阅读说明：本技术 在下一代移动通信系统中执行无线电链路故障报告的方法和设备 (Method and apparatus for performing radio link failure reporting in next generation mobile communication system ) 是由 金相范 金成勳 于 2020-04-29 设计创作，主要内容包括：本公开涉及一种用于融合支持比第四代(4G)系统更高的数据速率的第五代(5G)通信系统与物联网(IoT)技术的通信方法和系统。本公开可应用于基于5G通信技术和IoT相关技术的智能服务,诸如智能家居、智能建筑、智慧城市、智能汽车、联网汽车、健康护理、数字教育、智能零售、安保和安全服务。一种由终端执行的方法包括：检测无线电链路故障(RLF)并启动第一定时器；在第一定时器期满之前终端未找到能够与其连接的适合小区的情况下,进入IDLE状态；以及向其中终端能够从IDLE状态转变到连接模式状态的小区发送RLF报告消息,该RLF报告消息包括与适合小区相关联的信息。(The present disclosure relates to a communication method and system for fusing a fifth generation (5G) communication system supporting a higher data rate than a fourth generation (4G) system with internet of things (IoT) technology. The present disclosure is applicable to smart services based on 5G communication technologies and IoT related technologies, such as smart homes, smart buildings, smart cities, smart cars, networked cars, healthcare, digital education, smart retail, security, and security services. A method performed by a terminal comprising: detecting a Radio Link Failure (RLF) and starting a first timer; entering an IDLE state in case the terminal does not find a suitable cell to which it can connect before the expiration of the first timer; and transmitting an RLF report message including information associated with the suitable cell to the cell in which the terminal can transition from the IDLE state to the connected mode state.)

1. A method performed by a terminal in a mobile communication system, the method comprising:

detecting a radio link failure, RLF, and starting a first timer in case of a certain event;

entering an IDLE state if the terminal does not find a suitable cell to which it can connect before the first timer expires; and

transmitting an RLF report message including information associated with the suitable cell that can be CONNECTED with the terminal before the first timer expires, to a cell in which the terminal can transition from the IDLE state to a CONNECTED mode CONNECTED state.

2. The method of claim 1, wherein the information comprises information indicating that no suitable cell was found prior to expiration of the first timer.

3. The method of claim 1, further comprising:

in case the terminal finds a suitable cell to which it can connect before the first timer expires, attempting a radio connection re-establishment with the suitable cell and starting a second timer,

entering the IDLE state if the radio connection reestablishment with the suitable cell is not completed before the second timer expires; and

transmitting the RLF report message including information associated with the suitable cell that can be CONNECTED with the terminal before the first timer expires, to a cell that the terminal can transition from the IDLE state to the CONNECTED state.

4. The method of claim 1, wherein:

in case the specific event indicates a conditional handover, CHO, failure, the RLF report message further comprises information about the CHO failure; and

the information about the CHO failure includes at least one of: a cause value indicating that RLF occurs during CHO execution, information on at least one condition triggering the CHO, information on an elapsed time from a specific point in time to the occurrence of RLF, or ID list information of target cells in case of attempting handover to a plurality of target cells.

5. A method performed by a base station in a mobile communication system, the method comprising:

performing radio connection setup with the terminal; and

receiving a Radio Link Failure (RLF) report message from the terminal,

wherein a first timer associated with the terminal is started based on RLF detection of the terminal in case of occurrence of a specific event, an

Wherein the RLF report message includes information associated with a suitable cell that can be connected with the terminal before the first timer expires.

6. The method of claim 5, wherein the information comprises information indicating that no suitable cell was found prior to expiration of the first timer.

7. The method of claim 5, wherein:

in case the specific event indicates a conditional handover, CHO, failure, the RLF report message further comprises information about the CHO failure; and

the information about the CHO failure includes at least one of: a cause value indicating that RLF occurs during CHO execution, information on at least one condition triggering the CHO, information on an elapsed time from a specific point in time to the occurrence of RLF, or ID list information of target cells in case of attempting handover to a plurality of target cells.

8. A terminal of a mobile communication system, the terminal comprising:

a transceiver; and

a controller configured to:

detecting a radio link failure, RLF, and starting a first timer in case of a certain event;

entering an IDLE state if the terminal does not find a suitable cell to which it can connect before the first timer expires; and

controlling the transceiver to transmit an RLF report message to a cell in which the terminal can transition from the IDLE state to a CONNECTED mode CONNECTED state, the RLF report message including information associated with the suitable cell that can be CONNECTED with the terminal before expiration of the first timer.

9. The terminal of claim 8, wherein the information comprises information indicating that no suitable cell was found prior to expiration of the first timer.

10. The terminal of claim 8, wherein the controller is configured to: in the case where the terminal finds the suitable cell before the first timer expires, attempting a radio connection re-establishment with the suitable cell and starting a second timer.

11. The terminal of claim 10, wherein the controller is configured to:

entering the IDLE state if the radio connection reestablishment with the suitable cell is not completed before the second timer expires; and

controlling the transceiver to transmit the RLF report message to a cell to which the terminal can transition from the IDLE state to the CONNECTED state, the RLF report message including information associated with the suitable cell to which the terminal can connect before the first timer expires.

12. The terminal of claim 8, wherein:

in case the specific event indicates a conditional handover, CHO, failure, the RLF report message further comprises information about the CHO failure; and

the information about the CHO failure includes at least one of: a cause value indicating that RLF occurs during CHO execution, information on at least one condition triggering the CHO, information on an elapsed time from a specific point in time to the occurrence of RLF, or ID list information of target cells in case of attempting handover to a plurality of target cells.

13. A base station of a mobile communication system, the base station comprising:

a transceiver; and

a controller configured to:

performing radio connection setup with a terminal, an

Control the transceiver to receive a Radio Link Failure (RLF) report message from the terminal,

wherein a first timer associated with the terminal is started based on RLF detection of the terminal in case of occurrence of a specific event, an

Wherein the RLF report message includes information associated with a suitable cell that can be connected with the terminal before the first timer expires.

14. The base station of claim 13, wherein the information comprises information indicating that no suitable cell was found prior to expiration of the first timer.

15. The base station of claim 13, wherein:

in case the specific event indicates a conditional handover, CHO, failure, the RLF report message further comprises information about the CHO failure; and

the information about the CHO failure includes at least one of: a cause value indicating that RLF occurs during CHO execution, information on at least one condition triggering the CHO, information on an elapsed time from a specific point in time to the occurrence of RLF, or ID list information of target cells in case of attempting handover to a plurality of target cells.

Technical Field

The present disclosure relates to operations of a terminal and a base station in a next generation mobile communication system, and more particularly, to a method and apparatus for performing radio link failure reporting in a next generation mobile communication system.

Background

In order to meet the increasing demand for wireless data services since the deployment of 4G communication systems, efforts have been made to develop an improved 5G or quasi-5G communication system. Accordingly, the 5G or quasi-5G communication system is also referred to as a "super 4G network" or a "post-LTE system". The 5G communication system is considered to be implemented at a higher frequency (millimeter wave) band (for example, 60GHz band) in order to achieve a higher data rate. In order to reduce propagation loss of radio waves and increase transmission distance, beamforming, massive Multiple Input Multiple Output (MIMO), full-dimensional MIMO (FD-MIMO), array antenna, analog beamforming, large antenna technology are discussed in the 5G communication system. In addition, in the 5G communication system, development of system network improvement based on advanced small cells, cloud Radio Access Network (RAN), ultra dense network, device-to-device (D2D) communication, wireless backhaul, mobile network, cooperative communication, coordinated multipoint (CoMP), reception side interference cancellation, and the like is ongoing. In 5G systems, hybrid FSK and QAM modulation (FQAM) and Sliding Window Superposition Coding (SWSC) have been developed as Advanced Coding Modulation (ACM), and filter bank multi-carrier (FBMC), non-orthogonal multiple access (NOMA), and Sparse Code Multiple Access (SCMA) as advanced access techniques.

The internet, which is a human-centric connectivity network in which humans generate and consume information, is now evolving into the internet of things (IoT), where distributed entities, such as things, exchange and process information without human intervention. Internet of everything (IoE), which is a combination of IoT technology and big data processing technology through connection with a cloud server, has emerged. Since IoT implementations require technical elements such as "sensing technology", "wired/wireless communication and network infrastructure", "service interface technology", and "security technology", sensor networks, machine-to-machine (M2M) communication, Machine Type Communication (MTC), etc. have recently been studied. Such IoT environments can provide intelligent internet technology services that create new value for human life by collecting and analyzing data generated between connected things. IoT can be applied in a variety of fields including smart homes, smart buildings, smart cities, smart cars or networked cars, smart grids, healthcare, smart home appliances, and advanced medical services through fusion and combination between existing Information Technology (IT) and various industrial applications.

Accordingly, various attempts have been made to apply the 5G communication system to the IoT network. For example, technologies such as sensor networks, Machine Type Communication (MTC), and machine-to-machine (M2M) communication may be implemented through beamforming, MIMO, and array antennas. The application of cloud Radio Access Network (RAN) as the big data processing technology described above can also be considered as an example of the convergence of 5G technology with IoT technology.

The above information is presented merely as background information to aid in understanding the present disclosure. No determination has been made, nor has an assertion been made, as to whether any of the above can be applied as prior art to the present disclosure.

Disclosure of Invention

[ problem ] to

A technical problem to be solved in the embodiments is to provide a method and apparatus for performing radio link failure reporting in a next generation mobile communication system.

In addition, the technical problem to be solved in the embodiments is to provide a method and apparatus for supporting multiple DRX configuration information in a next generation mobile communication system.

In addition, a technical problem to be solved in the embodiments relates to a method and apparatus for improving radio link failure reporting in a next generation mobile communication system.

[ solution ]

In order to solve the above problem, a method performed by a terminal in a mobile communication system according to an embodiment includes: detecting a Radio Link Failure (RLF) and starting a first timer in case of a certain event; entering an IDLE state in case the terminal does not find a suitable cell to which it can connect before the expiration of the first timer; and transmitting an RLF report message including information associated with a suitable cell that can be CONNECTED with the terminal before expiration of the first timer, to a cell in which the terminal can transition from an IDLE state to a CONNECTED mode (CONNECTED) state.

According to another embodiment, a method performed by a base station in a mobile communication system, comprises: performing radio connection setup with the terminal; and receiving a Radio Link Failure (RLF) report message from the terminal, wherein a first timer associated with the terminal is started based on RLF detection by the terminal in case of occurrence of a specific event, and wherein the RLF report message includes information associated with a suitable cell that can be connected with the terminal before expiration of the first timer.

According to yet another embodiment, a terminal of a mobile communication system includes a transceiver and a controller. The controller is configured to: detecting a Radio Link Failure (RLF) and starting a first timer in case a certain event occurs; entering an IDLE state in case the terminal does not find a suitable cell to which it can connect before the expiration of the first timer; and controlling the transceiver to transmit an RLF report message including information associated with a suitable cell that can be CONNECTED with the terminal before expiration of the first timer, to a cell in which the terminal can transition from an IDLE state to a CONNECTED mode (CONNECTED) state.

According to yet another embodiment, a base station of a mobile communication system includes a transceiver and a controller. The controller is configured to: the method includes performing radio connection setup with a terminal, and controlling a transceiver to receive a Radio Link Failure (RLF) report message from the terminal, wherein a first timer associated with the terminal is started based on RLF detection of the terminal in case of occurrence of a specific event, and wherein the RLF report message includes information associated with a suitable cell that can be connected with the terminal before expiration of the first timer.

[ advantageous effects of the invention ]

Apparatuses and methods according to various embodiments provide a method for performing radio link failure reporting in a next generation mobile communication system, a method for supporting multiple DRX configuration information in a next generation mobile communication system, and a method for improving radio link failure reporting in a next generation mobile communication system.

Effects obtainable from the present disclosure may not be limited to the above-described effects, and other effects not mentioned may be clearly understood by those skilled in the art to which the present disclosure pertains through the following description.

Drawings

For a more complete understanding of the present disclosure and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which like reference numbers represent like parts:

fig. 1A shows a structure of an LTE system to which the present disclosure is applied;

fig. 1B illustrates a radio protocol structure in an LTE system to which the present disclosure is applied;

fig. 1C is a flowchart illustrating a procedure of performing a first handover operation in a mobile communication system;

fig. 1D shows a flowchart of a procedure for performing a second handover operation in the mobile communication system;

fig. 1E shows a flow chart of UE operation in the present disclosure;

fig. 1F shows a block diagram showing the internal structure of a UE to which the present disclosure is applied;

fig. 1G shows a block diagram showing a configuration of a base station according to the present disclosure;

fig. 2A shows the structure of a next-generation mobile communication system;

fig. 2B illustrates DRX operation in the prior art LTE;

figure 2C illustrates a flow chart of a method of providing preferred DRX configuration information by a UE in the present disclosure;

fig. 2D shows a flow chart of UE operation in the present disclosure;

fig. 2E shows a flow chart of base station operation in the present disclosure;

fig. 3A shows a structure of an LTE system to which the present disclosure is applied;

fig. 3B illustrates a radio protocol structure in an LTE system to which the present disclosure is applied;

fig. 3C illustrates a Radio Link Monitoring (RLM) operation in the present disclosure;

fig. 3D illustrates Radio Link Failure (RLF) operation in the present disclosure;

FIG. 3E illustrates a process in the present disclosure to collect useful information after RLF;

FIG. 3F illustrates a flow chart of a process in the present disclosure for collecting useful information after RLF; and

fig. 3G illustrates a flow chart of UE operation in this disclosure to collect useful information after RLF.

Detailed Description

Before proceeding with the following detailed description, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms "include" and "comprise," and derivatives thereof, mean including but not limited to; the term "or" is inclusive, meaning and/or; the phrases "associated with …" and "associated therewith," as well as derivatives thereof, may mean to include, be included within …, be interconnected with …, include, be included within …, be connected to or with …, be coupled to or with …, be capable of communicating with …, cooperate with …, be staggered, juxtaposed, proximate to, bound to or with …, have the nature of …, and the like; and the term "controller" means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely.

Further, the various functions described below may be implemented or supported by one or more computer programs, each formed from computer readable program code and embodied in a computer readable medium. The terms "application" and "program" refer to one or more computer programs, software components, sets of instructions, procedures, functions, objects, classes, instances, related data, or portions thereof adapted for implementation in suitable computer readable program code. The phrase "computer readable program code" includes any type of computer code, including source code, object code, and executable code. The phrase "computer readable medium" includes any type of medium capable of being accessed by a computer, such as Read Only Memory (ROM), Random Access Memory (RAM), a hard disk drive, a Compact Disc (CD), a Digital Video Disc (DVD), or any other type of memory. A "non-transitory" computer-readable medium does not include a wired, wireless, optical, or other communication link that transmits transitory electrical or other signals. Non-transitory computer readable media include media in which data can be permanently stored and media in which data can be stored and subsequently rewritten, such as rewritable optical disks or erasable memory devices.

Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.

Figures 1A through 3G, discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the disclosure may be implemented in any suitably arranged system or device.

In the following description of the present disclosure, a detailed description of known functions or configurations incorporated herein will be omitted when it may make the subject matter of the present disclosure unnecessarily obscure. Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings.

The present disclosure is prepared based on the LTE system, but is also applicable to other mobile communication systems, such as NR as a next generation mobile communication system. For example, in the present disclosure, eNB in LTE corresponds to gNB in NR, and MME in LTE corresponds to AMF in NR.

Fig. 1A shows a structure of an LTE system to which the present disclosure is applied.

Referring to fig. 1a, a radio access network of an LTE system includes next generation base stations (also referred to as evolved node bs, hereinafter referred to as ENBs, node bs, or base stations) 1a-05, 1a-10, 1a-15, and 1a-20, Mobility Management Entities (MMEs) 1a-25, and serving gateways (S-GWs) 1 a-30. User equipments (hereinafter referred to as UEs or terminals) 1a-35 access external networks through ENBs 1a-05, 1a-10, 1a-15 and 1a-20 and S-GWs 1 a-30.

In FIG. 1A, ENBs 1A-05, 1A-10, 1A-15, and 1A-20 correspond to existing node Bs of a UMTS system. The ENB is connected to the UEs 1a-35 via radio channels and acts in a more complex role than existing node Bs. In the LTE system, since all user traffic is served through a shared channel, including a real-time service such as voice over IP (VoIP) through an internet protocol, a means for collecting and scheduling status information such as a buffer status, an available transmission power status, and a channel status of a UE is required. The ENBs 1a-05, 1a-10, 1a-15, and 1a-20 are used to perform this function of the device. Generally, one ENB controls a plurality of cells. For example, to implement a transmission rate of 100Mbps, for example, in a 20MHz bandwidth, the LTE system uses Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology. In addition, the LTE system employs an adaptive modulation and coding (hereinafter, referred to as AMC) scheme, and determines a modulation scheme and a channel coding rate according to a channel status used by a terminal. The S-GW 1a-30 is a device for providing a data bearer, and generates or removes the data bearer under the control of the MME 1 a-25. The MME is a device for performing various control functions of the terminal and a mobility management function, and is connected to a plurality of base stations.

Fig. 1B shows a radio protocol structure in an LTE system to which the present disclosure is applied.

Referring to fig. 1B, radio protocols of the LTE system include Packet Data Convergence Protocols (PDCP)1B-05 and 1B-40, Radio Link Controls (RLC)1B-10 and 1B-35, and Medium Access Controls (MAC)1B-15 and 1B-30 in the UE and eNB, respectively. Packet Data Convergence Protocol (PDCP)1b-05 and 1b-40 are used to perform operations such as IP header compression/recovery. The MACs 1b-15 and 1b-30 are connected to a plurality of RLC layer devices configured in one terminal and can perform operations of multiplexing RLC PDUs with MAC PDUs and demultiplexing RLC PDUs with MAC PDUs. The physical layers 1b-20 and 1b-25 may perform the following operations: channel coding and modulating the higher layer data, generating the higher layer data into OFDM symbols, transmitting the OFDM symbols through a radio channel, or demodulating OFDM symbols received through a radio channel, channel decoding the OFDM symbols, and transmitting the OFDM symbols to the higher layer.

The present disclosure proposes to collect useful information in case of a condition-based handover failure and thus RLF occurring in a mobile communication system such as LTE or NR. In the present disclosure, the first switching operation refers to the following operations: if the terminal receives configuration information indicating that handover is performed from the base station, the terminal immediately performs a handover operation. On the other hand, the second switching operation refers to the following operations: if the terminal receives configuration information indicating that handover is performed from the base station, the terminal does not immediately perform a handover operation but performs a handover operation if a specific condition is satisfied. Due to the above features, the second switching operation is referred to as a condition-based switching or conditional switching (CHO). Since the terminal can identify the change of the channel quality state most quickly, the characteristic that the terminal determines the time point of initiating the switching operation is beneficial to minimizing the probability of switching failure. Therefore, the second handover is considered to be a more advanced technique than the first handover. Only one target cell may be considered in the first handover and one or more target cells may be considered in the second handover. The network decides the number of target cells considered in the second handover. To minimize the complexity of the neighboring target cells, only one target cell may be considered in the second handover. The second handover (condition-based handover) may also fail, where RLF is declared (or detected). At this time, useful information may be collected and then reported in case of switching the terminal mode to the connection mode, which is called RLF reporting. The present disclosure proposes to collect useful information when RLF occurs due to a second handover failure.

Fig. 1C shows a flowchart of a procedure for performing a first handover operation in a mobile communication system.

The UE 1c-05 receives an RRC message (indicated by reference numeral 1 c-25) including measurement configuration information from the source cell 1 c-10. The UE measures the signal quality of the serving cell and the neighbor cells by applying the measurement configuration information, and if a periodic or configured event (indicated by reference numerals 1 c-30) occurs, the UE reports the collected cell measurement information (indicated by reference numerals 1 c-35) to the source cell. The source cell determines whether to trigger a first handover operation (indicated by reference numerals 1 c-40) based on the reported cell measurement information. For example, the source cell may determine the first handover in case event a3 is satisfied (the offset of the neighboring cell becomes over the SpCell) and thus cell measurement information is reported. If it is determined that the first handover is triggered, the source cell requests the first handover from one target cell 1c-20 through an inter-node message (indicated by reference numeral 1 c-45). The target cell that received the request accepts the request and sends handover configuration information required for the first handover operation to the source cell (indicated by reference numerals 1 c-50). The source cell includes the handover configuration information and the additional configuration information received from the target cell in an RRC message and transmits the RRC message to the UE (indicated by reference numerals 1 c-55). The configuration information includes a target cell ID, frequency information, configuration information (dedicated preamble information, dedicated radio resource information, etc.) required for a random access operation to the target cell, transmission power information, and C-RNTI information used in the target cell.

Upon receiving the handover configuration information, the UE performs a random access procedure to the target cell and starts (or drives) a T304 timer (indicated by reference numerals 1 c-60). The UE transmits the received preamble (indicated by reference numerals 1 c-65). The UE transmits one of the contention-based preambles if the dedicated preamble is not provided. The target cell receiving the preamble transmits random access response information (RAR) to the UE (indicated by reference numerals 1 c-70). The UE sends message 3 (indicated by reference numerals 1 c-75) to the target cell using the UL grant information stored in the RAR. Message 3 stores rrcconnectionreconfiguration complete message under the LTE system and rrcconnectionreconfiguration complete message under the NR system. If the random access procedure is successfully completed, the first handover is considered successfully completed and the running T304 timer is stopped. If the first handover is not successfully completed until the timer expires at T304, the handover is considered to have failed.

Fig. 1D shows a flowchart of a procedure for performing a second handover operation in the mobile communication system.

The UE 1d-05 reports its own capability information (indicated by reference numeral 1 d-25) to the source cell 1 d-10. The capability information indicates whether the UE supports the second handover. The UE receives an RRC message (indicated by reference numerals 1 d-30) including measurement configuration information from the source cell. The UE measures the signal quality of the serving cell and the neighbor cells by applying the measurement configuration information, and if a periodic or configured event (indicated by reference numerals 1 d-35) occurs, the UE reports the collected cell measurement information (indicated by reference numerals 1 d-40) to the source cell. The source cell determines whether to trigger a second handover operation (indicated by reference numerals 1 d-45) based on the reported cell measurement information. To configure the second handover, the UE needs to support the second handover. If it is determined that the second handover is triggered, the source cell requests the second handover from one or more target cells 1d-20 through an inter-node message (indicated by reference numerals 1 d-50). The target cell that received the request accepts the request and sends handover configuration information required for the second handover operation to the source cell (indicated by reference numerals 1 d-55). The target cell that does not accept the request will not perform the second handover. The source cell includes the handover configuration information and the additional configuration information received from the target cell in an RRC message and transmits the RRC message to the UE (indicated by reference numerals 1 d-60). The configuration information includes each ID of the target cell, frequency information, configuration information (preamble information and dedicated radio resource information for each target cell, etc.) required for a random access operation to the target cell, transmission power information, C-RNTI information used in each target cell, conditions for triggering a random access operation to each target cell, etc. Each of the above conditions may be different for each target cell, and a plurality of conditions may be configured for one target cell.

Upon receiving the handover configuration information, the UE evaluates whether the received conditions (indicated by reference numerals 1 d-65) are met. If a condition related to a specific target cell is satisfied, the UE performs a random access procedure (indicated by reference numerals 1 d-70) with respect to the target cell and starts a first timer (indicated by reference numerals 1 d-75). For example, if an event a3 (the offset of the neighboring cell becomes more than the SpCell) is configured based on the above condition and the condition is satisfied, the UE transmits the received preamble to the relevant target cell. The UE transmits one of the contention-based preambles if the dedicated preamble is not provided. The target cell receiving the preamble transmits random access response information (RAR) to the UE. The UE sends message 3 to the target cell using the UL grant information stored in the RAR. Message 3 stores RRCConnectionReconfigurationComplete message in case of LTE system and rrcconfigurationcomplete message in case of NR system. And if the random access process is successfully completed, the second switching is considered to be successfully completed and the first timer is stopped running. If the second handover (indicated by reference numerals 1 d-80) is not successfully completed until (or before) the expiration of the first timer, the handover is considered to be failed. At this time, it is referred to as RLF (indicated by reference numerals 1 d-85) due to handover failure.

The UE collects and stores information (indicated by reference numerals 1 d-90) related to CHO failures. Thereafter, in case the UE successfully switches to the connected mode, the UE sends an RRC message to the base station, the RRC message including an indicator (indicated by reference numeral 1 d-95) indicating information collected at RLF due to conditional handover failure. The RRC message is a RRCSetupComplete or rrcreestablshmenticomplete message. The UE receives an RRC message (indicated by reference numerals 1 d-100) indicating report information from the base station. The UE reports the collected and stored information (indicated by reference numerals 1 d-105) to the base station.

And if the switching is successfully completed, the UE deletes the switching configuration information. And under the condition that the source cell receives the switching success report from the target cell, the source cell deletes the context information of the UE. Whether the handover is successful may be determined based on UE context release information, which is an inter-node message sent from the target cell to the source cell. In addition, the source cell indicates other candidate target cells included in the handover configuration information to delete the handover configuration information (or UE context information), or provides notification that the handover configuration information is no longer valid. The candidate target cell itself may delete the handover configuration information even if no instruction is received from the source cell if a predetermined time interval has elapsed after receiving the handover request.

Fig. 1E shows a flow chart of UE operation in the present disclosure.

In operation 1e-05, the UE enters a connected mode.

In operation 1e-10, the UE transmits capability information including an indicator indicating whether the UE itself supports the second handover (condition-based handover, Conditional Handover (CHO)) to the base station.

In operations 1e-15, the UE receives a configuration of the first handover or the second handover from the base station.

In operation 1e-20, the UE determines that the configured handover has failed.

In operations 1e-25, the UE determines whether the configured handover is a first handover or a second handover. That is, the UE determines whether the handover is triggered by a condition (random access to the target cell).

In operation 1e-30, if the first handover is triggered and thus RLF occurs, the following information is collected and stored.

-plmn-IdentityList

-measResultLastServCell

-measResultNeighCells

-locationInfo

-failedPCellId

-previousPCellId

-timeConnFailure

C-RNTI used in Source PCell

-connectionFailureType set to' hof

In operation 1e-35, if the second handover is triggered and thus RLF occurs, the following information is collected and stored.

-plmn-IdentityList

-measResultLastServCell

-measResultNeighCells

-locationInfo

-failedPCellId

-previousPCellId

-timeConnFailure

C-RNTI used in Source PCell

-connectionFailureType set to 'chof': which defines a new reason for the occurrence of RLF during the execution of the second handover.

CHO conditions triggering HO operation: condition information that triggers a handover operation, for example, an event type (event a3, etc.), threshold information applied to a corresponding event, a cell measurement value in case an event is satisfied, and the like.

-time that has elapsed since sending a preamble to a target candidate cell that has fulfilled the configured condition: time required for RLF to occur after transmitting a preamble to a target cell that has satisfied a configuration condition, or time required for RLF to occur after satisfying a configuration condition

Target candidate cell id in case multiple attempts are allowed: ID list information of target cells if handover to a plurality of target cells is performed

Thereafter, in operation 1e-40, the UE reports the stored information if the mode of the UE is successfully switched to the connected mode.

Fig. 1F shows the structure of the UE.

Referring to FIG. 1F, the UE includes Radio Frequency (RF) processors 1F-10, baseband processors 1F-20, memories 1F-30, and controllers 1F-40.

The RF processors 1f-10 perform signal transmission or reception functions such as band conversion and amplification of signals through radio channels. That is, the RF processors 1f-10 up-convert baseband signals provided from the baseband processors 1f-20 into RF band signals and transmit the RF band signals through the antennas, and down-convert RF band signals received through the antennas into baseband signals. For example, the RF processors 1f-10 may include transmit filters, receive filters, amplifiers, mixers, oscillators, digital-to-analog converters (DACs), analog-to-digital converters (ADCs), and so forth. In fig. 1F, only one antenna is shown, but the terminal may include a plurality of antennas. Further, the RF processors 1f-10 may include a plurality of RF chains. In addition, the RF processors 1f-10 may perform beamforming. For beamforming, the RF processors 1f-10 may adjust the phase and amplitude of each of the signals transmitted or received through the plurality of antennas or antenna elements. In addition, the RF processor may perform MIMO operation, and may perform MIMO operation by receiving a plurality of layers.

The baseband processors 1f-20 perform the function of converting between baseband signals and bit strings according to the physical layer standard of the system. For example, when transmitting data, the baseband processors 1f to 20 generate complex symbols by encoding and modulating a transmission bit stream. In addition, upon receiving data, the baseband processors 1f to 20 reconstruct the received bit string by demodulating and decoding the baseband signals supplied from the RF processors 1f to 10. For example, according to an Orthogonal Frequency Division Multiplexing (OFDM) scheme, when transmitting data, the baseband processors 1f-20 generate complex symbols by encoding and modulating a transmission bit stream, map the complex symbols onto subcarriers, and then configure the OFDM symbols by performing an Inverse Fast Fourier Transform (IFFT) operation and insertion of a Cyclic Prefix (CP). In addition, upon receiving data, the baseband processors 1f to 20 divide the baseband signals provided from the RF processors 1f to 10 into OFDM symbol units, reconstruct the signals mapped onto the subcarriers through a Fast Fourier Transform (FFT) operation, and then reconstruct the received bit string by demodulating and decoding the mapped signals.

As described above, the baseband processors 1f-20 and the RF processors 1f-10 transmit and receive signals. Thus, each of the baseband processors 1f-20 and the RF processors 1f-10 may be referred to as a transmitter, a receiver, a transceiver, or a communication unit. Further, at least one of the baseband processor 1f-20 and the RF processor 1f-10 may include a plurality of communication modules supporting different radio access technologies. In addition, at least one of the baseband processors 1f-20 and the RF processors 1f-10 may include different communication modules that process signals of different frequency bands. For example, the different radio access technologies may include wireless LAN (e.g., IEEE 802.11), cellular network (e.g., LTE), and so on. In addition, the different frequency bands may include the ultra high frequency (SHF) (e.g., 2.NRHz, NRHz) band and the millimeter wave (e.g., 60GHz) band.

The memories 1f-30 store data such as default programs, application programs, and configuration information for performing UE operations. In particular, the memories 1f-30 store information relating to a second access node for performing wireless communication using a second radio access technology. The memory 1f-30 provides the stored data at the request of the controller 1 f-40.

The controllers 1f-40 control the overall operation of the terminal. For example, the controller 1f-40 transmits or receives signals through the baseband processor 1f-20 and the RF processor 1 f-10. In addition, the controller 1f-40 records data in the memory 1f-30 or reads data from the memory 1 f-30. To this end, the controllers 1f-40 may include at least one processor. For example, the controllers 1f-40 may include a Communication Processor (CP) for performing communication control and an Application Processor (AP) for controlling higher layers such as application programs.

Fig. 1G shows a block configuration of a base station in a wireless communication system according to an embodiment.

As shown in fig. 1G, the base station includes RF processors 1G-10, baseband processors 1G-20, backhaul communication units 1G-30, memories 1G-40, and controllers 1G-50.

The RF processors 1g-10 perform functions of transmitting or receiving signals such as band conversion and amplification of the signals through radio channels. That is, the RF processors 1g-10 up-convert baseband signals provided from the baseband processors 1g-20 into RF band signals and transmit the RF band signals through the antennas, and down-convert RF band signals received through the antennas into baseband signals. For example, the RF processors 1g-10 may include transmit filters, receive filters, amplifiers, mixers, oscillators, digital-to-analog converters (DACs), analog-to-digital converters (ADCs), and so forth. In fig. 1G, only one antenna is shown, but the first access node may comprise multiple antennas. Further, the RF processors 1g-10 may include multiple RF chains. In addition, the RF processors 1g-10 may perform beamforming. For beamforming, the RF processors 1g-10 may adjust the phase and amplitude of each of the signals transmitted or received through the plurality of antennas or antenna elements. In addition, the RF processor may perform MIMO operations by transmitting one or more layers.

The baseband processors 1g-20 perform the function of converting between baseband signals and bit strings according to the physical layer standard of the first radio access technology. For example, when transmitting data, the baseband processors 1g to 20 generate complex symbols by encoding and modulating a transmission bit stream. In addition, upon receiving data, the baseband processors 1g-20 reconstruct a received bit string by demodulating and decoding the baseband signals supplied from the RF processors 1 g-10. For example, according to an Orthogonal Frequency Division Multiplexing (OFDM) scheme, when transmitting data, the baseband processors 1g-20 generate complex symbols by encoding and modulating a transmission bit stream, map the complex symbols onto subcarriers, and then configure the OFDM symbols by performing an IFFT operation and CP insertion. In addition, upon receiving data, the baseband processors 1g-20 divide the baseband signals provided from the RF processors 1g-10 into OFDM symbol units, reconstruct signals mapped onto subcarriers through FFT operations, and then reconstruct received bit strings by demodulating and decoding the mapped signals. The baseband processors 1g-20 and the RF processors 1g-10 transmit or receive signals as described above. Accordingly, each of the baseband processors 1g-20 and the RF processors 1g-10 may be referred to as a transmitter, a receiver, a transceiver, a communication unit, or a wireless communication unit.

The backhaul communication units 1g-30 provide interfaces for communicating with other nodes in the network. That is, the backhaul communication units 1g-30 convert a bit string transmitted from the main base station to another node (e.g., a sub base station, a core network, etc.) into a physical signal, and convert a physical signal received from another node into a bit string.

The memories 1g-40 store data such as basic programs, application programs, and configuration information for performing the operation of the main base station. In particular, the memories 1g-40 may store information about bearers allocated to the connected UE, measurement results reported by the connected UE, and the like. In addition, the memories 1g-40 may store information serving as criteria for determining whether to provide or terminate a plurality of connections to the terminal. In addition, the memories 1g-40 provide the stored data at the request of the controllers 1 g-50.

The controllers 1g-50 control the overall operation of the main base station. For example, the controller 1g-50 transmits or receives signals through the baseband processor 1g-20 and the RF processor 1g-10 or through the backhaul communication unit 1 g-30. In addition, the controller 1g-50 records data in the memory 1g-40 or reads data from the memory 1 g-40. To this end, the controllers 1g-50 may include at least one processor.

Fig. 2A shows an EN-DC structure of a next generation mobile communication system.

The EN-DC refers to a dual connection between EUTRAN (LTE system) and NR (next generation mobile communication system), and corresponds to a case where one UE is simultaneously connected to two heterogeneous systems to receive a service.

Referring to fig. 2A, a radio access network of a next generation mobile communication system includes next generation base stations (new radio node bs (hereinafter referred to as "gnbs") 2A-10 and AMFs (new radio core networks) 2A-05 user equipments (hereinafter referred to as NR UEs or terminals) 2A-15 access an external network via the gnbs 2A-10 and the AMFs 2A-05.

In fig. 2A, the gnbs 2A-10 correspond to evolved node bs (enbs) of a legacy LTE system. The gNB is connected to the NR UEs 2a-15 via radio channels and can provide superior service compared to legacy node Bs. In the next generation mobile communication system, since all types of user traffic are served through a shared channel, an apparatus for performing scheduling by collecting state information such as a buffer state, an available transmission power state, and a channel state of a UE is required. Further, the gNB 2a-10 is used to perform such functions of the device. Generally, one gNB controls a plurality of cells. To implement ultra-high speed data transmission beyond conventional LTE, the gNB may have a conventional maximum bandwidth or greater, and additionally may employ a beamforming technique using Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technique. In addition, an adaptive modulation and coding (hereinafter, referred to as AMC) scheme employed by the gNB determines a modulation scheme and a channel coding rate according to a state of a channel of the UE. The AMFs 2a-05 perform functions such as mobility support, bearer setup and QoS setup. The AMF 2a-05 is a device for performing various control functions and mobility management functions for the UE, and is connected to a plurality of base stations. In addition, the next generation mobile communication system may also operate in conjunction with a conventional LTE system, and the AMF is connected to the MMEs 2a-25 via a network interface. The MME may be connected to eNBs 2a-30, legacy base stations. In the EN-DC case, the gNB is connected to the eNB to be controlled.

Fig. 2B illustrates DRX operation. The DRX operation is for minimizing the amount of power consumed by the UE, and is a technique for performing monitoring only in a predetermined PDCCH to obtain scheduling information. DRX operation can operate in idle mode and connected mode with slightly different methods of operation. The present disclosure relates to a connection mode. Continuous monitoring of the PDCCH by the UE to acquire scheduling information may increase power consumption. For basic DRX operation, a DRX cycle 2b-00 is defined and the PDCCH is monitored only during the on duration 2 b-05. In connected mode, two types of values are configured for the DRX cycle, namely long DRX and short DRX. The long DRX cycle applies to the general case and the base station can use the MAC Control Element (CE) to trigger the short DRX cycle if needed. After a predetermined period of time has elapsed, the UE switches the short DRX cycle to the long DRX cycle. Initial scheduling information of a specific UE is provided only in a predetermined PDCCH. Accordingly, the UE can minimize power consumption by monitoring only the PDCCH periodically. If scheduling information for a new packet is received through the PDCCH during the on-duration period 2b-05 (indicated by reference numeral 2 b-10), the UE starts a DRX inactivity timer (indicated by reference numeral 2 b-15). The UE maintains an active state during the DRX inactivity timer, that is, the UE continues to perform PDCCH monitoring. Further, the UE starts a HARQ RTT timer (indicated by reference numerals 2 b-20). The HARQ RTT is applied in order to prevent the UE from unnecessarily monitoring the PDCCH within a HARQ Round Trip Time (RTT) period, and the UE does not need to monitor the PDCCH within a timer operation period of the HARQ RTT timer. However, when the DRX inactivity timer and the HARQ RTT timer are simultaneously running, the UE continues PDCCH monitoring based on the DRX inactivity timer. If the HARQ RTT timer expires, a DRX retransmission timer is started (indicated by reference numerals 2 b-25). When the DRX retransmission timer is running, the UE needs to perform PDCCH monitoring. Generally, scheduling information for HARQ retransmissions is received during the run time of the DRX retransmission timer (indicated by reference numerals 2 b-30). Upon receiving the scheduling information, the UE immediately stops the DRX retransmission timer and restarts the HARQ RTT timer. The above operations continue until a packet (indicated by reference numerals 2 b-35) is successfully received.

Configuration information related to DRX operation in the connected mode is transmitted to the UE via an RRCConnectionReconfiguration message. The on-duration timer, the DRX inactivity timer, and the DRX retransmission timer are defined according to the number of PDCCH subframes. The timer expires after a predetermined number of subframes defined as PDCCH subframes have elapsed since the timer started. All downlink subframes belong to PDCCH subframes in FDD, and downlink subframes and special subframes correspond to PDCCH subframes in TDD. In TDD, a downlink subframe, an uplink subframe, and a special subframe exist in the same frequency band. Among the downlink subframe, the uplink subframe, and the special subframe, the downlink subframe and the special subframe are considered as PDCCH subframes.

The base station may configure two states, long DRX and short DRX. In general, the base station may use one of two states in consideration of the characteristics of the configured DRBs reported from the UE, the UE mobility record information, and the power preference indication information. The transition between the two states is performed by transmitting a specific MAC CE to the UE or whether a specific timer expires.

Since only two types of DRX cycles can be configured in the existing LTE technology, the DRX cycle cannot be dynamically changed according to various DRB characteristics, traffic patterns, buffer conditions, and the like.

In the present disclosure, a plurality of DRX may be configured, and one of the configured DRX may be applied to one or more serving cells. In particular, in order to minimize UE power consumption, a group including one or more serving cells corresponds to one DRX configuration, and the serving cells belonging to the group apply the DRX configuration. For example, in case the serving cells operate in the same RF chain, it is desirable to apply the same DRX configuration to minimize UE power consumption. For this, the UE needs to provide the base station with preferred group information. In the present disclosure, the group information is referred to as DRX group information.

Fig. 2C illustrates a flowchart of a method of providing preferred DRX configuration information by a UE in the present disclosure.

The UE 2c-05 reports its own capability information (indicated by reference numeral 2 c-13) to the base station (i.e. eNB or gNB)2 c-10. The capability information includes an indicator indicating that the UE is capable of providing information of the preferred DRX group. The base station configures an SCell (indicated by reference numerals 2 c-15) for a connected mode UE. At this time, the base station provides one DRX in case of Carrier Aggregation (CA), and provides DRX for MCG and DRX for SCG, respectively, in case of Dual Connectivity (DC). The UE applies DRX by default. In case of CA, the UE applies one DRX to all serving cells. Or, even in case of CA, the UE may provide the base station with DRX applied to a serving cell belonging to frequency range 1(FR1) and DRX applied to a serving cell belonging to frequency range 2(FR 2). In this case, the UE applies two DRX by default to serving cells belonging to FR1 and FR2, respectively. At this time, the DRX group corresponds to a group of serving cells belonging to FR1 or a group of serving cells belonging to FR 2. In case of DC, the UE applies DRX for MCG and DRX for SCG to serving cells belonging to MCG and SCG, respectively, by default (indicated by reference numerals 2 c-20). At this time, the DRX group corresponds to a group of serving cells belonging to the MCG or a group of serving cells belonging to the SCG. The DRX applied to each serving cell minimizes UE power consumption, which may be non-optimized, depending on the serving cell to which the UE's RF chain is applied.

The base station reports an indication to the UE that DRX group information reporting is possible using an RRC message (indicated by reference numerals 2 c-25). Upon receiving the RRC message, the UE reports the preferred DRX group information (indicated by reference numerals 2 c-30) to the base station, either immediately or if it is determined that the DRX group needs to be re-adjusted. For example, if a UE is configured with serving cells belonging to FR1 and some serving cells operate using different RF chains, the UE may report a new DRX group using RRC messages. At this time, the UE may configure a serving cell using the same RF chain as one DRX group, and may report the DRX group to the base station (indicated by reference numerals 2 c-35). Preferred DRX configuration information corresponding to one DRX group may be reported to the base station along with serving cell ID list information belonging to the group. To reduce signaling overhead, the UE may provide only one DRX group information, and may assume that a serving cell not belonging to one DRX group implicitly belongs to another group. The DRX configuration information indicates an on duration timer, a DRX inactivity timer, a HARQ RTT timer, and a DRX retransmission timer.

The UE transmits the configured DRX group information to the base station (indicated by reference numerals 2 c-40). The base station re-adjusts the DRX using the RRC message in consideration of the group information and transmits the re-adjusted DRX to the UE (indicated by reference numerals 2 c-45). The base station provides a list of serving cells belonging to each DRX group and their corresponding DRX configuration information. In case of DC, the packet and its corresponding DRX configuration information can be independently configured by MAC entities of the MN and the SN and then provided to the UE. The UE may apply the configured DRX to serving cells belonging to the corresponding group (indicated by reference numerals 2 c-50).

An index for mapping information and a group is proposed as a method for mapping DRX configuration information to each DRX group. Each group has an ID, and a group ID to which DRX is applied may be stored for each DRX configuration information. At this time, the ID of the group to which the PCell belongs is always configured to be 0 or 1. Or, the first configuration information in the list of the plurality of DRX configuration information is always applied to the group to which the PCell belongs.

In case of indicating a serving cell belonging to each DRX group, a serving cell index or ID may be used. Alternatively, a band index to which each DRX group belongs may be indicated; that is, a serving cell belonging to each frequency band implicitly belongs to each DRX group. For example, DRX cell group 0 ═ FB1, FB2], DRX cell group 1 ═ FB3, and so on.

The method of mapping DRX configuration information to each DRX group and the method of indicating a serving cell belonging to a DRX group are applicable to both a case where a base station configures DRX for each DRX group of a UE and a case where the UE reports a preferred DRX group and its corresponding DRX to the base station.

Fig. 2D shows a flow chart of UE operation in the present disclosure.

In operation 2d-05, the UE reports its own capability information to the base station, the capability information including an indicator indicating that the UE is able to provide information of a preferred DRX group.

In operation 2d-10, the UE receives a configuration of the SCell from the base station.

In operation 2d-15, the UE receives an RRC message indicating that DRX group information reporting is possible.

In operation 2d-20, upon receiving the message, the UE receiving the message transmits preferred DRX group information to the base station immediately or in case of determining that the DRX group needs to be re-adjusted.

In operation 2d-25, the UE is provided with the readjusted DRX and the DRX group from the base station. In operation 2d-30, the UE applies the configured DRX to the serving cells belonging to the corresponding group.

Fig. 2E shows a flow chart of the base station operation in the present disclosure.

In operation 2e-05, the base station receives capability information reported from one UE.

In operation 2e-10, the base station configures an SCell for a connected mode UE.

In operation 2e-15, the base station reports an indication to the UE that DRX group information reporting is possible using an RRC message.

In operation 2e-20, the base station receives preferred DRX group information from the UE.

In operation 2e-25, the base station reconfigures the DRX using the RRC message in consideration of the set of information.

In operation 2e-30, the base station applies the configured DRX to the serving cells belonging to the corresponding group.

According to another method, in case that the UE reports the capability information to the base station, information of frequency bands operable in the same RF chain is stored in the capability information. That is, for each frequency band supported, an index indicating an RF chain may be stored. If the two bands have the same index, the serving cells of the bands may operate in the same RF chain. The base station may configure the same DRX for one or more serving cells that may operate in the same RF chain based on the capability information.

Fig. 3A shows a structure of an LTE system to which the present disclosure is applied.

Referring to fig. 3A, the radio access network of the LTE system includes next generation base stations (also referred to as evolved node bs, hereinafter referred to as ENBs, node bs, or base stations) 3A-05, 3A-10, 3A-15, and 3A-20, Mobility Management Entities (MMEs) 3A-25, and serving gateways (S-GWs) 3A-30. User equipment (hereinafter referred to as UE or terminal) 3a-35 accesses an external network through ENBs 3a-05, 3a-10, 3a-15 and 3a-20 and S-GW 3 a-30.

In FIG. 3A, the ENBs 3A-05, 3A-10, 3A-15, and 3A-20 correspond to existing node Bs of a UMTS system. The ENB is connected to the UEs 3a-35 via radio channels and acts in a more complex role than existing node Bs. In the LTE system, since all user traffic is served through a shared channel, including a real-time service such as voice over IP (VoIP) transmitted through an internet protocol, a means for collecting and scheduling status information such as a buffer status, an available transmission power status, and a channel status of a UE is required. The ENBs 3a-05, 3a-10, 3a-15, and 3a-20 are used to perform this function of the device. Generally, one ENB controls a plurality of cells. For example, to implement a transmission rate of 100Mbps, for example, in a 20MHz bandwidth, the LTE system uses Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology. In addition, the LTE system employs an adaptive modulation and coding (hereinafter, referred to as AMC) scheme, and determines a modulation scheme and a channel coding rate according to a channel status used by a terminal. The S-GW 3a-30 is a device for providing a data bearer, and generates or removes the data bearer under the control of the MME 3 a-25. The MME is a device for performing various control functions of the terminal and a mobility management function, and is connected to a plurality of base stations.

Fig. 3B shows a radio protocol structure in an LTE system to which the present disclosure is applied.

Referring to fig. 3B, the radio protocols of the LTE system include Packet Data Convergence Protocols (PDCP)3B-05 and 3B-40, Radio Link Controls (RLC)3B-10 and 3B-35, and Medium Access Controls (MAC)3B-15 and 3B-30 in the UE and eNB, respectively. Packet Data Convergence Protocols (PDCP)3b-05 and 3b-40 are used to perform operations such as IP header compression/recovery, and radio link control (hereinafter, RLC)3b-10 and 3b-35 reconfigure a PDCP Packet Data Unit (PDU) to an appropriate size to perform an ARQ operation, etc. The MACs 3b-15 and 3b-30 are connected to a plurality of RLC layer devices configured in one terminal, and can perform operations of multiplexing RLC PDUs with MAC PDUs and demultiplexing RLC PDUs with MAC PDUs. The physical layers 3b-20 and 3b-25 may perform the following operations: channel coding and modulating the higher layer data, generating the higher layer data into OFDM symbols, transmitting the OFDM symbols through a radio channel, or demodulating OFDM symbols received through a radio channel, channel decoding the OFDM symbols, and transmitting the OFDM symbols to the higher layer.

Fig. 3C illustrates a Radio Link Monitoring (RLM) operation in the present disclosure.

The physical layer (PHY) of the UE measures the downlink signal quality from the CRS of the serving cell (indicated by reference numeral 3 c-05). The physical layer determines whether the signal quality is below a certain threshold Qout (indicated by reference numerals 3 c-10). The threshold is a signal quality value corresponding to a specific BLER measured in the PDCCH. If the signal quality is below a certain threshold Qout, the physical layer passes an "out-of-sync" indicator to higher layers. In LTE technology, the above operation is referred to as "RLM". If the indicator is transmitted to the higher layer a certain number of times or more, the higher layer starts a certain timer, and if the timer expires, the higher layer announces RLF (indicated by reference numerals 3 c-15).

Fig. 3D illustrates Radio Link Failure (RLF) in the present disclosure.

As described above, RLF may be announced based on the results of RLM. The physical layer of the UE determines whether the downlink signal quality is below a certain threshold Qout based on the CRS of the serving cell at a certain period or each Qout assessment period. If the signal quality is below a certain threshold Qout, the physical layer sends an "out-of-sync" indicator to higher layers. After transmitting the first indicator to the higher layer (indicated by reference numeral 3 d-05), if the indicator is transmitted to the higher layer a certain number of times N310, a certain timer T310 is started (indicated by reference numeral 3 d-10). The physical layer determines whether the downlink signal quality is above a certain threshold Qin based on the CRS of the serving cell. If the signal quality is above a certain threshold Qin, the physical layer sends an "in-sync" indicator to higher layers. The running timer T310 is stopped if the indicator is sent to the higher layer a certain number of times. If the timer T310 is not stopped but expires, the higher layers declare RLF (indicated by reference numerals 3 d-15). After announcing (or detecting) the RLF, the UE starts another timer T311. The UE finds a new suitable cell. If the UE does not find a suitable cell before the timer T311 expires, the UE enters idle mode (indicated by reference numerals 3 d-25). If the UE finds a new suitable cell before the timer expires, the UE starts a timer T301 and performs a re-establishment procedure (indicated by reference numerals 3 d-20) for the new cell. If the re-establishment is not successfully completed until the timer T301 expires, the UE enters an idle mode (indicated by reference numerals 3 d-30). If the reestablishment is successful, the UE continues to maintain the connected mode to the cell. The RLF may be announced by RLM operation, or may be announced under another condition. If the random access fails, RLF may be declared (indicated by reference numerals 3 d-35). Furthermore, if the maximum number of retransmissions is reached in the RLC layer but the packet has not yet been successfully transmitted, RLF is declared (indicated by reference numerals 3 d-40). The operations of T301 and T311 are as follows.

[ TABLE 1 ]

Another case of declaring RLF corresponds to a case of handover failure. If the UE receives an RRCConnectionReconfiguration message (indicated by reference numerals 3 d-45) including handover configuration information and mobility control info IE, the UE starts a timer T304. The value of timer T304 is provided by mobilityControlInfo. If the random access to the target cell is not successfully completed until the timer expires, the handover is deemed to have failed and an RLF is declared (indicated by reference numerals 3 d-50).

The specific information collected in case of RLF occurring in the UE can be used to optimize the cell area. Therefore, specific information is stored in the UE in case of RLF, and then reported to the base station in case of successful handover of the UE to the connected mode. The report is called an RLF report, and specific information reported at this time is as follows.

-plmn-IdentityList

-measResultLastServCell

-measResultNeighCells

-locationInfo

-failedPCellId

-previousPCellId

-timeConnFailure

C-RNTI used in Source PCell

-connectionFailureType

After the RLF occurs, the UE performs cell selection and RRC reestablishment operations. At this time, the collected information may also be used to optimize the cell area. Accordingly, the present disclosure proposes a method of collecting specific information even after the occurrence of RLF, and defines information to be collected at this time. In addition, the present disclosure proposes a method of stopping an information collection operation based on an event or a timer. The operations proposed in this disclosure are referred to as enhanced RLF recording.

Fig. 3E illustrates the process of collecting useful information after RLF occurs in the present disclosure.

In order for a connected mode UE to normally perform a transmission/reception operation, minimum channel quality needs to be satisfied in both uplink and downlink. In this disclosure, it is referred to as DL availability 3e-05 in the downlink and UL availability 3e-10 in the uplink. For example, if DL availability or UL availability is not satisfied, RLF may be declared (indicated by reference numerals 3 e-15). At this time, the UE collects and stores valid information at a time point when the RLF occurs. In the present disclosure, it is proposed to collect useful information periodically or based on events according to the configuration of the base station even after the occurrence of RLF. One option for collecting useful information periodically is to collect and store useful information periodically after RLF occurs and continue to record periodically until a new first timer expires or certain conditions are met. A new first timer is started at the point in time when RLF occurs. If the first timer expires, the periodic recording is stopped. The first timer is provided by the network. According to another method, the first timer is stopped if a certain condition is met. For example, the specific condition represents:

-case of UE switching to Idle mode (RRC _ Idle)

-case of UE switching to Connected mode (RRC _ Connected)

-case where the UE finds a suitable cell by cell selection operation (indicated by reference numerals 3 e-20)

Case of UE starting the reestablishment operation (indicated by reference numerals 3 e-25)

Case of successful completion of the re-establishment operation by the UE

Case of UE starting T301 timer

-case of T301 timer expiration

Case of UE starting T311 timer

-case of T311 timer expiration

In the present disclosure, the UE stops recording if at least one of the above listed conditions is satisfied. Additionally, the first timer and the one or more conditions may be applied together.

Event-based selection refers to collecting valid information at a point in time after RLF where a specific event occurs. For example, such selection refers to an event that the UE finds a suitable cell through a cell selection operation performed after the occurrence of RLF, or an event that the UE starts a re-establishment operation in the suitable cell. The UE collects and stores useful information only in case of an event occurrence.

Fig. 3F illustrates a flow chart of a process in the present disclosure to collect useful information after RLF occurs.

The UE 3f-05 sends UE capability information (indicated by reference numeral 3 f-13) to the base station 3 f-10. The capability information includes an indicator indicating whether the UE supports enhanced RLF recording. Enhanced RLF recording refers to the operation of recording information that a UE can collect at a particular point in time after RLF occurs.

The base station provides configuration information (indicated by reference numerals 3 f-15) related to enhanced RLF recording to UEs supporting the enhanced RLF recording operation using RRC messages. The configuration information includes an indicator indicating that an enhanced RLF recording operation is performed, a first timer value, a condition(s) to stop the recording operation, a first threshold of RSRP (or RSRQ) for DL availability evaluation, an indicator indicating periodic recording or event-based recording, and event information. Since the provided configuration information itself indicates the enhanced RLF recording operation, an indicator indicating the enhanced RLF recording operation may be omitted.

If RLF occurs (indicated by reference numerals 3 f-20), the UE determines whether an enhanced RLF recording operation has been previously configured. If enhanced RLF logging operations have been configured, the UE collects useful information periodically or conditionally after RLF has occurred (indicated by reference numerals 3 f-25). The UE starts a first timer and periodically collects and stores useful information until the timer expires or a certain condition is met (indicated by reference numerals 3 f-30). In case of performing logging based on an event, the UE may collect and store useful information only in case of occurrence of a specific event.

Another method may be considered instead of the method of configuring the enhanced RLF record using the RRC message. A UE supporting enhanced RLF recording performs enhanced RLF recording if RLF occurs without previous configuration. According to this method, the configuration information is provided as system information broadcast by the base station, rather than as a dedicated RRC message. For example, the configuration information may be stored in the SIB 1.

In the present disclosure, the following information collected and stored by the enhanced RLF recording operation is proposed.

DL availability information, e.g.,

an indicator indicating whether cell selection has been successfully completed,

an indicator indicating whether a suitable cell has been found,

an indicator indicating that a cell satisfying the S-criterion has been found, an

An indicator indicating detection of an SS/PBCH that provides better signal quality than the configured first threshold.

UL availability information, e.g.,

an indicator indicating whether the maximum transmission power of the UE is higher than the P-max value, an

An indicator indicating whether the value of Pcompensition in the S-standard is non-zero.

Time information for each record (point in time when the storing operation is performed)

Starting and ending time points of the T301 and T311 timers

-channel quality information such as uplink RSRP, RSRQ and downlink RSRP, RSRQ, etc. of the best cell, suitable cells found by cell selection, PCell and neighboring neighbor cells in case of RLF

-starting and ending points in time of a first timer

Fig. 3G illustrates a flow chart of UE operations for collecting useful information after RLF occurs in the present disclosure.

In operation 3g-05, the UE provides its own capability information to the base station.

In operation 3g-10, the base station configures an enhanced RLF recording operation for the UE using the RRC message.

If RLF occurs in operations 3g-15, the UE stores specific information.

In operation 3g-20, the UE determines whether an enhanced RLF recording operation has been previously configured, and if the enhanced RLF recording operation has been previously configured, the UE records specific information even after RLF occurs. At this time, the UE starts a first timer.

In operation 3g-25, the UE records specific information periodically or based on an event until the first timer expires or a specific condition is satisfied.

While the present disclosure has been described in terms of various embodiments, various alterations and modifications will occur to those skilled in the art. The disclosure is intended to embrace such alterations and modifications as fall within the scope of the appended claims.