmission PDCP layer entity.

[0303] The function of the receiving PDCP layer entity, which is the function of the lower layer receiving PDCP layer entities 1i-21 and 1i-22 for the source BS or the target BS, can independently perform out-of-window data detection or a duplicate detection process on multiple data items received from each lower layer entity, especially multiple data items received from two RLC layer entities for the source BS and the target BS, respectively, based on the PDCP SN or the count value. In another method, for convenience of implementation, the receiving PDCP layer entity can perform out-of-window data detection or a duplicate detection process on all received data based on the PDCP SN or the count value without distinguishing the RLC layer entities. In another method, for more accurate duplicate detection, the receiving PDCP layer entity can perform out-of-window data detection on all received data based on the PDCP SN or the count value without distinguishing the RLC layer entities, and can independently perform a duplicate detection process on multiple data items received from each RLC layer entity. In another method, when multiple data items received from different BSs overlap with each other, in order to prevent data loss of the header compression protocol, the receiving PDCP layer entity can perform out-of-window data detection on all received data based on the PDCPSN or count value without distinguishing the RLC layer entity, and for multiple data items received from each RLC layer entity, a repeated detection process can be performed on all data after receiving the decryption process or integrity protection process or header (or data).

[0304] The sub-function of the receiving PDCP layer entity can be used to immediately apply the decryption process to multiple received data items by using a separate header (or data) compression context or security key configured for each of the source BS and the target BS, and when integrity protection is configured, the integrity verification process can be applied to the PDCP header and data (PDCP SDU).

[0305] In a 2-2PDCP layer entity architecture, a reordering process may be performed on a plurality of data items received from an RLC layer entity of a source BS corresponding to a first bearer and a plurality of data items received from an RLC layer entity of a target BS corresponding to a second bearer, and then a header (or data) decompression process may be performed on the plurality of data items received from each BS (source BS or target BS) in ascending order of a PDCP SN or a count value by applying a header (or data) compression context of each BS (source BS or target BS). In addition, in order to distinguish between data received from an RLC layer entity of a source BS corresponding to a first bearer and data received from an RLC layer entity of a target BS corresponding to a second bearer, the 2-2PDCP layer entity may define an indicator for each data so that the UE may identify whether the data is received from the source BS or the target BS. In another method, the 2-2PDCP layer entity may define a 1-bit indicator in a PDCP header or an SDAP header or an RLC header so that the UE may identify whether the data is received from the source BS or the target BS. In addition, the 2-2PDCP layer entity may perform a duplicate detection process (a process in which only one data (including pre-received data or data provided to an upper layer) is allocated to each PDCP SN or each count value and the other data are discarded) on all of the multiple data items received from the RLC layer entity of the source BS corresponding to the first bearer and the multiple data items received from the RLC layer entity of the target BS corresponding to the second bearer based on the PDCP SN or the count value, wherein the header (or data) compression process has been completed for the multiple data items. Then, the 2-2PDCP layer entity may sequentially provide the upper layer entity with all of the multiple data items received from the RLC layer entity of the source BS corresponding to the first bearer and the multiple data items received from the RLC layer entity of the target BS corresponding to the second bearer in ascending order based on the PDCP SN or the count value. Because one PDCP layer entity may receive data out of order from different BSs (i.e., from the first bearer or the second bearer), the PDCP layer entity may always have to perform a reordering process.

[0306] In order to speed up data processing, the functions of the two lower-layer receiving PDCP layer entities can perform parallel processing or encryption processes of header compression or integrity protection based on each PDCP SN or each count value, and can perform integrity protection or encryption processes or header decompression processes by using different header (or data) compression contexts or different security keys. The integrity protection or encryption process or decompression process can be performed on multiple different data items by applying different header (or data) compression contexts or different security keys or different security algorithms in the logically integrated sending PDCP layer entity. In addition, the function of the lower-layer receiving PDCP layer entity can perform an out-of-order decryption or integrity verification process on each of the multiple received data items, which is independent of the order of the PDCP SN or count value.

[0307] When a PDCP layer entity distinguishes between a first bearer layer entity and a second bearer layer entity, considering that the first bearer layer entity and the second bearer layer entity are connected to different MAC layer entities, or have different logical channel identifiers, or are connected to different MAC layer entities or different RLC layer entities using different encryption keys, the PDCP layer entity can separate the first bearer layer entity (or the first RLC layer entity) from the second bearer layer entity (or the second RLC layer entity), and then, an encryption process or a decryption process can be performed on the UL data and the DL data by using different security keys, and the UL data and the DL data can be compressed or decompressed by using different compression protocol contexts.

[0308] The present disclosure provides a third PDCP layer entity architecture 1i-30 for performing a switching process. The third PDCP layer entity structure can be applied to embodiment 2 of the switching method to minimize the data interruption time. In addition, the PDCP layer entity function in the third PDCP layer entity architecture can be equivalent to the second PDCP layer entity architecture. However, the third PDCP layer entity architecture may correspond to an architecture for releasing the first bearer for the source BS in the second PDCP layer entity architecture. Specifically, the third PDCP layer entity architecture may have the same function as the second PDCP layer entity architecture, but may have an architecture for releasing the first bearer for the source BS (e.g., SDAP layer entity or PDCP layer entity or RLC layer entity or MAC layer entity). Therefore, the third PDCP layer entity architecture may correspond to an architecture for releasing the QoS mapping information of the SDAP layer entity of the source BS or the security key information of the PDCP layer entity of the source BS or the header (or data) compression context information of the source BS or the RLC layer entity or MAC layer entity of the source BS.

[0309] Figure 1Jis a diagram for describing an architecture of a valid SDAP layer entity to be applied to a DAPS switching method and an embodiment 2 of a switching method using the architecture according to an embodiment of the present disclosure.

[0310] Reference Figure 1J , provides a specific architecture and function of an effective SDAP layer entity to be applied to the DAPS switching method of embodiment 2 of the switching method according to an embodiment of the present invention, and when the DAPS switching process is performed, different SDAP layer entity architectures can be applied to corresponding bearers at different time points.

[0311] For example, before the UE receives a switching command message from the BS, the UE can process and send or receive data by applying the first SDAP layer entity architecture and function 1j-10 proposed in the present disclosure to each bearer (1j-01). In the first SDAP layer entity architecture, the SDAP layer entity can perform data processing (e.g., a process of reading SDAP header information and updating mapping information or configuring an SDAP header, or a process of routing or providing processed data to an appropriate upper layer entity or lower layer entity based on the first QoS flow and bearer mapping information) on multiple UL data items to be sent or multiple DL data items received (e.g., multiple data items received from a source BS) by maintaining and applying the first QoS flow and bearer mapping information for the source BS.

[0312] However, when the UE receives a switching command message from the BS, and the switching command message indicates the DAPS switching method proposed in the present disclosure or indicates the DAPS switching method for a specific bearer, the UE can process and send or receive data by applying the second PDCP layer entity architecture and function 1j-20 proposed in the present disclosure to each bearer or the bearer indicating the DAPS switching method (1j-02). That is, when the UE receives a switching command message from the BS and the switching command message indicates the DAPS switching method proposed in the present disclosure or indicates the DAPS switching method for a specific bearer, the UE can switch from the first SDAP layer entity architecture or function 1j-10 used for each bearer to the second SDAP layer entity architecture or function 1j-20 proposed in the present disclosure for each bearer or the bearer indicating the DAPS switching method. In another method, when the first condition proposed in the present disclosure is met, the UE can switch from the first SDAP layer entity architecture or function 1j-10 used for each bearer to the second SDAP layer entity architecture or function 1j-20 proposed in the present disclosure for each bearer or the bearer indicating the DAPS switching method (1j-02). In addition, when the UE receives a handover command message from the BS, and the handover command message indicates the DAPS handover method proposed in the present disclosure, indicates the DAPS handover method for a specific bearer, or newly configures the QoS flow and bearer mapping information, the UE can switch from the first SDAP layer entity architecture or function 1j-10 to the second SDAP layer entity architecture or function 1j-20 proposed in the present disclosure for each bearer or the bearer indicating the DAPS handover method. In addition, the second SDAP layer entity architecture can be configured in such a way that the existing first QoS flow and the existing bearer mapping information for the source BS are maintained to process the UL data to be sent to the source BS and the DL data to be received from the source BS, and the second QoS flow and bearer mapping information newly configured in the handover command message can be configured for the target BS, and the UE can use the configured information to process the UL data to be sent to the target BS and the DL data to be received from the target BS. That is, in the second SDAP layer entity architecture proposed in the present disclosure, the first QoS flow and bearer mapping information for the source BS or the second QoS flow and bearer mapping information for the target BS are maintained, so that the data for the source BS and the data for the target BS can be processed separately. In the second SDAP layer entity architecture, the SDAP layer entity can identify whether the data received from the lower layer is received from the source BS or the target BS through a 1-bit indicator in the SDAP header or a 1-bit indicator in the PDCP header or information indicated by the PDCP layer entity.When the BS indicates the DAPS switching method for each bearer to the UE through the switching command message, the BS can always indicate the DAPS switching method for the default DRB, and therefore, when data occurs in a new QoS flow that does not correspond to the QoS flow and bearer mapping information when performing the DAPS switching procedure, the BS can instruct the UE to always send UL data via the default bearer. When the DAPS switching method is not configured for the default bearer, UL data transmission for the new QoS flow occurring in the switching is unavailable, so that a data interruption time may occur.

[0313] In the case where the second condition is satisfied when the UE performs the DAPS switching method, the UE may release the first bearer of the source BS relative to the second SDAP layer entity architecture and function 1j-20 that has been applied to each bearer or the bearer indicating the DAPS switching method, and may switch back to the first SDAP layer entity architecture or function 1j-10 and apply the first SDAP layer entity architecture or function 1j-10. In addition, when the second condition is satisfied, the UE may switch from the second SDAP layer entity architecture and function 1j-20 to the first SDAP layer entity architecture or function 1j-10 proposed in the present disclosure for each bearer or the bearer indicating the DAPS switching method, and may maintain the second bearer or the second QoS flow and the bearer mapping information for the target BS. And before the UE releases the first bearer or the first QoS flow and the bearer mapping information for the source BS, the UE may complete the data processing by applying the first QoS flow and the bearer mapping information to multiple data items received from the source BS (e.g., all data received from the source BS), and then may release the first QoS flow and the bearer mapping information or the first bearer. Then, the multiple processed data items may be provided to the upper layer in ascending order. That is, when the second condition is met, the UE may process the data (e.g., a process of reading SDAP header information and updating mapping information or configuring an SDAP header, or a process of routing or providing the processed data to an appropriate upper layer entity or lower layer entity based on the first QoS flow and bearer mapping information) by applying the first QoS flow and bearer mapping information of the source BS to a plurality of data items stored in a buffer (e.g., a plurality of data items received from the source BS), and then may discard the first QoS flow and bearer mapping information of the source BS. The SDAP layer entity may define and apply information indicated by a 1-bit indicator of a new SDAP header or a new PDCP header or SDAP control data (e.g., a DL end marker) or a 1-bit indicator of a PDCP layer entity, and may identify which data was recently received from the source BS based on the information. Therefore, the SDAP layer entity may perform data processing on the data, which was recently received from the source BS, by applying the first QoS flow and bearer mapping information of the source BS, and then may discard the first QoS flow and bearer mapping information of the source BS. Then, the SDAP layer entity may continuously maintain the second QoS flow and the bearer mapping information, and may process UL data or DL ​​data with respect to the target BS based on the second QoS flow and the bearer mapping information.

[0314] In the above mentioned Figure 1F In the present invention, when the UE receives a handover command message and applies the bearer configuration information included in the handover command message, the UE may apply the bearer configuration information using different schemes according to the handover type indicated in the handover command message.

[0315] -When the UE receives the handover command message, if the ReconfigWithSync information indicates the first handover method (for example, embodiment 1 of the present disclosure or the normal handover method),

[0316] -When the default bearer is configured in the SDAP layer entity configuration information configured in the handover command message, the UE may configure the default bearer of the source BS as the default bearer of the target BS indicated in the configuration information.

[0317] -When the second QoS flow and bearer mapping information are configured in the SDAP layer entity configuration information configured in the handover command message, the UE may release the first QoS flow and bearer mapping information applied to the source BS, and may apply the second QoS flow and bearer mapping information. In another method, the UE may replace the first QoS flow and bearer mapping information applied to the source BS with the second QoS flow and bearer mapping information.

[0318] - When the data discard timer value is configured in the PDCP layer entity configuration information configured in the handover command message, the UE may apply the discard timer value to the PDCP layer entity corresponding to the bearer identifier of the configuration information.

[0319] -When the drb-ContinueROHC indicator is configured as false (False) in the PDCP layer entity configuration information configured in the handover command message, the UE may reset the context of the header compression or decompression protocol in the PDCP layer entity corresponding to the bearer identifier of the configuration information. When the drb-ContinueROHC indicator is configured as true (True), the UE does not reset the context of the header compression or decompression protocol in the PDCP layer entity corresponding to the bearer identifier of the configuration information.

[0320] - When the reordering timer value is configured in the PDCP layer entity configuration information configured in the handover command message, the UE may immediately apply the reordering timer value to the PDCP layer entity corresponding to the bearer identifier of the configuration information.

[0321] -When the security key configuration information or the security algorithm is configured in the security configuration information configured in the handover command message, the UE may derive a new security key or new security configuration information by using the configuration information and release the existing security key or existing security configuration information, or may replace the existing security key or existing security configuration information with the new security key or new security configuration information.

[0322] -When a new logical channel identifier is configured in the RLC layer entity configuration information configured in the handover command message, the UE may release the existing logical channel identifier corresponding to the bearer identifier indicated in the RLC layer entity configuration information, or may replace the existing logical channel identifier with the new logical channel identifier.

[0323] - When the RLC re-establishment procedure is configured in the RLC layer entity configuration information configured in the handover command message, the UE may perform the RLC re-establishment procedure on the RLC layer entity corresponding to the bearer identifier indicated in the RLC layer entity configuration information.

[0324] -When the RLC layer entity configuration information configured in the handover command message is newly configured, the UE may perform an RLC re-establishment procedure on the RLC layer entity corresponding to the bearer identifier indicated in the RLC layer entity configuration information.

[0325] -When a second priority regarding a logical channel is newly configured in the MAC layer entity configuration information configured in the switching command message, the UE may release the first priority corresponding to the identifier of the logical channel indicated in the configuration information, or may replace the first priority corresponding to the logical channel identifier with the newly configured second priority.

[0326] -When a second prioritisedBitRate (PBR) about a logical channel is newly configured in the MAC layer entity configuration information configured in the handover command message, the UE may release the first prioritisedBitRate (PBR) corresponding to the identifier of the logical channel indicated in the configuration information, or may replace the first prioritisedBitRate (PBR) corresponding to the identifier of the logical channel with the newly configured second prioritisedBitRate (PBR). PrioritisedBitRate refers to a value that increases at a predetermined time interval (e.g., at each TTI) with respect to each logical channel. When the UE receives UL transmission resources, the UE may perform a logical channel prioritization (LCP) procedure, and may send data about the logical channel in consideration of the priority and the prioritisedBitRate. At this point, the higher the priority, or the larger the value of the prioritisedBitRate, the more data that may be sent.

[0327] -When a second bucketSizeDuration about a logical channel is newly configured in the MAC layer entity configuration information configured in the handover command message, the UE may release the first bucketSizeDuration corresponding to the identifier of the logical channel indicated in the configuration information, or may perform the configuration by replacing the first bucketSizeDuration corresponding to the identifier of the logical channel with the newly configured second bucketSizeDuration. In the above description, the bucket capacity duration indicates a maximum value that the prioritisedBitRate may have when the prioritisedBitRate is accumulated.

[0328] -When the second available SCell information or available subcarrier spacing information or maximum PUSCH duration or logical channel group configuration information is configured in the MAC layer entity configuration information configured in the switching command message, the UE may release the pre-configured first available SCell information or the pre-configured available subcarrier spacing information or the pre-configured maximum PUSCH duration or the pre-configured logical channel group configuration information, or may perform the configuration by replacing the pre-configured first available SCell information or the pre-configured available subcarrier spacing information or the pre-configured maximum PUSCH duration or the pre-configured logical channel group configuration information with the newly configured second available SCell information or the newly configured available subcarrier spacing information or the newly configured maximum PUSCH duration or the newly configured logical channel group configuration information.

[0329] -When the UE receives the handover command message, when the ReconfigWithSync information indicates the second handover method (for example, the DAPS handover method of Embodiment 2 or the present disclosure) or indicates the DAPS handover method for each bearer identifier, then

[0330] -When the default bearer is configured in the SDAP layer entity configuration information configured in the handover command message, the UE can perform the DAPS handover method proposed in the present disclosure, and can maintain the existing default bearer of the source BS by applying the second SDAP layer entity architecture, and can configure the default bearer information indicated in the configuration information as the default bearer of the target BS. In another method, when the first condition proposed in the present disclosure is met, the UE can switch from the existing default bearer of the source BS to the default bearer of the target BS indicated in the configuration information.

[0331] -When the second QoS flow and bearer mapping information are configured in the SDAP layer entity configuration information configured by the switching command message, the UE can perform the DAPS switching method proposed in the present disclosure, and can maintain the first QoS flow and bearer mapping information for the source BS by applying the second SDAP layer entity architecture, and can apply the second QoS flow and bearer mapping information to the data for the target BS.

[0332] -When a data discard timer value is configured in the PDCP layer entity configuration information configured by the handover command message, the UE can perform the DAPS handover method proposed in the present disclosure, and can apply the discard timer value to the PDCP layer entity corresponding to the bearer identifier of the configuration information by applying the second PDCP layer entity architecture.

[0333] -When the drb-ContinueROHC indicator is configured as false (False) in the PDCP layer entity configuration information indicated by the handover command message, the UE can perform the DAPS handover method proposed in the present disclosure, and can use the header compression or decompression protocol context of the source BS unchanged in the PDCP layer entity corresponding to the bearer identifier of the configuration information by applying the second PDCP layer entity architecture, reset the header compression or decompression protocol context of the target BS, and start in an initial state (e.g., IR state). When the drb-ContinueROHC indicator is configured as true (True), the UE can perform the DAPS handover method proposed in the present disclosure, and can use the header compression or decompression protocol context of the source BS unchanged in the PDCP layer entity corresponding to the bearer identifier of the configuration information by applying the second PDCP layer entity architecture, and can apply the header compression or decompression protocol context of the target BS identically as the header compression or decompression protocol context of the source BS. For example, the UE can copy and apply the header compression or decompression protocol context of the source BS unchanged to the header compression or decompression protocol context of the target BS. In another method, the UE may apply the same header compression or decompression protocol context to the target BS or the source BS.

[0334] -When the reordering timer value is configured in the PDCP layer entity configuration information configured by the handover command message, the UE can perform the DAPS handover method proposed in the present disclosure, and can apply the reordering timer value to the PDCP layer entity corresponding to the bearer identifier of the configuration information by applying the second PDCP layer entity architecture.

[0335] -When security key configuration information or a security algorithm is configured in the security configuration information configured by the handover command message, or when an indicator indicating a new procedure is included in the PDCP layer entity configuration information, the UE can derive a new security key or new security configuration information by using the configuration information, and can perform the DAPS handover method proposed in the present disclosure, and can maintain the existing security key or existing security configuration information of the source BS by applying the second PDCP layer entity architecture, and can configure the security key or security configuration information of the target BS as a new security key or new security configuration information.

[0336] -When a new logical channel identifier is configured in the RLC layer entity configuration information configured by the handover command message, the UE can perform the DAPS handover method proposed in the present disclosure, and can maintain the existing logical channel identifier for the RLC layer entity or MAC layer entity of the first bearer of the source BS by applying the second PDCP layer entity architecture, which logical channel identifier corresponds to the bearer identifier indicated in the RLC layer entity configuration information, and can configure the new logical channel identifier indicated in the configuration information for the RLC layer entity or MAC layer entity of the target BS corresponding to the second bearer.

[0337] -When the RLC reestablishment process is configured in the RLC layer entity configuration information configured by the handover command message, the UE can perform the DAPS handover method proposed in the present disclosure, and can perform the RLC reestablishment process on the RLC layer entity of the first bearer for the source BS by applying the second PDCP layer entity architecture, wherein the first bearer corresponds to the bearer identifier indicated in the RLC layer entity configuration information.

[0338] -When the RLC layer entity configuration information configured in the handover command message is newly configured, the UE can perform the DAPS handover method proposed in the present disclosure, and by applying the second PDCP layer entity architecture, the existing RLC configuration information can be maintained for the RLC layer entity of the first bearer of the source BS, which first bearer corresponds to the bearer identifier indicated in the RLC layer entity configuration information, and new RLC layer entity configuration information can be configured, which new RLC layer entity configuration information is indicated in the configuration information, for the RLC layer entity of the target BS corresponding to the second bearer.

[0339] -When a second priority regarding a logical channel is newly configured in the MAC layer entity configuration information configured by the handover command message, the UE may perform the DAPS handover method proposed in the present disclosure and may apply the second PDCP layer entity architecture, may maintain the existing configuration information regarding the MAC layer entity of the first bearer of the source BS, the existing configuration information corresponding to the bearer identifier indicated above, may configure a new logical channel identifier, which is indicated in the configuration information to the MAC layer entity of the target BS corresponding to the second bearer, and may configure the newly configured second priority corresponding to the logical channel identifier indicated in the configuration information. In another method, when the first condition proposed in the present disclosure is met, the UE may apply the priority order to the MAC layer entity of the target BS corresponding to the second bearer according to each logical channel identifier.

[0340] -When a second prioritisedBitRate (PBR) about a logical channel is newly configured in the MAC layer entity configuration information configured by the handover command message, the UE may perform the DAPS handover method proposed in the present disclosure, may apply the second PDCP layer entity architecture, and maintain the existing configuration information for the MAC layer entity of the first bearer of the source BS, the existing configuration information corresponding to the bearer identifier indicated above, may configure a new logical channel identifier for the MAC layer entity of the target BS corresponding to the second bearer, the new logical channel identifier is indicated in the configuration information, and may configure the newly configured second prioritisedBitRate (PBR) corresponding to the new logical channel identifier indicated in the configuration information. In another method, the UE may start applying the second prioritisedBitRate to the logical channel identifier in the MAC layer entity of the target BS corresponding to the second bearer after the first condition proposed in the present disclosure is met (by doing so, when different handover methods are indicated for each bearer, UL transmission resources can be distributed fairly). PrioritisedBitRate refers to a value that is increased at a predetermined time interval (e.g., at each TTI) relative to each logical channel when the prioritisedBitRate is applied to each logical channel identifier. When the UE receives UL transmission resources, the UE may perform an LCP process and may send data on the logical channel in consideration of the priority and the prioritizedBitRate. At this point, the higher the priority, or the larger the value of prioritizedBitRate, the more data may be sent.

[0341] - In the case where the DAPS switching method is applied in the above description, when the UE must send UL data via the first bearer for the source BS because the first condition proposed in the present disclosure has not been satisfied, the UE may select the MAC layer entity for the first bearer as the target of the LCP procedure only with respect to the bearer or logical channel identifier indicating the DAPS switching method (or the switching method that can also continuously send data to the source BS after receiving the switching command message), and may perform the LCP procedure. When the UE receives a switching command message with respect to a bearer or logical channel identifier to which the DAPS switching method is not applied, the UE cannot send UL data to the source BS, and therefore, the bearer or logical channel identifier should not be selected as the target of the LCP procedure.

[0342] -When a second bucketSizeDuration about a logical channel is newly configured in the MAC layer entity configuration information configured by the handover command message, the UE may perform the DAPS handover method proposed in the present disclosure, may apply the second PDCP layer entity architecture and maintain the existing configuration information about the MAC layer entity of the first bearer of the source BS, the existing configuration information corresponding to the bearer identifier indicated above, may configure a new logical channel identifier for the MAC layer entity of the target BS corresponding to the second bearer, the new logical channel identifier is indicated in the configuration information, and may configure the newly configured second bucketSizeDuration corresponding to the new logical channel identifier indicated in the configuration information. In another method, the UE may start applying the second bucketSizeDuration to the logical channel identifier in the MAC layer entity of the target BS corresponding to the second bearer after the first condition proposed in the present disclosure is met (by doing so, when different handover methods are indicated for each bearer, UL transmission resources can be distributed fairly). In the above description, the bucket capacity duration indicates the maximum value that the prioritisedBitRate can have when the prioritisedBitRate is accumulated.

[0343] -When the second available SCell information or available subcarrier spacing information or maximum PUSCH duration or logical channel group configuration information is configured in the MAC layer entity configuration information configured by the handover command message, the UE can perform the DAPS handover method proposed in the present disclosure, and can apply the second PDCP layer entity architecture, and maintain the existing configuration information of the MAC layer entity of the first bearer of the source BS corresponding to the bearer identifier indicated above, and can configure the second available SCell information or available subcarrier spacing information or maximum PUSCH duration or logical channel group configuration information indicated in the configuration information relative to the MAC layer entity of the target BS corresponding to the second bearer.

[0344] The present invention provides a method, when a UE performs embodiment 2 (DAPS handover method) of an efficient handover method according to an embodiment of the present invention and fails to perform handover, the UE quickly falls back to the source BS by using the features of the above-mentioned DAPS handover method to reconfigure the link to the source BS. The DAPS handover method according to an embodiment of the present disclosure may indicate that even when the UE performs a handover process, the UE still maintains a link to the source BS to perform data transmission or reception, and even when the UE fails to perform handover, the UE may fall back by using the previous wireless link established with the source BS.

[0345] As reference Figure 1H As described, according to Embodiment 2 of the effective handover method (DAPS handover method), even when the UE receives a handover command message from the source BS, as proposed in 1h-02, the UE can perform a handover procedure on the target BS while the UE maintains sending data to the source BS or receiving data from the source BS. In addition, in the present disclosure, when the UE fails to perform a handover procedure on the target BS, the UE can fall back to the source BS.

[0346] If the UE fails to perform a handover procedure to the target BS, there must be a method by which the UE can identify whether the radio link to the source BS is active. If the UE fails to perform a handover and does not perform a fallback on the source BS even when the radio link to the source BS is inactive, the UE cannot perform a fallback procedure on the source BS, so that the long data interruption time increases, thereby causing significant data interruption. In addition, in the case where the radio link to the source BS is active, the SRB configured between the UE and the source BS must be maintained.

[0347] First, the present disclosure proposes a new timer applicable to a handover method and proposes a detailed operation of each timer. In addition, the detailed operation of each timer may include different operations according to the type of handover method indicated by a handover command message from a BS. In addition, a method for releasing or maintaining a link or SRB configuration to a source BS according to a handover method is provided.

[0348] In order to effectively perform the handover process, the present disclosure may introduce a first timer (e.g., T304) or a second timer (e.g., T310) or a third timer (e.g., T312) or a fourth timer (e.g., a timer for backoff), and at least one of the first timer to the fourth timer may be run and applied to the handover process. According to the present disclosure, the first timer (e.g., T304) or the second timer (e.g., T310) or the third timer (e.g., T312) or the fourth timer (e.g., a timer for backoff) may perform the following different operations according to the type of handover method indicated by the handover command message. The first timer (e.g., T304) is a timer configured to determine whether the handover has been successfully performed, the second timer (e.g., T310) is a timer configured to determine whether the radio link is active, and the third timer (e.g., T312) is an auxiliary timer configured to determine whether the radio link is active, trigger a frequency measurement process, and report a frequency measurement report. When the UE performs embodiment 2 (DAPS handover method) of the handover method according to an embodiment of the present invention and then fails to perform handover, the UE performs a fallback procedure on the source BS, thereby sending a message indicating the handover failure to the source BS. In this regard, the fourth timer (e.g., a timer for fallback) is a timer configured to determine whether the fallback procedure has been successfully performed or has failed.

[0349] The detailed operation of the first timer (e.g., T304) or the second timer (e.g., T310) or the third timer (e.g., T312) or the fourth timer (e.g., a timer for fallback) proposed in the present disclosure is presented below according to the type of switching method indicated to support an effective switching method.

[0350] 1> When the UE receives an indicator indicating asynchrony (asynchronous indication) of a radio link signal from a lower layer entity (e.g., a MAC layer entity or a PHY layer entity) a preset number of times (e.g., the preset number of times can be set by the BS), and thus detects a problem with the PHY layer entity, the UE can start a second timer (e.g., T310) when the first timer is not running. Then, the UE receives an indicator indicating synchronization (synchronous indication) of a radio link signal from a lower layer entity a preset number of times (e.g., the preset number of times can be set by the BS), or triggers (starts) a handover process, or starts an RRC connection reestablishment process, and the UE stops the second timer. If the second timer expires, the UE triggers or starts the RRC connection reestablishment process. Alternatively, the UE switches to RRC inactive mode and triggers or starts the RRC connection reestablishment process.

[0351] 1> When a frequency measurement process is triggered for a frequency measurement identifier configured for a third timer while the second timer is running, the UE starts the third timer. Then, when the UE receives an indicator indicating synchronization (synchronization indication) of a radio link signal from a lower layer entity for a preset number of times (for example, the preset number of times can be set by the BS) or a handover process is triggered (started) or an RRC connection reestablishment process is started, the UE stops the third timer. If the third timer expires, the UE triggers or starts the RRC connection reestablishment process. Alternatively, the UE switches to the RRC inactive mode and triggers or starts the RRC connection reestablishment process.

[0352] 1> If the UE receives a handover command message from the BS (including a mobility indication (MobilityControl information or ReconfigurationWithSync) or an RRCReconfiguration message of a handover indication), and the handover command message indicates a first handover method (eg, Embodiment 1 or a general handover method).

[0353] 2> According to the present disclosure, when the UE receives a handover command message (including a mobility indication (MobilityControl information or ReconfigurationWithSync) or an RRCReconfiguration message of a handover indication), the UE triggers the handover process and starts a first timer.

[0354] 2> When the UE triggers the handover process, the UE releases the SRB configured for the source BS (e.g., SRB1), and configures the SRB (e.g., SRB1) for the target BS based on the configuration information configured in the handover command message.

[0355] 2> When the UE triggers the handover process, if the second timer is running, the UE may stop the second timer. While the first timer is running, the UE does not start the second timer even when the condition for starting the second timer is met (an indicator indicating asynchrony of the radio link signal is received from a lower layer entity a predetermined number of times). That is, when the first timer is running, the UE does not use the second timer.

[0356] 2> When the UE triggers the handover process, if the third timer is running, the UE may stop the third timer. Then, only when the second timer is running, the UE may start the third timer when the condition for starting the third timer is met (when the frequency measurement process is triggered for the frequency measurement identifier configured for the third timer). That is, because the second timer is not used when the first timer is running, the third timer is not used either.

[0357] 2> If the UE successfully performs a handover procedure or successfully completes a random access procedure with respect to a target BS, the UE stops the first timer.

[0358] 2> If the first timer expires (for example, if the handover process with respect to the target BS fails), the UE performs the RRC connection reestablishment process (the UE may release the link to the BS and may perform the RRC connection process from the starting point, i.e., may perform a cell selection or reselection process, perform a random access process, and send an RRC connection reestablishment request message).

[0359] 2> If the UE receives a handover command message from the BS (including a mobility indication (MobilityControl information or ReconfigurationWithSync) or an RRCReconfiguration message of a handover indication), and the handover command message indicates a second handover method (e.g., Embodiment 2 or a DAPS handover method) (or this can be broadly applied to situations where a conditional handover method is also indicated).

[0360] 2> According to the present disclosure, when the UE receives a handover command message (including a mobility indication (MobilityControl information or ReconfigurationWithSync) or an RRCReconfiguration message of a handover indication), the UE triggers the handover process and starts the first timer. If a conditional handover method is also indicated, when the UE selects a cell from multiple target cells, the UE can start the first timer and start the handover process or perform a random access process.

[0361] 2> In the case where the UE triggers the handover process, when the UE starts the DAPS handover method, the UE maintains or suspends the SRB (e.g., SRB1) configured for the source BS, and configures the SRB (e.g., SRB1) for the target BS based on the configuration information configured in the handover command message. In another method, in the case where the UE triggers the handover process, when the UE starts the DAPS handover method, the UE may maintain or suspend the SRB (e.g., SRB1) configured for the source BS, may reset the window state variable by reestablishing a PDCP layer entity or an RLC layer entity for the SRB corresponding to the source BS, may stop the timer, may indicate a plurality of stored data items (PDCP SDU or PDCP PDU) to be discarded (this may be performed when the fallback process proposed in the present disclosure is triggered), and may configure the SRB (e.g., SRB1) for the target BS based on the configuration information configured in the handover command message. In another method, the UE may apply the second PDCP layer entity architecture proposed in the present disclosure to the SRB, thereby configuring the first bearer for the source BS, and configuring the second bearer for the target BS. In another method, when the UE applies the second PDCP layer entity architecture to the SRB, the UE may reset the window state variable by reestablishing the PDCP layer entity or the RLC layer entity for the first bearer, may stop the timer, and may indicate a plurality of stored data items (PDCP SDU or PDCP PDU) to be discarded (this may be performed when the fallback procedure proposed in the present disclosure is triggered).

[0362] 2> In the case where the UE triggers the handover process, when the UE starts the DAPS handover method, the UE does not stop the second timer for the source BS even when the second timer is running. When the first timer is running (or even when the first timer is not running), if the condition for starting the second timer is met (an indicator indicating asynchrony of the radio link signal is received from the lower layer a predetermined number of times), the UE starts the second timer. The second timer can be operated with respect to the radio link between the UE and the source BS. In another method, two second timers can be operated so that one second timer can be operated with respect to the radio link between the UE and the source BS, and another second timer can be operated with respect to the radio link between the UE and the target BS. That is, even when the first timer is running, the UE uses the second timer with respect to the radio link to the source BS or the target BS. However, even when the second timer expires, if the first timer has not expired but is running, the UE does not trigger the RRC connection reestablishment process. That is, specifically, even when the second timer for the source BS expires or a radio link failure (RLF) occurs, if the first timer has not expired but is running, or a random access process is being performed on the target BS, or a handover process is being performed on the target BS, the UE may not trigger the RRC connection reestablishment process, and may release the radio link to the source BS, may not release the RRC configuration information (e.g., bearer configuration information, etc.) configured by the source BS, and may use the RRC configuration information again when the RRC connection reestablishment process is triggered later. In addition, when the first timer is still running even if the second timer expires and the first timer has not expired, the UE may not trigger the RRC connection reestablishment process, and the source BS may report the source link failure to the target BS, or the UE may release the link to the source BS (e.g., the UE may release the first bearer for the source BS) or may suspend the first bearer for the source BS. However, when the second timer expires, if the first timer expires or stops or is not running because the first timer is not started, the UE may trigger the RRC connection reestablishment process. The reason why the second timer runs even when the handover procedure is performed is to allow the UE to perform a fallback procedure when the radio link to the source BS is active when a handover failure occurs while monitoring the radio link to the source BS. When the second timer for the target BS expires or the radio link to the target BS fails, if the first timer expires or stops or is not running because the first timer is not started, or a random access procedure with respect to the target BS has been successfully performed, the RRC connection reestablishment procedure may be triggered.

[0363] 2> In the case where the UE triggers the handover process, when the UE triggers the DAPS handover method, the UE will not stop the third timer of the source BS even when the third timer is running. Then, only when the second timer is running, the UE can start the third timer when the condition for starting the third timer is met (when the frequency measurement process is triggered for the frequency measurement identifier for which the third timer is configured). That is, since the UE uses the second timer even when the first timer is running, the UE can also use the third timer. The third timer can be operated with respect to the wireless link between the UE and the source BS. In another method, two third timers can be operated so that one third timer can be operated with respect to the wireless link between the UE and the source BS, and another third timer can be operated with respect to the wireless link between the UE and the target BS. That is, even when the first timer is running, the third timer can be used with respect to the wireless link to the source BS or the target BS. However, even when the third timer expires, if the first timer is not expired and is running, the UE may not trigger the RRC connection reestablishment process. In addition, when the first timer is still running even if the third timer expires and the first timer has not expired, the UE may not trigger the RRC connection reestablishment process, and the source BS may report the source link failure to the target BS, or the UE may release the link to the source BS (for example, the UE may release the first bearer of the source BS) or may suspend the first bearer of the source BS. However, when the third timer expires, if the first timer expires or stops or is not running because the first timer is not started, the UE may trigger the RRC connection reestablishment process. The reason why the third timer runs even when the handover process is performed is that when a handover failure occurs while monitoring the wireless link to the source BS, the UE is allowed to perform a fallback process and report the frequency measurement result in the fallback process.

[0364] 2> When the UE successfully completes the handover procedure with respect to the target BS, the UE stops the first timer.

[0365] 2> When the first timer expires (for example, when the handover procedure to the target BS fails), or when the maximum number of retransmissions of the RLC layer entity relative to the target BS is exceeded, or when the UE receives a handover command message but the configuration information of the handover command message exceeds the capability of the UE, or when an error occurs in the application of the configuration information causing the handover to fail, or when a problem occurs in the random access to the target BS and the UE continues to attempt the random access procedure but the first timer expires, so the UE cannot perform the handover procedure, or when the UE runs the second timer or the third timer for the target BS but the second timer or the third timer expires before the handover procedure is completed, or when the T304 timer stops or expires, the UE determines that the handover procedure has failed.

[0366] 3> If the second timer or the third timer of the radio link between the UE and the source BS has not expired (or if the second timer or the third timer of the radio link between the UE and the source BS is not started or is running), or if the radio link between the UE and the source BS is active, then

[0367] 4> The UE may determine that the radio link between the UE and the source BS is active, and may perform the backoff procedure proposed in the present disclosure.

[0368] 4> When the UE starts the fallback process, if the SRB configured for the source BS (e.g., SRB1 or a MAC layer entity, an RLC layer entity, or a PDCP layer entity for SRB1) has been suspended, the UE may resume or reconfigure the SRB, and may perform the fallback process on the SRB (e.g., SRB1). In another method, when the second PDCP layer entity architecture proposed in the present disclosure is applied to the SRB, the UE may perform the fallback process via the first bearer of the source BS, and may release the second bearer of the target BS. For example, the UE may switch UL data transmission to the first bearer for the source BS, may indicate the existence of an RLC layer entity or a MAC layer entity for the first bearer to send data, and may send a handover failure report message for the fallback process via the first bearer.

[0369] 4> In the above, the fallback process refers to the process in which the UE configures a message reporting a handover failure and reports the handover failure to the source BS via the SRB configured for the source BS (e.g., SRB1). When the UE sends a message reporting a handover failure to the source BS, the UE may also report the result of the frequency measured by the UE, thereby supporting rapid recovery of the link to the source BS. In another method, the UE may define and send MAC control information (e.g., the UE may indicate the presence of data to be sent, or may define and indicate new MAC control information or a specific value in a buffer status report, thereby indicating a handover failure) or RLC control information or PDCP control information, thereby indicating a handover failure to the source BS. In another method, the UE may send an RRC connection reestablishment request message via the SRB used for the source BS (e.g., SRB0 or ​​SRB1). In another method, in the above method, the fallback process may refer to the following process: when the handover fails, the UE releases the second bearer of the target BS in the second PDCP layer entity architecture of each bearer or the bearer configured using the DAPS handover method, or switches to the first PDCP layer entity architecture, and then resumes data transmission or reception of the source BS via the first bearer, and the UE may indicate the MAC layer entity of the first bearer where there is data to be sent, may request scheduling or report the data to be sent (e.g., buffer status report) to the source BS, or may send a new MAC CE or RLC control data or PDCP control data to the source BS, thereby indicating to the source BS that the UE will fall back to the source BS and resume data transmission. The UE may then reconfigure or resume the SRB for the source BS. In addition, when the handover fails, the fallback process may be performed for each bearer. Because the bearer not configured with the DAPS switching method does not have a second PDCP layer entity architecture, the UE may release the previously configured PDCP layer entity or RLC layer entity or bearer configuration information or logical channel identifier information from the MAC layer entity for the target BS, and then reconfigure in the configuration information of the switching command message, or may switch to the MAC layer entity for the source BS and configure the MAC layer entity for the source BS, and may resume sending data to the source BS or receiving data from the source BS via each bearer. This is because, when the UE receives the switching command message, the UE may apply the bearer configuration information configured in the switching command message to the MAC layer entity of the target BS corresponding to the bearer, which bearer is not configured with the DAPS switching method, and may switch the link of the PDCP layer entity or RLC layer entity of the bearer not configured with the DAPS switching method from the MAC layer entity of the source BS to the MAC layer entity of the target BS.For example, when the UE receives the handover command message, the upper layer entity of the UE (e.g., the RRC layer entity) may instruct the MAC layer entity of the source BS to perform MAC reconfiguration by using the configuration information of the current MAC layer entity that does not include the configuration information related to the bearer for which the DAPS handover method is not indicated, or the upper layer entity of the UE (e.g., the RRC layer entity) may instruct the MAC layer entity of the source BS to perform MAC reconfiguration by using the configuration information of the current MAC layer entity that only includes the configuration information related to the bearer for which the DAPS handover method is indicated. That is, since when the UE receives the handover command message, the UE may release the configuration information of the PDCP layer entity or the RLC layer entity or the MAC layer entity of the bearer not configured with the DAPS handover method from the MAC layer entity of the source BS, and may be applied to the MAC layer entity of the target BS or connected according to the bearer configuration of the target BS, therefore, if the UE performs the fallback procedure, the UE must reconfigure the bearer that is not configured with the DAPS handover method to the MAC layer entity of the source BS. For example, when the UE performs the fallback procedure, the upper layer entity of the UE (e.g., the RRC layer entity) may instruct the MAC layer entity of the source BS to perform MAC reconfiguration by using the configuration information of the current MAC layer entity, wherein the configuration information includes configuration information related to the bearer for which the DAPS switching method is not indicated and configuration information related to the bearer for which the DAPS switching method is indicated. Alternatively, when performing the fallback procedure, the UE may reconfigure or restore the bearer configuration (e.g., PDCP layer entity configuration information or RLC layer entity configuration information or MAC layer entity configuration information or PHY layer entity configuration information) before receiving the handover command message, and may apply them to the bearer for the source BS (PDCP layer entity configuration information or RLC layer entity configuration information or MAC layer entity configuration information or PHY layer entity configuration information for SRB or AM DRB or UM DRB).

[0370] 4> During the fallback process, when the UE sends a handover failure report message to the source BS (for example, the RRC message or MAC CE or RLC control data or PDCP control data proposed above), the UE may start a fourth timer. When the UE receives an indication or message from the source BS, in response to the handover failure report message sent by the UE, the UE may stop the fourth timer. However, when the fourth timer expires or a response message is not received until the fourth timer expires, the UE performs an RRC connection reestablishment process (the UE may release the link to the BS and may perform an RRC connection process from the starting point, i.e., may perform a cell selection or reselection process, perform a random access process, and send an RRC connection reestablishment request message). When the UE triggers the RRC connection reestablishment process due to the expiration of the fourth timer, if the second timer or the third timer is running, the UE may stop the second timer or the third timer.

[0371] u 3> When the second timer or the third timer of the radio link between the UE and the source BS or the target BS expires or the radio link between the UE and the source BS or the target BS is inactive, then

[0372] l 4>UE performs the RRC connection reestablishment process (UE can release the link to the BS and can perform the RRC connection process from the beginning, that is, can perform a cell selection or reselection process, perform a random access process, and send an RRC connection reestablishment request message).

[0373] n 2> When the UE performs a DAPS handover procedure, if the second condition proposed in the present disclosure is met, the UE may release the link to the source BS, or may release the SRB for the source BS, and if the second timer or the third timer is running, the second timer or the third timer for the source BS may be stopped and reset. Only when the UE stops the second timer or the third timer can the UE prevent an unnecessary RRC connection reestablishment procedure due to the expiration of the second timer or the third timer. Since satisfying the second condition may mean that the handover procedure is successfully performed, the expiration of the second timer or the expiration of the third timer due to the stop of the first timer may trigger an unnecessary RRC connection reestablishment procedure. In another method, when the first condition proposed in the present disclosure is met or the handover procedure is successfully performed, the UE may release the SRB for the source BS, or if the second timer or the third timer is running, the second timer or the third timer for the source BS may be stopped and reset. Only when the UE stops the second timer or the third timer can the UE prevent an unnecessary RRC connection reestablishment procedure due to the expiration of the second timer or the third timer. Since the first condition being satisfied may mean that the handover procedure is successfully performed, the expiration of the second timer or the expiration of the third timer due to the stopping of the first timer may trigger an unnecessary RRC connection reestablishment procedure.

[0374] According to the method proposed in the present disclosure, when the proposed conditions are met and the UE determines that a handover failure has occurred and performs a fallback process, the UE adds information indicating that a handover failure has occurred to an RRC message (e.g., a ULInformationTransferMRDC message or a FailureInformation message), and sends the RRC message via or through SRB1 to which the second PDCP layer entity architecture is applied, thereby allowing the source BS to identify the UE's handover failure. When the source BS detects a handover failure of the UE, the source BS may configure an RRC message (e.g., an RRCReconfiguration message or an RRCRelease message) in response thereto and send an RRC message (e.g., an RRCReconfiguration message or an RRCRelease message) to the UE, and when the UE receives the RRCReconfiguration message as an RRC message in response to a handover failure report (via SRB1 to which the second PDCP layer entity architecture is applied or an RRC message received via SRB1), the UE may apply and complete its configuration information, and in response thereto, may send an RRCReconfigurationComplete message to the source BS via SRB1 to which the second PDCP layer entity architecture is applied or via SRB1, and if the RRCReconfiguration message indicates handover or access to another cell, the UE may complete the random access procedure with respect to the cell and may send an RRCReconfigurationComplete message via SRB1. However, when the UE receives the RRCRelease message as an RRC message in response to a handover failure report, the UE may transition to the RRC idle mode or may transition to the RRC inactive mode according to the configuration information indicated by the RRCRelease message, and may not send an additional RRC message to the BS in response to the RRC message.

[0375] Figure 1K is a flow chart illustrating the operation of a UE according to an embodiment of the present invention.

[0376] Reference Figure 1K, UE 1k-01 can send data to the source BS or receive data from the source BS via each bearer by using the first PDCP layer entity architecture. However, when the switching command message received by UE 1k-01 indicates the DAPS switching method of embodiment 2 proposed in the present disclosure or indicates the DAPS switching method for each bearer, UE 1k-01 can switch to the second PDCP layer entity architecture for each bearer or the bearer indicating the DAPS switching method for the target BS indicated by the switching command message, and can configure and establish a protocol layer entity for the second bearer, and even when UE 1k-01 performs a random access procedure to the target BS via the established protocol layer entities (1k-10 and 1k-15), UE 1k-01 can continuously send data to the source BS or receive data from the source BS (UL data transmission and DL data reception) by using the protocol layer entity (1k-20) for the first bearer.

[0377] If the first condition (1k-25) is met, the UE 1k-01 may stop UL data transmission to the source BS via the protocol layer entity for the first bearer, and may switch UL data transmission, and thus may send UL data to the target BS via the protocol layer entity for the second bearer, and may continuously receive DL data from the source BS and the target BS via the protocol layer entities for the first and second bearers (1k-30). In addition, the PDCP layer entity for the second bearer may continuously and uninterruptedly perform sending and receiving of data to and from the source BS by using data to be sent or received or SN information or information such as header compression and decompression context (which is stored in the PDCP layer entity for the first bearer). If the first condition is not met, the UE 1k-01 may continuously check the first condition while continuously executing the ongoing process (1k-35).

[0378] In addition, if the second condition is met, UE 1k-01 can stop receiving DL data from the source BS via the protocol layer entity for the first bearer (1k-45). In addition, the PDCP layer entity for the second bearer can continuously and uninterruptedly perform sending and receiving data to and from the source BS by using data to be sent or received or SN information or information such as header compression and decompression context (which is stored in the PDCP layer entity for the first bearer).

[0379] If the second condition is not satisfied, the UE 1k-01 may continuously check the second condition while continuously performing the ongoing process (1k-50).

[0380] The PDCP layer entity according to certain embodiments of the present disclosure may perform different processes according to the handover type indicated by the handover command message received by the UE.

[0381] - If the handover type indicated by the handover command message received by the UE from the source BS is the handover of Embodiment 1 (eg, the normal handover method), then

[0382] The UE may perform a PDCP reestablishment procedure on a PDCP layer entity for each bearer.

[0383] - If the handover type indicated by the handover command message received by the UE from the source BS is the handover of embodiment 2 (or indicated for each bearer), then

[0384] When the first condition is met, the UE may perform the process proposed in the present disclosure for each bearer (or for a bearer indicating Embodiment 2).

[0385] In addition, when the source BS indicates to the UE that the handover proposed in the present disclosure is applied, the source BS may start forwarding data to the target BS when the following third condition is met. The third condition may mean that one or more of the following conditions are met.

[0386] - In the case where the source BS receives an indication from the target BS indicating that the UE successfully completed the handover

[0387] - In the case where the source BS sends a handover command message to the UE

[0388] - In the case where the source BS sends a handover command message to the UE and recognizes successful delivery of the handover command message (HARQ ACK or NACK, or RLC ACK or NACK)

[0389] - When the source BS receives an indication (e.g., an RRC message (e.g., an RRCReconfiguration message)) or a MAC CE or an RLC control PDU or a PDCP control PDU from the UE indicating that the link to the source BS is to be released

[0390] - When the source BS sends a handover command message to the UE and starts a timer, and then the timer expires

[0391] - In the case where no confirmation (HARQ ACK or NACK, or RLC ACK or NACK) about the successful delivery of DL data is received from the UE within a certain period of time

[0392] Figure 1L is a flowchart illustrating an operation of a UE performing a fallback procedure when a handover fails in a DAPS handover method according to an embodiment of the present disclosure.

[0393] Reference Figure 1L, UE 1l-05 can perform data transmission to the source BS or data reception from the source BS via each bearer by using the first PDCP layer entity architecture. However, when the switching command message received by UE 1l-05 indicates the DAPS switching method of embodiment 2 proposed in the present disclosure or indicates the DAPS switching method for each bearer, UE 1l-05 can switch to the second PDCP layer entity architecture for each bearer or the bearer indicating the DAPS switching method relative to the target BS indicated in the message, and can configure and establish a protocol layer entity for the second bearer. In addition, when UE 1l-05 performs a random access procedure to the target BS (1l-10 and 1l-15) via establishing a protocol layer entity, UE 1l-05 can continuously perform data transmission to the source BS or data reception from the source BS (UL data transmission and DL data reception) via the protocol layer entity for the first bearer (1l-20).

[0394] When UE 11-05 successfully completes the handover process (11-35), UE 11-05 ends the handover process according to Embodiment 2 (DAPS handover method) of the handover method proposed in the present disclosure.

[0395] However, when UE 11-05 fails in the handover procedure (11-25) (for example, if the first timer expires in the above situation (for example, if the handover procedure to the target BS fails), or when the maximum number of retransmissions in the RLC layer entity exceeds, or when UE 11-05 receives a handover command message but the configuration information of the handover command message exceeds the capability of the UE, or when an error occurs in applying the configuration information so that the handover fails, or when a problem occurs in random access to the target BS so that the handover procedure fails, or when the second timer or the third timer is running for the target BS and then the second timer or the third timer expires before the handover procedure is completed, so that the UE 1l-05 stops or terminates the T304 timer and determines that the switching process has failed), when the second timer or the third timer of the wireless link between the UE and the source BS has not expired (or, when the second timer or the third timer of the wireless link between the UE and the source BS is not started or is running (1l-40), or when the wireless link between the UE and the source BS is active, the UE can determine that the wireless link between the UE and the source BS is active and can perform the fallback process proposed in the present disclosure (1l-45). When the second timer or the third timer for the wireless link between the UE and the source BS expires or the wireless link between the UE and the source BS is inactive (1l-30), the UE performs the RRC connection reestablishment process (the UE can release the link to the BS and can perform the RRC connection process from the starting point, that is, can perform a cell selection or reselection process, perform a random access process, and send an RRC connection reestablishment request message) (1l-45).

[0396] Figure 1M is a block diagram showing the configuration of a UE according to an embodiment of the present disclosure.

[0397] Reference Figure 1M , the UE may include a radio frequency (RF) processor 1m-10, a baseband processor 1m-20, a memory 1m-30, and a controller 1m-40.

[0398] The RF processor 1m-10 performs functions for sending and receiving signals through wireless channels, such as frequency band conversion and amplification of signals. That is, the RF processor 1m-10 up-converts the baseband signal provided by the baseband processor 1m-20 into an RF band signal, then transmits the RF band signal through an antenna, and down-converts the RF band signal received through the antenna into a baseband signal. For example, the RF processor 1m-10 may include a transmitting filter, a receiving filter, an amplifier, a mixer, an oscillator, a digital-to-analog converter (DAC), an analog-to-digital converter (ADC), and the like. Although in Figure 1MOnly one antenna is shown in the figure, and the UE may include multiple antennas. Moreover, the RF processor 1m-10 may include multiple RF chains. In addition, the RF processor 1m-10 can perform beamforming. For beamforming, the RF processor 1m-10 can adjust the phase and strength of each signal sent or received through multiple antennas or antenna elements. In addition, the RF processor 1m-10 can perform MIMO operations and can receive multiple layers in MIMO operations. The RF processor 1m-10 can perform receive beam scanning by appropriately configuring multiple antennas or antenna elements, or can adjust the direction and beam width of the receive beam under the control of the controller to coordinate with the transmit beam.

[0399] The baseband processor 1m-20 performs conversion between baseband signals and bit strings based on the physical layer specifications of the system. For example, for data transmission, the baseband processor 1m-20 generates complex symbols by encoding and modulating the transmitted bit string. For data reception, the baseband processor 1m-20 reconstructs the received bit string by demodulating and decoding the baseband signal provided from the RF processor 1m-10. For example, according to the OFDM scheme, for data transmission, the baseband processor 1m-20 generates complex symbols by encoding and modulating the transmitted bit string, maps the complex symbols to subcarriers, and then configures OFDM symbols by performing an inverse fast Fourier transform (IFFT) and inserting a cyclic prefix (CP). For data reception, the baseband processor 1m-20 segments the baseband signal provided from the RF processor 1m-10 into OFDM symbol units, reconstructs the signal mapped to the subcarrier by performing a fast Fourier transform (FFT) calculation, and then reconstructs the received bit string by demodulating and decoding the signal.

[0400] The baseband processor 1m-20 and the RF processor 1m-10 send and receive signals in the above manner. Therefore, the baseband processor 1m-20 and the RF processor 1m-10 may also be referred to as a transmitter, a receiver, a transceiver or a communicator. At least one of the baseband processor 1m-20 or the RF processor 1m-10 may include multiple communication modules supporting a variety of different wireless access technologies. At least one of the baseband processor 1m-20 and the RF processor 1m-10 may include different communication modules to process signals of different frequency bands. For example, different wireless access technologies may include LTE networks, NR networks, etc. Different frequency bands may include ultra-high frequency (SHF) (e.g., 2.5 GHz or 5 GHz) bands and millimeter wave (mmWave) (e.g., 60 GHz) bands.

[0401] The memory 1m-30 may store basic programs, applications, and data such as configuration information for the operation of the UE. The memory 1m-30 may provide the stored data according to a request of the controller 1m-40.

[0402] The controller 1m-40 controls the overall operation of the UE. For example, the controller 1m-40 sends and receives signals through the baseband processor 1m-20 and the RF processor 1m-10. In addition, the controller 1m-40 records data on the memory 1m-40 or reads data from the memory 1m-40. To this end, the controller 1m-40 may include at least one processor. For example, the controller 1m-40 may include a communication processor (CP) for controlling communication and an application processor (AP) for controlling an upper layer such as an application.

[0403] Figure 1N is a block diagram illustrating a configuration of a network entity according to an embodiment of the present disclosure.

[0404] Specifically, Figure 1N 2 is a diagram showing a configuration of a Tx / Rx point (TRP) in a wireless communication system to which an embodiment of the present disclosure can be applied.

[0405] like Figure 1N As shown, the TRP includes an RF processor 1n-10, a baseband processor 1n-20, a backhaul communicator 1n-30, a memory 1n-40 and a controller 1n-50.

[0406] The RF processor 1n-10 performs functions for transmitting and receiving signals through a wireless channel, such as frequency band conversion and amplification of signals. That is, the RF processor 1n-10 up-converts the baseband signal provided by the baseband processor 1n-20 into an RF band signal, then transmits the RF band signal through an antenna, and down-converts the RF band signal received through the antenna into a baseband signal. For example, the RF processor 110 may include a transmission filter, a reception filter, an amplifier, a mixer, an oscillator, a DAC, an ADC, etc. Although in Figure 1N Only one antenna is shown in FIG. In this case, the first access node may include multiple antennas. In addition, the RF processor 110 may include multiple RF chains. In addition, the RF processor 110 may perform beamforming. For beamforming, the RF processor 110 may adjust the phase and strength of each signal sent or received through multiple antennas or antenna elements. The RF processor may perform DL MIMO operations by sending one or more layers.

[0407] The baseband processor 1n-20 performs conversion between baseband signals and bit strings based on the physical layer specification of the first wireless access technology. For example, for data transmission, the baseband processor 1n-20 generates complex symbols by encoding and modulating the transmission bit string. For data reception, the baseband processor 1n-20 reconstructs the received bit string by demodulating and decoding the baseband signal provided from the RF processor 1n-10. For example, according to the OFDM scheme, for data transmission, the baseband processor 1n-20 generates complex symbols by encoding and modulating the transmission bit string, maps the complex symbols to subcarriers, and then configures OFDM symbols by performing IFFT calculations and inserting CPs. For data reception, the baseband processor 1n-20 segments the baseband signal provided from the RF processor 1n-10 into OFDM symbol units, reconstructs the signal mapped to the subcarriers by performing FFT calculations, and then reconstructs the received bit string by demodulating and decoding the signal. As described above, the baseband processor 1n-20 and the RF processor 1n-10 send and receive signals. Therefore, the baseband processor 1n-20 and the RF processor 1n-10 may also be referred to as a transmitter, a receiver, a transceiver, a communicator, or a wireless communicator.

[0408] The communicator 1n-30 provides an interface for communicating with other nodes in the network.

[0409] The memory 1n-40 may store basic programs, applications, and data for the operation of the TRP, such as configuration information. In particular, the memory 1n-40 may store, for example, information about the bearers allocated to the connected UE and measurement results reported from the connected UE. The memory 1n-40 may store standard information for determining whether to provide multiple connections to the UE or release multiple connections from the UE. The memory 1n-40 provides the stored data in response to a request from the controller 1n-50.

[0410] The controller 1n-50 controls the overall operation of the TRP. For example, the controller 1n-50 sends and receives signals through the baseband processor 1n-20 and the RF processor 1n-10 or the backhaul communicator 1n-30. The controller 1n-50 records data on the memory 1n-40 or reads data from the memory 1n-40. To this end, the controller 1n-50 may include at least one processor.

[0411] Methods according to embodiments of the present invention as described herein or in the appended claims may be implemented as hardware, software or a combination of hardware and software.

[0412] When implemented as software, a computer-readable storage medium storing one or more programs (e.g., software modules) may be provided. One or more programs stored in the computer-readable storage medium are configured to be executed by one or more processors in an electronic device. One or more programs include instructions that instruct the electronic device to perform a method according to an embodiment of the present disclosure as described in the claims or the specification.

[0413] The program (e.g., software module or software) may be stored in a non-volatile memory, including random access memory (RAM) or flash memory, read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), magnetic disk storage device, compact disk (CD)-ROM, digital versatile disk (DVD), other optical storage device or magnetic tape cassette. Alternatively, the program may be stored in a memory including a combination of some or all of the above storage media. A plurality of such memories may be included.

[0414] In addition, the program can be stored in an attachable storage device that can be accessed through any one or a combination of communication networks such as the Internet, an intranet, a local area network (LAN), a wide area network (WLAN), a storage area network (SAN), etc. Such a storage device can access the device that performs the embodiments of the present disclosure via an external port. In addition, a separate storage device on a communication network can access the electronic device that performs the embodiments of the present disclosure.

[0415] In the above-mentioned embodiment of the present disclosure, according to the embodiment of the present disclosure, the configuration elements included in the present disclosure are expressed in singular or plural form. However, for the convenience of description, the singular or plural form is appropriately selected, and the present disclosure is not limited thereto. In this way, the configuration element expressed in plural form can also be configured as a singular element, and the configuration element expressed in singular form can also be configured as a plural element.

[0416] The embodiments of the present disclosure described with reference to this specification and the accompanying drawings are merely for the convenience of describing and understanding the specific examples of the present disclosure, and are not intended to limit the scope of the present disclosure. That is, it will be understood by those of ordinary skill in the art that other modifications based on the technical ideas of the present disclosure are feasible. In addition, the embodiments of the present disclosure can be combined to be implemented when necessary. For example, the BS and the UE can operate in a manner that a part of an embodiment of the present disclosure is combined with a part of another embodiment of the present disclosure. In addition, modifications based on the technical scope of the embodiments of the present invention can be applied to various systems, such as FDD LTE systems, TDD LTE systems, 5G or NR systems, etc.

Claims

1. A method performed by a user equipment UE in a wireless communication system, the method include: receiving a message including a reconfiguration with synchronization from a source base station; Starting a first timer; In a case where a dual active protocol stack DAPS bearer is configured based on the message, a radio link control RLC entity for the target base station is established, and a signaling radio bearer SRB for the source base station is suspended; When the first timer expires and the radio link between the UE and the source base station is active, release the RLC entity for the target base station, restore the suspended SRB for the source base station, and send a DAPS handover failure report to the source base station; as well as When the first timer expires and the radio link between the UE and the source base station is inactive, a radio resource control RRC reestablishment procedure is performed.

2. The method according to claim 1, further comprising: include: When the condition for starting the third timer is met, the third timer is started while the second timer is running.

3. The method according to claim 1, further comprising: include: In case the DAPS bearer is configured, an out-of-sync indication is received from a lower layer entity, and the first timer is running, a second timer for the source base station is started.

4. The method according to claim 3, further comprising: include: In case a synchronization indication is received from the lower layer entity while the second timer is running, the second timer is stopped, and if the third timer is running, the third timer is stopped.

5. The method according to claim 1, in, When the third timer expires, the RRC reestablishment procedure is performed. 6 . The method according to claim 2 , wherein the first timer is a T304 timer, the second timer is a T310 timer, and the third timer is a T312 timer.

7. A user equipment UE in a wireless communication system, the UE include: Transceiver; as well as at least one processor connected to the transceiver, wherein the at least one processor is configured to: receiving a message including a reconfiguration with synchronization from a source base station; Starting a first timer; In a case where a dual active protocol stack DAPS bearer is configured based on the message, a radio link control RLC entity for the target base station is established, and a signaling radio bearer SRB for the source base station is suspended; When the first timer expires and the radio link between the UE and the source base station is active, release the RLC entity for the target base station, restore the suspended SRB for the source base station, and send a DAPS handover failure report to the source base station; as well as When the first timer expires and the radio link between the UE and the source base station is inactive, a radio resource control RRC reestablishment procedure is performed.

8. The UE of claim 7, wherein the at least one processor is configured to: When a condition for starting the third timer is met, the third timer is started while the second timer is running.

9. The UE of claim 7, wherein the at least one processor is configured to: In case the DAPS bearer is configured, an out-of-sync indication is received from a lower layer entity, and the first timer is running, a second timer for the source base station is started.

10. The UE of claim 9, wherein the at least one processor is configured to: In case a synchronization indication is received from the lower layer entity while the second timer is running, the second timer is stopped, and if the third timer is running, the third timer is stopped.

11. The UE according to claim 7, in, When the third timer expires, the RRC reestablishment procedure is performed. 12 . The UE according to claim 8 , wherein the first timer is a T304 timer, the second timer is a T310 timer, and the third timer is a T312 timer.

13. A method performed by a source base station in a wireless communication system, the method include: sending a message including a reconfiguration with synchronization to a user equipment UE, wherein a first timer is started at the UE; Wherein, when a dual active protocol stack DAPS bearer is configured based on the message, a radio link control RLC entity for the target base station is established at the UE, and a signaling radio bearer SRB for the source base station is suspended at the UE; Wherein, when the first timer expires and the radio link between the UE and the source base station is active, the RLC entity for the target base station is released at the UE, the suspended SRB for the source base station is restored at the UE, and a DAPS handover failure report is sent at the UE to the source base station; and Wherein, when the first timer expires and the radio link between the UE and the source base station is inactive, a radio resource control RRC reestablishment process is performed at the UE.

14. A source base station in a wireless communication system, the source base station include: Transceiver; as well as at least one processor coupled to the transceiver and configured to: sending a message including a reconfiguration with synchronization to a user equipment UE, Wherein, a first timer is started at the UE; Wherein, when a dual active protocol stack DAPS bearer is configured based on the message, a radio link control RLC entity for the target base station is established at the UE, and a signaling radio bearer SRB for the source base station is suspended at the UE; Wherein, when the first timer expires and the radio link between the UE and the source base station is active, the RLC entity for the target base station is released at the UE, the suspended SRB for the source base station is restored at the UE, and a DAPS handover failure report is sent at the UE to the source base station; and Wherein, when the first timer expires and the radio link between the UE and the source base station is inactive, a radio resource control RRC reestablishment process is performed at the UE.

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