Abstract:
Provided is a method for executing a minimization drive test (MDT) carried out by a terminal in a wireless communication system. The method comprises: receiving a logged measurement setting; logging an MDT measurement and a measurement result on the basis of the logged measurement setting; and reporting the whole or a part of the logged measurement result to a network. The logged measurement setting includes logging termination time information for indicating the termination time of the MDT measurement or the measurement result logging, wherein the MDT measurement and the measurement result is logged by the time indicated by the logging termination time information.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the National Phase of PCT/KR2012/003265 filed on Apr. 26, 2012, which claims priority under 35 U.S.C. 119(e) to U.S. Provisional Application No. 61/479,835 filed on Apr. 27, 2011, all of which are hereby expressly incorporated by reference into the present application. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a wireless communication system and, more particularly, to a method of performing a Minimization Drive Test (MDT) based on time-related measurement configuration information in a wireless communication system. 
     BACKGROUND ART 
     3 rd  Generation Partnership Project (3GPP) Long Term Evolution (LTE), that is, the improvement of an LTE Universal Mobile Telecommunications System (UMTS), has been introduced as 3GPP release 8. 3GPP LTE uses Orthogonal Frequency Division Multiple Access (OFDMA) in downlink and uses Single Carrier-Frequency Division Multiple Access (SC-FDMA) in uplink. 3GPP LTE adopts Multiple Input Multiple Output (MIMO) having a maximum of 4 antennas. Recently, 3GPP LTE-Advanced (LTE-A), that is, the evolution of 3GPP LTE, is being discussed. 
     A Minimization Drive Test (MDT) is that service providers perform a test using UE instead of a vehicle for coverage optimization. Coverage varies depending on the location of a BS, the deployment of peripheral buildings, and a user use environment. Accordingly, a service provider needs to periodically perform a driving test, and lots of costs and resources are consumed. An MDT is that a service provider measures coverage using UE. 
     An MDT can be divided into a logged MDT and an immediate MDT. In accordance with the logged MDT, UE performs MDT measurement and then transfers logged measurement to a network at a specific point of time. In accordance with the immediate MDT, UE performs MDT measurement and then transfers measurement to a network when a report condition is satisfied. In the logged MDT, MDT measurement is performed in an RRC idle mode, whereas in the immediate MDT, MDT measurement is performed in an RRC connected mode. 
     Meanwhile, in a wireless communication system, the usage of a wireless network can be suddenly increased at a specific time. In this case, to measure radio environments or UE capability criteria for a specific time, log the radio environments or UE capability criteria, and report the log can be preferred. Furthermore, if UE reports a logged measurement result to a network during the time when the usage of a wireless network is suddenly increased, a problem in which the congestion of a wireless communication network is further increased can occur. Suitability in executing logging and/or a report by UE can vary in each time interval as described above. Accordingly, there is a need for a method in which UE performs an MDT or reports radio link failure information suitably for a wireless communication network situation. 
     DISCLOSURE 
     Technical Problem 
     An object of the present invention is to provide a method of performing a Minimization Drive Test (MDT) based on time-related measurement configuration information in a wireless communication system. 
     Technical Solution 
     In an aspect, a method of performing, by user equipment (UE), a Minimization Drive Test (MDT) in a wireless communication system is provided. The method includes receiving a logged measurement configuration, performing MDT measurement and logging a measurement result based on the logged measurement configuration, and reporting all or part of the logged measurement result to a network. The logged measurement configuration comprises logging end time information indicating that the MDT measurement and the measurement result logging are ended. The steps of performing the MDT measurement and logging the measurement result are performed until the time indicated by the logging end time information. 
     The method may further include discarding the logged measurement configuration when the end time indicated by the logging end time information elapses. 
     The logged measurement configuration may further include logging start time information indicative of a start time when performing the MDT measurement and logging the measurement result are allowed. 
     The steps of performing the MDT measurement and logging the measurement result may be performed when the start time indicated by the logging start time information is reached and until the end time indicated by the logging end time information. 
     The logged measurement configuration may include logging interval information, the logging interval information indicates a specific time interval. The step of performing the MDT measurement and logging the measurement result may be periodically performed in accordance with the specific time interval. 
     The UE may be in a Radio Resource Control (RRC) idle state at a point of time at which the MDT measurement is performed and the measurement result is logged. 
     The logged measurement configuration may further include log report end time information. The log report end time information may indicate an end time when the report is ended. 
     The method may further include discarding remaining logged measurement result if the end time indicated by the log report end time information elapses and the remaining logged measurement result not reported is present in the logged measurement result. 
     The logged measurement configuration may further include log report start time information. The log report start time information may indicate a start time when the report is allowed. 
     The report may be performed when the start time indicated by the log report start time is reached and until the end time indicated by the log report end time. 
     At a point of time at which part of or the entire logged measurement is reported, the UE may be in an RRC_connected state. 
     In another aspect, a wireless apparatus is provided. The wireless apparatus includes a Radio Frequency (RF) unit transmitting and receiving radio signals, and a processor operably coupled to the RF unit. The processor is configured to receive a logged measurement configuration, perform a Minimization Driving Test (MDT) measurement and log a measurement result based on the logged measurement configuration, and report all or a part of the logged measurement result to a network. The logged measurement configuration comprises logging end time information indicating that the MDT measurement and the measurement result logging are ended The performing the MDT measurement and the logging the measurement result are performed until the time indicated by the logging end time information. 
     The processor may be configured to discard the logged measurement configuration when the end time indicated by the logging end time information elapses. 
     The logged measurement configuration may further include logging start time information indicative of a start time when performing the MDT measurement and logging the measurement result are allowed. 
     The performing the MDT measurement and the logging the measurement result may be performed when the start time indicated by the logging start time information is reached and until the end time indicated by the logging end time information. 
     Wherein the wireless apparatus may be in a Radio Resource Control (RRC) idle state at a point of time at which the MDT measurement is performed and the measurement result is logged. 
     The logged measurement configuration may further include log report end time information. The log report end time information may indicate an end time when the report is ended. 
     The processor may be further configured to discard remaining logged measurement result if the end time indicated by the log report end time information elapses and the remaining logged measurement result not reported is present in the logged measurement result. 
     The logged measurement configuration may further include log report start time information. The log report start time information may indicate a start time when the report is allowed. 
     The report may be performed when the start time indicated by the log report start time is reached and until the end time indicated by the log report end time. 
     At a point of time at which all or a part of the logged measurement is reported, the UE may be in an RRC_connected state. 
     Advantageous Effects 
     UE can generate logging or radio link failure information for a specific time interval and report logged measurement and the generated radio link failure information to a network for a specific time interval. Accordingly, a network can obtain a logged measurement result more efficiently because UE intensively performs logging at a necessary time. Or, a network can efficiently obtain corresponding information because UE detects a radio link failure and generates and reports radio link failure information at a necessary time. UE reports logged measurement or generated radio link failure information to a network within a specific time interval, so-called within a time zone in which occurrence frequency of traffic is low. Accordingly, a loss of information due to a lapse of time can be prevented from occurring. A network can implement the optimization of a wireless communication system more effectively based on information obtained as described above. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a wireless communication system to which the present invention is applied. 
         FIG. 2  is a block diagram showing a radio protocol architecture for a user plane. 
         FIG. 3  is a block diagram showing a radio protocol architecture for a control plane. 
         FIG. 4  is a flowchart illustrating the operation of UE in the RRC_idle state. 
         FIG. 5  is a flowchart illustrating a process of setting up an RRC connection. 
         FIG. 6  is a flowchart illustrating a process of reconfiguring an RRC connection. 
         FIG. 7  is an exemplary diagram showing a radio link failure. 
         FIG. 8  is a flowchart illustrating the success of a connection reestablishment process. 
         FIG. 9  is a flowchart illustrating the failure of a connection reestablishment process. 
         FIG. 10  is a flowchart illustrating an existing method of performing measurement. 
         FIG. 11  shows an example in which a measurement identity is deleted. 
         FIG. 12  shows an example in which a measurement object is deleted. 
         FIG. 13  is a flowchart illustrating an existing measurement procedure. 
         FIG. 14  is a flowchart illustrating a method of performing a logged MDT. 
         FIG. 15  is a diagram showing an example of logged MDT measurement according to a logging area. 
         FIG. 16  is a diagram showing an example of logged MDT measurement according to a change of RAT. 
         FIG. 17  is a diagram showing an example of logged measurement. 
         FIG. 18  is a diagram showing an example of an immediate MDT. 
         FIG. 19  is a diagram showing an example of a method of performing a logged MDT in accordance with an embodiment of the present invention. 
         FIG. 20  is a flowchart illustrating a method of reporting radio link failure information in accordance with an embodiment of the present invention. 
         FIG. 21  is a block diagram showing a radio device in which an embodiment of the present invention can be implemented. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a wireless communication system to which the present invention is applied. The wireless communication system may also be called an Evolved-UMTS Terrestrial Radio Access Network (E-UTRAN) or a Long Term Evolution (LTE)/LTE-A system. 
     The E-UTRAN includes a Base Station (BS)  20  which provides User Equipment (UE)  10  with a control plane and a user plane. The UE  10  may be fixed or may have mobility. The UE  10  may be called another term, such as a Mobile Station (MS), a User Terminal (UT), a Subscriber Station (SS), a Mobile Terminal (MT), or a wireless device. The BS  20  refers to a fixed station communicating with the UE  10  and may be called another term, such as an evolved-NodeB (eNB), a Base Transceiver System (BTS), or an access point. 
     The BSs  20  may be interconnected through an X2 interface. The BS  20  is connected to an Evolved Packet Core (EPC)  30  through an S1 interface, more particularly, to a Mobility Management Entity (MME) through an S1-MME and a Serving Gateway (S-GW) through an S1-U. 
     The EPC  30  includes the MME, the S-GW, and a Packet Data Network-Gateway (P-GW). The MME has access information about UE or information about the capabilities of UE. Such information is chiefly used in the mobility management of UE. The S-GW is a gateway having an E-UTRAN as an end point, and the P-GW is a gateway having a PDN as an end point. 
     The layers of a radio interface protocol between UE and a network may be classified into L1 (first layer), L2 (second layer), and L3 (third layer) based on lower 3 layers of an Open System Interconnection (OSI) reference model which is widely known in communication systems. From among the layers, the PHY layer belonging to the first layer provides information transfer service using physical channels, and a Radio Resource Control (RRC) layer placed in the third layer functions to control radio resources between UE and a network. To this end, the RRC layer exchanges RRC messages between UE and a BS. 
       FIG. 2  is a block diagram showing a radio protocol architecture for a user plane.  FIG. 3  is a block diagram showing a radio protocol architecture for a control plane. A data plane is a protocol stack for user data transmission, and a control plane is a protocol stack for control signal transmission. 
     Referring to  FIGS. 2 and 3 , a physical (PHY) layer provides a higher layer with information transfer service using physical channels. The PHY layer is connected to a Medium Access Control (MAC) layer, that is, a higher layer, through a transport channel. Data is moved between the MAC layer and the PHY layer through the transport channel. The transport channel is classified depending on how data is transmitted through a radio interface according to what characteristics. 
     Data is transferred between different PHY layers, that is, between the PHY layers of a transmitter and a receiver, through a physical channel. The physical channel may be modulated according to an Orthogonal Frequency Division Multiplexing (OFDM) scheme, and the physical channel uses the time and frequency as radio resources. 
     The functions of the MAC layer include mapping between a logical channel and a transport channel and multiplexing/de-multiplexing to a transport block that is provided to a physical channel on the transport channel of an MAC Service Data Unit (SDU) that belongs to a logical channel. The MAC layer provides service to a Radio Link Control (RLC) layer through a logical channel. 
     The functions of the RLC layer include the concatenation, segmentation, and reassembly of an RLC SDU. In order to guarantee various types of Quality of Service (QoS) required by a Radio Bearer (RB), the RLC layer provides three operation modes: a Transparent Mode (TM), an Unacknowledged Mode (UM), and an Acknowledged Mode (AM). AM RLC provides error correction through an Automatic Repeat Request (ARQ). 
     The functions of a Packet Data Convergence Protocol (PDCP) layer in the user plane include the transfer, header compression, and ciphering of user data. The functions of the PDCP layer in the user plane include the transfer and ciphering/integrity protection of control plane data. 
     The Radio Resource Control (RRC) layer is defined only in the control plane. The RRC layer is related to the configuration, re-configuration, and release of RBs and responsible for control of logical channels, transport channels, and physical channels. An RB means a logical path that is provided by the first layer (PHY layer) and the second layers (the MAC layer, the RLC layer, and the PDCP layer) for the transfer of data between UE and a network. 
     What an RB is configured means a process of regulating the characteristics of a radio protocol layer and channel and configuring each detailed parameter and operation method in order to provide specific service. An RB may be divided into a Signaling RB (SRB) and a Data RB (DRB). The SRB is used as a passage along which an RRC message is transported in the control plane, and the DRB is used as a passage along which user data is transported in the user plane. 
     If an RRC connection is established between the RRC layer of UE and the RRC layer of an E-UTRAN, the UE is in an RRC connected state. If not, the UE is in the R_RC idle state. 
     A downlink transport channel through which data is transmitted from a network to UE includes a broadcast channel (BCH) through which system information is transmitted and a downlink shared channel (SCH) through which user traffic or control messages are transmitted. Traffic or a control message for downlink multicast or broadcast service may be transmitted through a downlink SCH or may be transmitted through an additional downlink multicast channel (MCH). Meanwhile, an uplink transport channel through which data is transmitted from UE to a network includes a random access channel (RACH) through which an initial control message is transmitted and an uplink shared channel (SCH) through which user traffic or a control message is transmitted. 
     Logical channels placed over a transport channel and mapped to the transport channel include a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH), a multicast traffic channel (MTCH), and so on. 
     A physical channel includes several OFDM symbols in a time domain and several subcarriers in a frequency domain. One subframe consists of a plurality of OFDM symbols in the time domain. A resource block is a resource allocation unit, and a resource block consists of a plurality of OFDM symbols and a plurality of subcarriers. Furthermore, each subframe may use specific subcarriers of specific OFDM symbols (e.g., the first OFDM symbol) of a corresponding subframe for a physical downlink control channel (PDCCH), that is, an L1/L2 control channel. A Transmission Time Interval (TTI) is a unit time for subframe transmission. 
     An RRC state and RRC connection method of UE are described below. 
     An RRC state means whether or not the RRC layer of UE has been logically connected with the RRC layer of an E-UTRAN. If the RRC layer of UE is logically connection with the RRC layer of an E-UTRAN, it is called the RRC_connected state. If the RRC layer of UE is not logically connection with the RRC layer of an E-UTRAN, it is called the RRC_idle state. Since UE in the RRC_connected state has an RRC connection, an E-UTRAN may check the presence of the UE in a cell unit, and thus the UE may be effectively controlled. In contrast, an E-UTRAN cannot check UE in the RRC_idle state, and UE in the RRC_idle state is managed by a Core Network (CN) in a tracking area, that is, an area unit greater than a cell. That is, the presence of UE in the RRC_idle state is checked in a larger area unit, and the UE in the RRC_idle state needs to move to the RRC_connected state in order to receive common mobile communication service, such as voice or data. 
     When a user first powers on UE, the UE first searches for a suitable cell and remains in the RRC_idle state in a corresponding cell. UE in the RRC_idle state establishes an RRC connection with an E-UTRAN through an RRC connection procedure when it needs to establish the RRC connection and shifts to the RRC_connected state. A case where the UE in the RRC_idle state needs to establish the RRC connection includes several cases. For example, the UE in the RRC_idle state establishes an RRC connection when it needs to send uplink data for a reason, such as a call attempt by a user, or when it sends a message in response to a paging message received from an E-UTRAN. 
     A Non-Access Stratum (NAS) layer placed over the RRC layer performs functions, such as session management and mobility management. 
     In order to manage the mobility of UE in the NAS layer, two states: EPS Mobility Management-REGISTERED (EMM-REGISTERED) and EMM-DEREGISTERED are defined. The two states are applied to UE and an MME. Initial UE is in the EMM-DEREGISTERED state. In order for the UE to access a network, the UE performs a process of being registered with the network through an initial attach procedure. If the attach procedure is successfully performed, the UE and the MME become the EMM-REGISTERED state. 
     In order to manage signaling connection between UE and an EPC, two states: an EPS Connection Management (ECM)-IDLE state and an ECM-CONNECTED state are defined. The two states are applied to UE and an MME. If UE in the ECM-IDLE state establishes an RRC connection with an E-UTRAN, the UE becomes the ECM-CONNECTED state. If an MME in the ECM-IDLE state establishes an S1 connection with the E-UTRAN, the MME becomes the ECM-CONNECTED state. When UE is in the ECM-IDLE state, an E-UTRAN does not have context information about the UE. Accordingly, the UE in the ECM-IDLE state performs a mobility-based procedure based UE, such as cell selection or cell reselection, without a need to receive a command from a network. In contrast, when UE is in the ECM-CONNECTED state, the mobility of the UE is managed by a command from a network. If the location of UE is different from a location known to a network in the ECM-IDLE state, the UE informs the network of the location of the UE through a tracking area update procedure. 
     System information is described below. 
     System information includes essential information that needs to be known by UE in order to access a BS. Accordingly, the UE needs to have received all pieces of system information before accessing the BS and needs to always have recent system information. Furthermore, system information is information that needs to be known by all pieces of UE within one cell, and thus a BS periodically transmits the system information. 
     In accordance with Paragraph 5.2.2 of 3GPP TS 36.331 V8.7.0 (2009-09) “Radio Resource Control (RRC); Protocol specification (Release 8)”, system information is classified into a Master Information Block (MIB), a Scheduling Block (SB), and a System Information Block (SIB). The MIB enables UE to know the physical configuration of a corresponding cell, for example, a bandwidth. The SB informs information about the transmission of SIBs, for example, a transport period. The SIB is a collection of pieces of correlated system information. For example, any SIB includes only information about neighboring cells, and any SIB includes only information about an uplink radio channel used by UE. 
     In general, service provided from a network to UE may be classified into three types as follows. Furthermore, UE differently recognizes the type of cell depending on what service may be received. A service type is first described, and the type of cell is then described. 
     1) Limited service: this service provides an emergency call and an Earthquake and Tsunami Warning System (ETWS), and this service can be provided by an acceptable cell. 
     2) Normal service: this service means public use for common purposes, and this service can be provided by a suitable or normal cell. 
     3) Operator service: this service means service for a communication network service provider, and this cell can be used only by a communication network service provider, but cannot be used by a common user. 
     In relation to a service type provided by a cell, the type of cell can be classified as follows. 
     1) Acceptable cell: a cell from which UE can be provided with limited service. This cell is a cell which is not barred from a viewpoint of corresponding UE, but satisfies a cell selection criterion for the UE. 
     2) Suitable cell: a cell from which UE can be provided with suitable service. This cell satisfies a condition for an acceptable cell and also satisfies an additional condition. The additional condition includes that this cell needs to belong to a Public Land Mobile Network (PLMN) accessible to corresponding UE and this cell must be a cell on which a tracking area update procedure is not barred from being executed by UE. If a corresponding cell is a CSG cell, the CSG cell needs to be a cell that UE can access as a CSG member. 
     3) Barred cell: a cell that broadcasts that this cell is a cell barred through system information. 
     4) Reserved cell: a cell that broadcasts that this cell is a cell reserved through system information. 
       FIG. 4  is a flowchart illustrating the operation of UE in the RRC_idle state.  FIG. 4  shows a procedure in which UE that is initially powered on is registered with a network through a cell selection process and the UE performs cell reselection, if necessary. 
     Referring to  FIG. 4 , the UE selects Radio Access Technology (RAT) for communicating with a Public Land Mobile Network (PLMN), that is, a network from which the UE is provided with service (S 410 ). Information about the PLMN and the RAT may be selected by the user of the UE, and information stored in a Universal Subscriber Identity Module (USIM) may be used. 
     The UE selects a cell having the greatest value from cells whose measured BS and signal intensity or quality is greater than a specific value (S 420 ). In this case, powered-on UE performs cell selection, which may be called initial cell selection. Cell selection procedure is described in detail later. After the cell selection, the UE receives system information that is periodically transmitted by a BS. The specific value refers to a value defined in a system in order to guarantee quality for a physical signal in data transmission/reception. Accordingly, the value may be different depending on applied RAT. 
     If the UE needs to be registered with a network, the UE performs a network registration procedure (S 430 ). The UE registers its own information (e.g., IMSI) with the network in order to receive service (e.g., paging) from the network. The UE does not register its information with an accessing network whenever the UE selects a cell, but registers its information with a network if information (e.g., Tracking Area Identity (TAI)) about the network received from system information is different from information about the network which has been known to the UE. 
     The UE performs cell reselection based on a service environment provided by a cell, the environment of the UE, etc. (S 440 ). If a value of the intensity or quality of a signal measured from a BS that provides service to the UE is lower than a value measured from a BS of a neighboring cell, the UE selects a cell from other cells that provide better signal characteristics than the cell of the BS that the UE has accessed. This process is called cell reselection differently from the initial cell selection of second process. Here, in order to prevent a cell from being frequently reselected depending on a change of signal characteristics, a temporal restriction condition is imposed. The cell reselection procedure is described in detail later. 
       FIG. 5  is a flowchart illustrating a process of setting up an RRC connection. 
     UE transmits an RRC connection request message, requesting an RRC connection, to a network (S 510 ). The network transmits an RRC connection setup message in response to the RRC connection request (S 520 ). After receiving the RRC connection setup message, the UE enters an RRC connected mode. 
     The UE transmits an RRC connection setup complete message, used to check the successful completion of the RRC connection setup, to the network (S 530 ). 
       FIG. 6  is a flowchart illustrating a process of reconfiguring an RRC connection. An RRC connection reconfiguration is used to modify an RRC connection. This is used to set up, modify/release an RB, perform handover, and set up, modify/release measurement. 
     A network transmits an RRC connection reconfiguration message for modifying an RRC connection to UE (S 610 ). The UE transmits an RRC connection reconfiguration complete message used to check the successful completion of an RRC connection reconfiguration to the network in response to the RRC connection reconfiguration (S 620 ). 
     A radio link failure is described below. 
     UE continues to perform measurement in order to maintain the quality of a radio link to a serving cell from which service is received. The UE determines whether or not communication is impossible in a current situation due to the deterioration of quality of a radio link to a serving cell. If communication is almost impossible because the quality of a serving cell is too low, the UE determines a current situation to be a radio connection failure. 
     If a radio link failure is determined, the UE forgives maintaining communication with a current serving cell, selects a new cell through a cell selection (or cell reselection) procedure, and attempts an RRC connection reestablishment to the new cell. 
       FIG. 7  is an exemplary diagram showing a radio link failure. An operation related to a radio link failure may be described in two phases. 
     In the first phase, UE performs a normal operation and checks whether or not there is a problem in a current communication link. If a problem is detected, the UE declares a radio link problem and waits for the recovery of a radio link for a first standby time T1. If the radio link is recovered before the first standby time elapses, the UE performs a normal operation again. If the radio link is not recovered until the first standby time expires, the UE declares a radio link failure and enters the second phase. 
     In the second phase, the UE waits for the recovery of the radio link for a second standby time T2. If the radio link is not recovered until the second standby time expires, the UE enters the RRC_idle state. Or, the UE may perform an RRC reestablishment procedure. 
     The RRC connection reestablishment procedure is a procedure of reestablishing an RRC connection again in the RRC_connected state. Since the UE remains in the RRC_connected state, that is, since the UE does not enter the RRC_idle state, the UE does not initialize all its radio configurations (e.g., radio bearer configurations). Instead, the UE suspends the use of all radio bearers other than an SRB0 when starting an RRC connection reconfiguration procedure. If an RRC connection reconfiguration is successful, the UE resumes the use of suspended radio bearers. 
       FIG. 8  is a flowchart illustrating the success of a connection reestablishment process. 
     UE selects a cell by performing cell selection. The UE receives system information in order to receive basic parameters for cell access in the selected cell. Furthermore, the UE sends an RRC connection reestablishment request message to a BS (S 810 ). 
     If the selected cell is a cell having the context of the UE, that is, a prepared cell, the BS accepts the RRC connection reestablishment request of the UE and transmits an RRC connection reestablishment message to the UE (S 820 ). The UE transmits an RRC connection reestablishment complete message to the BS, so the RRC connection reestablishment procedure can be successful (S 830 ). 
       FIG. 9  is a flowchart illustrating the failure of a connection reestablishment process. UE transmits an RRC connection reestablishment request message to a BS (S 810 ). If a selected cell is not a prepared cell, the BS transmits an RRC connection reestablishment reject message to the UE in response to the RRC connection reestablishment request (S 815 ). 
     A procedure of UE selecting a cell is described in detail below. 
     When UE is powered on or the UE remains in a cell, the UE performs procedures for receiving service by selecting/reselecting a cell having suitable quality. 
     UE in the RRC_idle state always needs to select a cell having suitable quality and to be prepared to receive service through this cell. For example, UE that is just powered on needs to select a cell having suitable quality in order to be registered with a network. When the UE in the RRC_connected state enters the RRC_idle state, the UE needs to select a cell in which the UE will remain in the RRC_idle state. A process in which the UE selects a cell that satisfies any condition in order to remain in a service standby state, such as the RRC_idle state, as described above is called cell selection. An important point is that the UE needs to select a cell as rapidly as possible because cell selection is performed in the state in which the UE has not determined a cell in which the UE will remain in the RRC_idle state. Accordingly, if a cell provides the quality of a radio signal higher than a certain reference, although the cell is not a cell that provides the best quality of a radio signal to the UE, the cell may be selected in the cell selection process of the UE. 
     A method and procedure of UE selecting a cell in 3GPP LTE are described below with reference to 3GPP TS 36.304 V8.5.0 (2009-03) “User Equipment (UE) procedures in idle mode (Release 8)”. 
     When UE is initially powered on, the UE searches for an available Public Land Mobile Network (PLMN) and selects a suitable PLMN from which service may be received. The PLMN is a network that is deployed or managed by a mobile network operator. Each mobile network operator manages one or more PLMNs. Each PLMN can be identified by Mobile Country Code (MCC) and Mobile Network Code (MNC). Information about the PLMN of a cell is included in system information and broadcasted. UE attempts to register a selected PLMN. If the registration is successful, the selected PLMN becomes a Registered PLMN (RPLMN). A network may signal a PLMN list to the UE. PLMNs included in the PLMN list may be considered to be the same PLMNs as RPLMNs. UE registered with a network needs to be always reachable by the network. If UE is in the ECM-CONNECTED state (identically an RRC_connected state), a network recognizes that the UE is provided with service. If the UE is in the ECM-IDLE state (identically the RRC_idle state), the situation of the UE is not valid in an eNB, but is stored in an MME. In this case, only the MME is informed of the location of the UE in the ECM-IDLE state as the granularity of a list of Tracking Areas (TAs). A single TA is identified by a Tracking Area Identity (TAI) including a PLMN identity to which the TA belongs and Tracking Area Code (TAC) that uniquely represent a TA within a PLMN. 
     Next, the UE selects a cell having signal quality and characteristics from which the UE can be provided with suitable service from cells provided by the selected PLMN. 
     A cell selection process is chiefly divided into two types. 
     First, as an initial cell selection process, in this process, UE does not have preliminary information about a radio channel. Accordingly, in order to find a suitable cell, the UE searches all radio channels. The UE searches each channel for the strongest cell. Next, the UE selects a corresponding cell only if it has only to find a suitable cell that satisfies a cell selection reference. 
     Next, the UE may select a cell using stored information or using information broadcasted by a cell. Accordingly, cell selection may be faster than an initial cell selection process. The UE selects a corresponding cell if it has only to find a cell that satisfies a cell selection reference. If a suitable cell that satisfies the cell selection reference is not found through this process, the UE performs an initial cell selection process. 
     After UE selects any cell through a cell selection process, the intensity or quality of a signal between the UE and a BS may be changed due to the mobility of the UE or a change of a radio environment. Accordingly, if the quality of a selected cell is deteriorated, the UE may select another cell that provides better quality. If a cell is selected again as described above, the UE selects a cell that provides better signal quality than a currently selected cell. This process is called cell reselection. In general, the cell reselection process has a basic object to select a cell that provides the best quality to UE from a viewpoint of the quality of a radio signal. 
     In addition to the viewpoint of the quality of a radio signal, a network can determine priority by the frequency and inform UE of the determined priority. The UE which has received the priority preferentially considers this priority to be higher than a radio signal quality reference in a cell reselection process. 
     There is a method of selecting or reselecting a cell based on the signal characteristics of a radio environment as described above. Upon cell reselection, in selecting a cell for reselection, the following cell reselection method may be used depending on the RAT and frequency characteristics of a cell.
         Intra-frequency cell reselection: A reselected cell is a cell having the same center-frequency and the same RAT as those used in a cell on which the UE is currently being camped.   Inter-frequency cell reselection: A reselected cell is a cell having the same RAT and a different center-frequency with respect to those used in the cell on which the UE is currently being camped.   Inter-RAT cell reselection: A reselected cell is a cell using a different RAT from a RAT used in the cell on which the UE is currently being camped.       

     The principle of a cell reselection process is as follows. 
     First, UE measures the quality of a serving cell and a neighboring cell for cell reselection. 
     Second, cell reselection is performed based on a cell reselection reference. The cell reselection reference has the following characteristics in relation to the measurement of the serving cell and the neighboring cell. 
     Intra-frequency cell reselection is basically based on ranking. Ranking is a task for defining a criterion value for cell reselection evaluation and ranking cells using the criterion value in order of a higher criterion value. A cell having the best criterion is commonly called the best ranked cell. The cell criterion value is a value to which a frequency offset or a cell offset has been applied, if necessary, on the basis of a value measured by UE for a corresponding cell. 
     Inter-frequency cell reselection is based on frequency priority provided by a network. UE attempts to camp on a frequency having the highest frequency priority. A network may provide frequency priority that will be in common applied by pieces of UEs within a cell through broadcast signaling or may provide frequency-dedicated priority according to each piece of UE through UE-dedicated signaling. 
     For inter-frequency cell reselection, a network may provide UE with a parameter (e.g., a frequency-specific offset) used in cell reselection by the frequency. 
     For intra-frequency cell reselection or inter-frequency cell reselection, a network may provide UE with a Neighboring Cell List (NCL) used in cell reselection. The NCL includes a cell-specific parameter (e.g., a cell-specific offset) used in cell reselection. 
     For intra-frequency or inter-frequency cell reselection, a network may provide UE with a cell reselection black list used in cell reselection. UE does not perform cell reselection on a cell that is included in the black list Ranking performed in a cell reselection evaluation process is described below. 
     A ranking criterion used to assign priority to a cell is defined as in Equation 1.
 
 R   s   =Q   meas,s   +Q   hyst   ,R   n   =Q   meas,n   +Q   offset   [Equation 1]
 
     Here, R s  is a ranking criterion for a serving cell, R n  is a ranking criterion for a neighboring cell, Q meas,s  is a quality value measured by UE in relation to a serving cell, Q meas,n  is a quality value measured by UE in relation to a neighboring cell, Q hyst  is a hysteresis value for ranking, and Q offset  is an offset between two cells. 
     In Intra-frequency, if UE receives an offset Q offsets,n  between a serving cell and a neighboring cell, Q offset =Q offsets,n . If UE does not receive Q offsets,n , Q offset =0. 
     In Inter-frequency, if UE receives an offset Q offsets,n  for a corresponding cell, Q offset =Q offsets,n +Q frequency . If UE does not receive Q offsets,n , Q offset =Q frequency . 
     If the ranking criterion R s  of a serving cell and the ranking criterion R n  of a neighboring cell are shifted in a similar state, UE may alternately reselect two cells because ranking priority is frequently changed as a result of the shift. Q hyst  is a parameter that prevents UE from alternately reselecting two cells by giving a hysteresis in cell reselection. 
     UE measures R s  of a serving cell and R n  of a neighboring cell according to the above equation, considers a cell having the greatest ranking criterion value to be the best ranked cell, and reselects the cell. 
     In accordance with the reference, it may be seen that the quality of a cell functions as the most important reference in cell reselection. If a reselected cell is not a suitable cell, UE excludes a corresponding frequency or a corresponding cell from the subject of cell reselection. 
     Measurement and a measurement report are described below. 
     In a mobile communication system, the mobility support of UE is essential. Accordingly, UE consistently measures quality for a serving cell which now provides service and quality for a neighboring cell. UE reports a measurement result to a network on a suitable time, and a network provides the UE with the optimal mobility through handover, etc. Measurement for this purpose is called Radio Resource Management (RRM) Measurement. 
     In addition to the mobility support object, in order to provide information that may help a service provider to manage a network, UE may perform measurement for a specific object set by the network and report a measurement result thereof to the network. For example, UE may receive broadcast information about a specific cell that has been determined by a network. UE may report a cell identity of the specific cell (this is also called a global cell identity), location identity information (e.g., tracking area code) to which the specific cell belongs and/or other cell information (e.g., whether or not the specific cell is a member of a Closed Subscriber Group (CSG) cell) to a serving cell. 
     If UE in motion checks that the quality of a specific area is very poor through measurement, the UE may report location information and a measurement result of cells having poor quality to a network. The network may attempt to optimize the network based on a report on measurement results from pieces of UE that help the operation of the network. 
     In a mobile communication system whose frequency reuse factor is 1, mobility is chiefly performed between different cells in the same frequency band. Accordingly, in order to well guarantee the mobility of UE, the UE needs to be able to well measure the quality of neighboring cells having the same center frequency as a serving cell and information about the cells. As described above, measurement for a cell having the same center frequency as a serving cell is called intra-frequency measurement. UE performs intra-frequency measurement and reports a measurement result to a network on a suitable time so that the object of a corresponding measurement result is achieved. 
     A mobile communication service provider may manage a network using a plurality of frequency bands. If a communication system provides service through a plurality of frequency bands, in order to guarantee optimal mobility for UE, the UE needs to be able to well measure the quality of neighboring cells having a different center frequency from a serving cell and information about the cells. As described above, measurement for a cell having a different center frequency from a serving cell is called inter-frequency measurement. UE needs to be able to perform inter-frequency measurement and report a measurement result to a network on a suitable time. 
     If UE supports measurement for a heterogeneous network, measurement may be performed on a cell of a heterogeneous network based on a BS configuration. Such measurement for a heterogeneous network is called inter-Radio Access Technology (RAT) measurement. For example, RAT may include a UMTS Terrestrial Radio Access Network (UTRAN) and a GSM EDGE Radio Access Network (GERAN) that comply with the 3GPP standard and may also include a COMA 2000 system that complies with the 3GPP2 standard. 
       FIG. 10  is a flowchart illustrating an existing method of performing measurement. 
     UE receives measurement configuration information from a BS (S 1010 ). A message including the measurement configuration information is called a measurement configuration message. The UE performs measurement based on the measurement configuration information (S 1020 ). The UE reports a measurement result to the BS if the measurement result satisfies a report condition within the measurement configuration information (S 1030 ). A message including the measurement result is called a measurement report message. 
     The measurement configuration information may include the following information. 
     (1) Measurement object information: information about the object on which measurement will be performed by UE. The measurement object includes at least one of an intra-frequency measurement object that is the object of intra-cell measurement, an inter-frequency measurement object that is the object of inter-cell measurement, and an inter-RAT measurement object that is the object of inter-RAT measurement. For example, the intra-frequency measurement object may indicate a neighboring cell having the same frequency band as a serving cell, the inter-frequency measurement object may indicate a neighboring cell having a different frequency band from a serving cell, and the inter-RAT measurement object may indicate a neighboring cell having different RAT from a serving cell. 
     (2) Reporting configuration information: information about a report condition regarding when UE reports a measurement result and about a report type. The report condition may include information about an event or period in which a report on a measurement result is triggered. The report type is information regarding that a measurement result will be configured according to what type. 
     (3) Measurement identity information: information about a measurement identity, which makes UE determine to report what measurement object according to what type when in association with a measurement object and a reporting configuration. The measurement identity information is included in a measurement report message, and it may indicate that a measurement result is about what measurement object and that a measurement report has been generated under what report condition. 
     (4) Quantity configuration information: information about a parameter for configuring the filtering a measurement unit, a report unit and/or a measurement result value. 
     (5) Measurement gap information: information about a measurement gap, that is, an interval which may be used by UE for only measurement without taking data transmission to a serving cell into consideration, because DL transmission or UL transmission has not been scheduled. 
     In order to perform a measurement procedure, UE has a measurement object list, a measurement report configuration list, and a measurement identity list. 
     In 3GPP LTE, a BS may con figure only one measurement object for one frequency band regarding UE. In accordance with Paragraph 5.5.4 of 3GPP TS 36.331 V8.5.0 (2009-03) “Evolved Universal Terrestrial Radio Access (E-UTRA) Radio Resource Control (RRC); Protocol specification (Release 8)”, events triggered by a measurement report, such as those in the following table, are defined. 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Events 
                 Report Conditions 
               
               
                   
               
             
             
               
                 Event A1 
                 Serving becomes better than threshold 
               
               
                 Event A2 
                 Serving becomes worse than threshold 
               
               
                 Event A3 
                 Neighbour becomes offset better than serving 
               
               
                 Event A4 
                 Neighbour becomes better than serving 
               
               
                 Event A5 
                 Serving becomes worse than threshold1 and neigbour becomes 
               
               
                   
                 better than threshold2 
               
               
                 Event B1 
                 Inter RAT neighbor becomes better than threshold 
               
               
                 Event B2 
                 Serving becomes worse than threshold1 and inter RAT 
               
               
                   
                 neighbor becomes better than threshold2 
               
               
                   
               
             
          
         
       
     
     If the measurement report of UE satisfies a set event, the UE transmits a measurement report message to a BS. 
       FIG. 11  shows an example of a measurement configuration configured for UE. 
     First, a measurement identity 1(1101) connects an intra-frequency measurement object and a reporting configuration 1. UE performs intra-frequency measurement, and the reporting configuration 1 is used to determine a reference and report type for a measurement result report. 
     Like the measurement identity 1(1101), a measurement identity 2(1102) is connected to the intra-frequency measurement object, but the measurement identity 2(1102) connects the intra-frequency measurement object and a reporting configuration 2. UE performs intra-frequency measurement, and the reporting configuration 2 is used to determine a reference and a report type for a measurement result report. 
     Based on the measurement identity 1(1101) and the measurement identity 2(1102), UE transmits a measurement result of the intra-frequency measurement object if the measurement result satisfies any one of the reporting configuration 1 and the reporting configuration 2. 
     A measurement identity 3(1103) connects an inter-frequency measurement object 1 and a reporting configuration 3. UE reports a measurement result of the inter-frequency measurement object 1 if the measurement result satisfies a report condition included in the reporting configuration 1. 
     A measurement identity 4(1104) connects an inter-frequency measurement object 2 and the reporting configuration 2. UE reports a measurement result of the inter-frequency measurement object 2 if the measurement result satisfies a report condition included in the reporting configuration 2. 
     Meanwhile, a measurement object, a reporting configuration and/or a measurement identity may be added, changed and/or deleted. This may be indicated when a BS transmits a new measurement configuration message to UE or transmits a measurement configuration change message to the UE. 
       FIG. 12  shows an example in which a measurement identity is deleted. When a measurement identity 2(1102) is deleted, measurement for a measurement object associated with the measurement identity 2(1102) is suspended, and a measurement report is not also transmitted. A measurement object or a reporting configuration associated with the deleted measurement identity may not be changed. 
       FIG. 13  shows an example in which a measurement object is deleted. When an inter-frequency measurement object 1 is deleted, UE also deletes an associated measurement identity 3(1103). Measurement for the inter-frequency measurement object 1 is suspended, and a measurement report is also not transmitted. However, a reporting configuration associated with the deleted intra-frequency measurement object 1 may not be changed or deleted. 
     When a reporting configuration is removed, UE also removes an associated measurement identity. UE suspends measurement for a measurement object associated by the associated measurement identity. However, a measurement object associated with the deleted reporting configuration may not be changed or deleted. 
     A measurement report may include a measurement identity, the measured quality of a serving cell, and a measurement result of a neighboring cell. The measurement identity identifies a measurement object whose measurement report has been triggered. The measurement result of the neighboring cell may include a cell identity and measured quality of the neighboring cell. The measured quality may include at least one of Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ). 
     A Minimization Drive Test (MDT) is described below. 
     An MDT enables UE to perform measurement and report a result thereof instead of a drive test in which conventional service providers measure the quality of cells using vehicles for cell coverage optimization. Coverage varies depending on the location of a BS, the deployment of peripheral buildings, and a use environment of a user. Accordingly, a service provider needs to periodically perform a drive test, which consumes lots of costs and resources. In order to overcome the disadvantages, there is proposed an MDT in which a service provider measures coverage using UE. 
     A service provider may write a coverage map, indicating whether or not service is possible over the entire area to which service is provided by a service provider and a distribution of QoS, by synthesizing MDT measurement values received from several pieces of UE and use the written coverage map in network operations and optimization. For example, when a report on a coverage problem for a specific area is received from UE, a service provider may enlarge the coverage of a corresponding area cell by increasing the transmission power of a BS that provides service to the corresponding area. The time and cost necessary for network optimization can be minimized through such a method. 
     An MDT has been produced based on the framework of a tracking function, that is, one of the tools of an operator for Operation, Administration, and Maintenance (OAM). The tracking function provides an operator with the ability to perform tracking and to log the behaviors of UE and thus can enable the operator to determine the major cause of a function failure on the UE side. Traced data is collected over a network, which is called a Trace Collection Entity (TCE). An operator uses data collected in a TCE for analysis and evaluation. A tracking function used for an MDT includes signaling based on a tracking function and management based on tracking function. Signaling based on a tracking function is used to activate an MDT task toward specific UE, whereas management based on tracking functions is used to activate an MDT task without being limited to specific UE. 
     An MDT may be divided into two types: a logged MDT and an immediate MDT depending on whether UE reports measured and stored log data in a non-real time or in real time. The logged MDT is a method of UE performing MDT measurement, logging corresponding data, and sending the logged data to a network. In contrast, the immediate MDT is a method of UE performing MDT measurement and immediately sending corresponding data to a network. In accordance with the logged MDT, UE performs MDT measurement in the RRC_idle state. In accordance with the immediate MDT, UE performs MDT measurement in the RRC_connected state. 
       FIG. 14  is a flowchart illustrating a method of performing a logged MDT. 
     Referring to  FIG. 14 , UE receives a logged measurement configuration (S 1410 ). The logged measurement configuration may be included in an RRC message and transmitted through a downlink control channel. The logged measurement configuration may include at least one of a TCE ID, information about a reference time that is a basis for logging, logging duration, a logging interval, and information about an area configuration. The logging interval indicates an interval in which a measurement result is stored. The logging duration indicates duration for which UE performs a logged MDT. The reference time indicates a reference time for duration for which a logged MDT is performed. The area configuration indicates an area that has been requested to be logged by UE. 
     Meanwhile, UE initiates a validity timer when a logged measurement configuration is received. The validity timer means the lifetime of the logged measurement configuration, which may be specified by information about logging duration. The duration of the validity timer may indicate the validity of measurement results owned by UE as well as the valid lifetime of a logged measurement configuration. 
     A procedure in which UE performs a logged measurement configuration and a corresponding overall procedure is performed as described above is called a configuration phase. 
     When the UE enters the RRC_idle state (S 1421 ), the UE logs the measurement result while the validity timer is driven (S 1422 ). A measurement result value may include RSRP, RSRQ, Received Signal Code Power (RSCP), Ec/No, etc. Information on which a measurement result has been logged is called logged measurement. A temporal interval in which UE logs a measurement result one or more times is called a logging phase. 
     The execution of a logged MDT based on a logged measurement configuration by UE may vary depending on the location of the UE. 
       FIG. 15  is a diagram showing an example of a logged MDT according to a logging area. 
     A network may configure a logging area that is an area in which UE has to log. The logging area may be represented as a cell list or a tracking area/location area list. If a logging area is configured in UE, the UE suspends logging when the UE gets out of the logging area. 
     Referring to  FIG. 15 , a first area  1510  and a third area  1530  are areas configured as logging areas, and a second area  1520  is an area in which logging is not permitted. UE performs logging in the first area  1510 , but does not perform logging in the second area  1520 . UE performs logging again when the UE moves from the second area  1520  to the third area  1530 . 
       FIG. 16  is a diagram showing an example of a logged MDT according to a change of RAT. 
     UE performs logging only when the UE camps on RAT from which a logged measurement configuration has been received and suspends logging in another RAT. However, the UE may log cell information about other RATs in addition to camp-on RAT. 
     A first area  1610  and a third area  1630  are E-UTRAN areas, and a second area  1620  is a UTRAN area. A logged measurement configuration is received from the E-UTRAN. When UE enters the second area  1620 , the UE does not perform MDT measurement. 
     Referring back to  FIG. 14 , the UE enters the RRC_connected state (S 1431 ). If logged measurement to be reported is present, the UE informs an eNB that the logged measurement to be reported is present (S 1432 ). The UE may inform the eNB that the logged measurement is present when an RRC connection is established, an RRC connection is reestablished, or an RRC connection is reconfigured. Furthermore, if the UE performs handover, the UE may inform a handover target cell of a presence of the logged measurement. Informing, by the UE, the eNB that the logged measurement is present may include including a logged measurement-available indicator, that is, indication information informing that the logged measurement is present, in an RRC message transmitted from the UE to the eNB and sending the RRC message. The RRC message may be an RRC connection configuration complete message, an RRC connection reestablishment complete message, an RRC reconfiguration complete message, or a handover complete message. 
     When the eNB receives a signal informing that the logged measurement is present from the UE, the eNB request the UE to report logged measurement (S 1433 ). Requesting the report on the logged measurement may include including a logged measurement report request parameter regarding information indicative of the request in an RRC message and sending the RRC message. The RRC message may be a UE information request message. 
     When the UE receives the request to report the logged measurement from the eNB, the UE reports the logged measurement to the eNB (S 1434 ). Reporting the logged measurement to the eNB may include including the logged measurement report, including pieces of logged measurement, in an RRC message and sending the RRC message to the eNB. The RRC message may be a UE information report message. In reporting the logged measurement, the UE may report all or some of pieces of logged measurement owned by the UE on a report time point to the eNB. If the UE reports some of pieces of logged measurement, the reported pieces of logged measurement may be discarded. 
     A phase of a process in which the UE informs the eNB that the logged measurement is present, receives a request to report the logged measurement from the eNB, and reports the logged measurement is performed as described above is called a report phase. 
     A radio environment is chiefly measured by the UE while a logged MDT is performed. MDT measurement may include a cell identity and the signal quality and/or signal intensity of the cell. MDT measurement may include a measurement time and a measurement place. The following table illustrates contents logged by UE. 
     
       
         
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Parameter (set) 
                 Contents 
               
               
                   
               
             
             
               
                 Serving 
                 Global cell identity of a serving cell 
               
               
                 cell identity 
               
               
                 Measurement result of 
                 Measured RSRP of serving cell 
               
               
                 serving cell 
                 Measured RSRQ of serving cell 
               
               
                 Measurement result of 
                 Cell identities of measured E-UTRA cells, 
               
               
                 neighbor cell 
                 Measurement results of UTRA cells 
               
               
                   
                 Cell identities of measured UTRA cells, 
               
               
                   
                 Measurement results of UTRA cells 
               
               
                   
                 Cell identities of measured GERAN cells, 
               
               
                   
                 Measurement results of GERAN cells 
               
               
                   
                 Cell identities of measured CDMA2000 cells, 
               
               
                   
                 Measurement results of CDMA200 cells 
               
               
                 Time stamp 
                 Time of logging measurement result, calculated 
               
               
                   
                 (current time-reference time), calculated in 
               
               
                   
                 seconds 
               
               
                 Location information 
                 Detailed location information of logging time 
               
               
                   
                 point 
               
               
                   
               
             
          
         
       
     
     Information logged at different logging time points can be classified and stored according to different log entries. 
       FIG. 17  is a diagram showing an example of logged measurement. 
     Logged measurement includes one or more log entries. 
     The log entry includes a logging location, a logging time, a serving cell identity, a serving cell measurement result, and a neighboring cell measurement result. 
     The logging location indicates the location where UE has performed measurement. The logging time indicates the time when UE has performed measurement. Pieces of information logged at different logging times are stored in different log entries. 
     The serving cell identity may include a cell identity in the layer  3 , which is called a Global Cell Identity (GCI). The GCI is a set of a Physical Cell Identity (PCI) and a PLMN identity. 
     Meanwhile, UE may perform logging by analyzing criteria related to the performance of UE in addition to a radio environment. For example, the criteria related to the performance of UE may include a throughput, an erroneous transmission/reception rate, etc. 
     Referring back to  FIG. 14 , the aforementioned logging phase and report phase may be present in plural times for logging duration (S 1441 , S 1442 ). 
     The eNB may record/store the logged measurement on/in a TCE when the logged measurement is reported. 
     If the UE has logged measurement that has not been reported after the validity timer expires, that is, after the logging duration elapses, the UE performs a procedure for reporting the logged measurement to the eNB. A phase in which the overall procedure for the procedure is called a post-reporting phase. 
     When the logging duration expires, the UE discards the logged measurement configuration and initiates a conservation timer. After the logging duration is terminated, the UE suspends MDT measurement. However, the already logged measurement remains intact without being discarded. The conservation timer indicates the lifetime of the remaining logged measurement. 
     When the UE enters the RRC_connected state (S 1451 ) before the conservation timer expires, the UE may report logged measurement to the eNB. In this case, the aforementioned procedure for a logged measurement report may be performed (S 1452 , S 1453 , S 1454 ). When the conservation timer expires, the remaining logged measurement may be discarded. When the logged measurement is reported, the eNB may record/store the logged measurement on/in the TCE. 
     The conservation timer may be fixed to a predetermined value in the UE and may be previously set in the UE. For example, a value of the conservation timer may be 48 hours. Or, a value of the conservation timer may be included in the logged measurement configuration and transferred to the UE or may be included in a different RRC message and transferred to the UE. 
     Meanwhile, when a new logged measurement configuration is transferred to the UE, the UE may update an existing logged measurement configuration into the newly obtained logged measurement configuration. In this case, the validity timer can be started again from the time when the logged measurement configuration is newly received. Furthermore, logged measurement based on the previous logged measurement configuration may be discarded. 
       FIG. 18  is a diagram showing an example of an immediate MDT. The immediate MDT is based on a Radio Resource Management (RRM) measurement and report mechanism. In addition, information related to the location upon a measurement report is added and reported to an eNB. 
     Referring to  FIG. 18 , UE receives an RRC connection reconfiguration message (S 1810 ) and transmits an RRC connection reconfiguration complete message (S 1820 ). Thus, the UE enters the RRC_connected state. The UE may receive a measurement configuration by receiving the RRC connection reconfiguration message. In the example of  FIG. 18 , the measurement configuration has been illustrated as being received through the RRC connection reconfiguration message, but the measurement configuration may be included in a different RRC message and transmitted. 
     The UE performs measurement and evaluation in the RRC_connected state (S 1831 ) and reports a measurement result to the eNB (S 1832 ). In the immediate MDT, the measurement result may provide precise location information, as possible, as in the example of location information provided by a Global Navigation Satellite System (GNSS). For location measurement, such as an RF fingerprint, the measurement result may provide measurement information about a neighboring cell, which may be used to determine the location of UE. 
     From  FIG. 18 , it may be seen that even after the executed measurement and evaluation (S 1831 ) and the report (S 1832 ), the UE reports a measurement result (S 1843 ) to the eNB right after performing measurement and evaluation (S 1842 ). This is the biggest difference between the logged MDT and the immediate MDT. 
     Meanwhile, when access to radio resources is exponentially increased at a specific time, service quality for UE can be deteriorated. In this case, it is preferred that UE log and report radio environments or UE performance criteria for a corresponding specific time. Reporting, by UE, log information to a network when access to radio resources is very much leads to a problem in that the congestion of the network is further increased because the UE also accesses the radio resources. If the reception of logged measurement information on the network side is delayed due to the congestion of the network, logged measurement owned by UE may be discarded. Accordingly, there is a need for a mechanism in which UE performs measurement for a specific time interval and the UE reports logged measurement for a specific time. 
       FIG. 19  is a diagram showing an example of a method of performing a logged MDT in accordance with an embodiment of the present invention. 
     Referring to  FIG. 19 , UE receives a logged measurement configuration from a network (S 1910 ). The logged measurement configuration may include the information included in the logged measurement configuration described with reference to  FIG. 14 . The logged measurement configuration may further include pieces of the following time-related information. 
     The logged measurement configuration may include logging start time information indicative of a point of time at which logging is started. 
     The logged measurement configuration may include logging end time information indicative of a start point of time at which logging is terminated. 
     The logged measurement configuration may include logging start time information and logging duration information indicative of a time interval in which logging is performed. In this case, UE may determine a point of time at which a logging execution time interval has elapsed from a logging start time to be a logging end time. 
     The logged measurement configuration may include log report start time information indicative of a point of time at which logged measurement is reported. 
     The logged measurement configuration may include log report end time information indicative of a point of time at which a logged measurement report is ended. 
     The logged measurement configuration may include one or more of the pieces of above-described time-related information. 
     The UE which has received logging start time information does not log a measurement result after MDT measurement although the UE enters an RRC idle state. The UE performs MDT measurement and logs a measurement result in the RRC idle state and from a logging start time (S 1921 ). The measurement result logged by the UE may include the pieces of information included in the measurement result described with reference to  FIG. 14 . 
     If the UE receives the logged measurement configuration including logging interval information, the UE can perform MDT measurement and log a measurement result in each logging interval (S 1922 ). 
     The UE which has received logging end time information terminates the MDT measurement and the measurement result logging and discards the existing logged measurement configuration at the logging end time (S 1930 ). However, the UE does not discard the logged measurement. 
     Meanwhile, if memory for logging is full before the logging end time is reached, the UE may consider that the logging end time has been reached, terminal the logging, and discard the existing logged measurement configuration. 
     The UE which has received log report start time information does not report logged measurement to a network if the log report start time is not reached although the UE enters an RRC_connected state. In contrast, the UE which has not received log report start time information may perform a procedure for reporting logged measurement when the UE enters an RRC_connected state after a logging end time is reached. 
     When a log report start time is reached, the UE reports the logged measurement (S 1940 ). The UE sends a logged measurement-available indicator, indicating that the logged measurement to be reported is present, to the network (S 1941 ). If the UE has already entered an RRC_connected state upon a report, the UE may include the logged measurement-available indicator in an RRC message and send the RRC message. If the UE has not entered an RRC_connected state when reporting a log, the UE can transmit and receive RRC messages that establish an RRC connection. Here, the UE may include a logged measurement-available indicator in a message that completes the establishment of the RRC connection and transmit the message to the network. The message that completes the establishment of the RRC connection may be an RRC connection setup complete message, an RRC connection reconfiguration complete message, or an RRC connection reestablishment complete message. 
     The UE receive a logged measurement report request, transmitted in response to the logged measurement-available indicator, from the network (S 1942 ) and reports the logged measurement to the network in response to the request (S 1943 ). The logged measurement report request may be included in a UE information request message transmitted from the network to the UE and be then transmitted. The logged measurement configuration may be included in a UE information report message transmitted from the UE to the network and be then transmitted. 
     If the UE reports part of the logged measurement to the network through the logged measurement report step (S 1940 ), the UE performs a logged measurement report again in order to report the remaining logged measurement (S 1950 ). To report the logged measurement may include performing steps S 1951  to S 1953  as described above. 
     When a log report end time is reached, the UE stops the report of the logged measurement. If logged measurement to be reported to the network is present, the UE may discard the remaining logged measurement (S 1960 ). 
     Meanwhile, when the log report end time is reached before a conservation timer expires, the UE may discard the remaining logged measurement. On the contrary, when the conservation timer expires prior to the log report end time, the UE may discard the remaining logged measurement when the conservation timer expires. 
     In a method of performing a logged MDT, such as that of  FIG. 19 , UE performs MDT measurement and logging only in an RRC idle state, but the UE may be implemented to perform MDT measurement and logging irrespective of a state. For example, UE may perform logging even in an RRC idle state and/or an RRC_connected state. In this case, when logging start time information is received, the UE may perform MDT measurement and logging from the logging start time. If a logging start time is not received, the UE may perform MDT measurement and logging from a point of time at which a logged measurement configuration is received. 
     In a method of performing a logged MDT, such as that of  FIG. 19 , a measurement result may not be logged based on a logging interval, but a measurement result may be logged if quality of a serving cell is a specific threshold or lower. 
     MDT measurement/report may be performed based on the above logging start time information and the logging end time information even when performing an immediate MDT. 
     In an immediate MDT method, such as that of  FIG. 18 , the measurement configuration transmitted to UE may further include measurement report start time information and/or measurement report end time information. 
     If UE receives measurement report start time information, the UE may perform measurement evaluation from the measurement report start time although the UE is in an RRC_connected state and report a measurement result to a network. 
     If UE receives measurement report end time information, the UE may stop measurement evaluation/report at the measurement report end time although the UE is in an RRC_connected state. 
     Meanwhile, in the case of an immediate MDT, a measurement result is reported right after MDT measurement. Therefore, information about a report start time and a report end time may not be included in a measurement configuration. 
     A method of reporting radio link failure information or handover failure information can be performed based on the above-described time-related information. An example of a method of reporting radio link failure information is described below with reference to  FIG. 20 . 
       FIG. 20  is a flowchart illustrating a method of reporting radio link failure information in accordance with an embodiment of the present invention. 
     Referring to  FIG. 20 , a network transmits RLF configuration information, including time-related information, to UE (S 2010 ). 
     The RLF configuration information may include generation start time information on which the UE detects a radio link failure and starts generating radio link failure information. 
     The RLF configuration information may include generation end time information on which the generation of radio link failure information is stopped. 
     The RLF configuration information may include generation start time information and generation duration information, that is, a time interval in which radio link failure information is generated from a generation start time. If the generation duration elapses from the generation start time, the UE may determine that it is a generation end time. 
     The RLF configuration information may include report start time information indicative of the time when the UE starts reporting generated radio link failure information to the network. 
     The RLF configuration information may include report end time information indicative of the time when the report of radio link failure information is stopped. 
     The RLF configuration information may include one or more of the above-described pieces of information. 
     The UE which has received generation start time information does not generate radio link failure information although it detects a radio link failure prior to the generation start time (S 2020 ). 
     The UE which has received generation start time information generates radio link failure information when it detects a radio link failure after the generation start time (S 2030 ). 
     Although a radio link is recovered and the UE enters an RRC_connected state, the UE may not report the generated radio link failure information to the network before a report start time is reached. 
     When the UE detects a radio link failure again prior to a generation end time, the UE gene rates new radio link failure information (S 2040 ). Here, the UE may update existing radio link failure information into the new radio link failure information. Or, the UE may store the new radio link failure information independently from the existing radio link failure information. 
     When a generation end time is reached, the UE may not generate radio link failure information although it detects a radio link failure. The UE which has not obtained information about a generation end time may generate radio link failure information until a report start time or a report end time. 
     When a report start time is reached and the UE enters an RRC_connected state, the UE may report radio link failure information (S 2050 ). The UE transmits a logged measurement-available indicator, indicating that logged measurement to be reported is present, to the network (S 2051 ). If the UE has already entered the RRC_connected state upon a report, the UE can include a radio link failure information-available indicator in an RRC message and transmit the RRC message. If the UE has not entered the RRC_connected state when reporting a log, the UE transmits and receives RRC messages for establishing an RRC connection. Here, the UE can include a message that completes the establishment of the RRC connection in the radio link failure information-available indicator and transmit the message to the network. The message that completes the RRC connection can be an RRC connection setup complete message, an RRC connection reconfiguration complete message, or an RRC connection reestablishment complete message. 
     Meanwhile, when a report end time elapses, the UE may discard radio failure information without reporting the radio failure information to the network although the UE has the radio failure information to be reported. 
     As described above, UE may perform logging or generate radio link failure information for a specific time interval and report logged measurement and the generated radio link failure information to a network for a specific time interval. Accordingly, the network may obtain a logged measurement result more efficiently because the UE intensively performs logging at a necessary time. Or, since UE detects a radio link failure and generates and reports radio link failure information at a necessary time, a network can obtain corresponding information efficiently. UE reports logged measurement or generated radio link failure information to a network within a specific time interval, so-called within a time zone in which occurrence frequency of traffic is low. Accordingly, a loss of information due to a lapse of time can be prevented from occurring. A network can implement the optimization of a wireless communication system more effectively based on information obtained as described above. 
       FIG. 21  is a block diagram showing a radio device in which an embodiment of the present invention can be implemented. The device can implement the operation of UE in the embodiments of  FIGS. 19 and 20 . 
     The radio device  2100  includes a processor  2110 , memory  2120 , and a Radio Frequency (RF) unit  2130 . The processor  2110  implements the proposed functions, processes and/or methods. The processor  2110  can be configured to perform a logged MDT based on a logged measurement configuration including time-related information. The processor  2110  can be configured to log MDT measurement in a specific time interval based on the time-related information and to report the logged measurement to a network in another specific time interval. The processor  2110  can be configured to perform a radio link failure information report based on the RLF configuration information. The processor  2110  can be configured to generate radio link failure information in a specific time interval and to report radio link failure information to a network in another specific time interval. The embodiments of  FIGS. 19 and 20  can be implemented by the processor  2110  and the memory  2120 . 
     The RF unit  2130  is connected to the processor  2110 , and it transmits and receives radio signals. 
     The processor  2110  may include Application-Specific Integrated Circuits (ASICs), other chipsets, logic circuits, and/or data processor  2110 s. The memory  2120  may include Read-Only Memory (ROM), Random Access Memory (RAM), flash memory  2120 , memory  2120  cards, storage media and/or other storage devices. The RF unit  2130  may include a baseband circuit for processing radio signals. When the above-described embodiment is implemented in software, the above-described scheme may be implemented into a module (process or function) configured to perform the above function. The module may be stored in the memory  2120  and executed by the processor  2110 . The memory  2120  may be placed inside or outside the processor  2110  and connected to the processor  2110  using a variety of well-known means. 
     In the above exemplary system, although the methods have been described based on the flowcharts in the form of a series of steps or blocks, the present invention is not limited to the sequence of the steps, and some of the steps may be performed in a different order from that of other steps or may be performed simultaneous to other steps. Furthermore, those skilled in the art will understand that the steps shown in the flowchart are not exclusive and the steps may include additional steps or that one or more steps in the flowchart may be deleted without affecting the scope of the present invention.