Abstract:
A method and apparatus for configuring a radio link of a terminal communicating via aggregated carriers including a primary cell and a secondary cell are provided. The method includes detecting a Radio Link Failure (RLF) for the secondary cell, deactivating the secondary cell, and reporting at least one of a measurement result of the secondary cell and a measurement result of neighboring cell of the secondary cell to a base station. The apparatus includes a transceiver for communicating with a base station, and a controller configured to detect a RLF for the secondary cell, to deactivate the secondary cell, and to report at least one of a measurement result of the secondary cell and a measurement result of neighboring cell of the secondary cell to the base station.

Description:
PRIORITY 
       [0001]    This is a continuation application of prior U.S. patent application Ser. No. 14/329,136, filed on Jul. 11, 2014, which issued as U.S. Pat. No. 9,344,324 on May 17, 2016; which is continuation of U.S. patent application Ser. No. 13/509,360, filed on May 11, 2012, which issued as U.S. Pat. No. 8,817,602 on Aug. 26, 2014, which is a National Stage application under 35 U.S.C. §371 of an International application filed on Nov. 10, 2010 and assigned application No. PCT/KR2010/007908, which claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed on Nov. 11, 2009 in the Korean Intellectual Property Office and assigned Serial No. 10-2009-0108751, the entire disclosure of which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a communication apparatus and method for a mobile communication system and, in particularly, to a radio link configuration method and apparatus of a terminal for communicating data with a base station over multiple carriers aggregated in a mobile communication system. 
       BACKGROUND 
       [0003]    Mobile communication systems have developed to provide the subscribers with voice communication services on the move. With the rapid advance of technologies, the mobile communication systems have evolved to support high speed data communication services as well as the standard voice communication services. Recently, as the next generation mobile communication system of the 3rd Generation Partnership Project (3GPP), Long Term Evolution (LTE) is under development. The LTE system is a technology for realizing high-speed packet-based communication at about 100 Mbps, aiming at commercialization in around 2010. Meanwhile, unlike the standard voice service, most of the data services are allocated resources according to the data amount to be transmitted and channel condition. Accordingly, in the wireless communication system such as cellular communication system, it is important to manage resource allocation based on the resource scheduled for data transmission, channel condition, and data amount to be transmitted. This is the fact even in the LTE system, and the base station scheduler manages and assigns radio resources. More recent studies have focused on the LTE-Advanced (LTE-A) for improving data rate with the adaptation of several new techniques. Carrier Aggregation (CA) is one of these newly introduced techniques. As compared to the data communication in which a terminal uses one downlink and one uplink carriers, the terminal supporting CA is capable of performing data communication over multiple downlink and multiple uplink carriers. 
         [0004]    Since the conventional radio link failure detection and recovery procedure has been designed in consideration of the terminal assigned a single downlink and a signal uplink carriers, it is impossible to adopt this procedure to the terminal which transmits and receives data on multiple aggregated uplink and downlink carriers without modification. There is therefore a need of defining an operation procedure of the CA-enabled terminal when per-carrier radio link failure is detected. 
         [0005]    The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure. 
       SUMMARY 
       [0006]    Aspects of the present disclosure are to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. 
         [0007]    In accordance with an aspect of the present invention, a method for configuring a radio link of a terminal communicating via aggregated carriers including a primary cell and a secondary cell is provided. The method includes detecting a Radio Link Failure (RLF) for the secondary cell, deactivating the secondary cell, and reporting at least one of a measurement result of the secondary cell and a measurement result of neighboring cell of the secondary cell to a base station. 
         [0008]    In accordance with another aspect of the present invention, an apparatus for configuring a radio link of a terminal communicating via aggregated carriers including a primary cell and a secondary cell is provided. The apparatus includes a transceiver for communicating with a base station, and a controller configured to detect a RLF for the secondary cell, to deactivate the secondary cell, and to report at least one of a measurement result of the secondary cell and a measurement result of neighboring cell of the secondary cell to the base station. 
         [0009]    The terminal for communicating data with a base station through multiple carriers aggregated in a mobile communication system is capable of performing radio link failure detection and recovery efficiently. That is, if radio link failure is detected on a secondary carrier other than the primary carrier among aggregated carriers and if the radio link is not recovered on the secondary carrier before expiry of a timer, the User Equipment (UE) does not perform measurement to discover a cell suitable for receiving the normal service, resulting in reduction of UE&#39;s power waste. If the probability of the radio link failure is high on the primary carrier, the UE performs measurement to discover the cell suitable for receiving the normal service on a secondary carrier so as to reduce the delay for radio link connection re-establishment when the radio link failure is detected on the primary carrier afterward. 
         [0010]    Other aspects, advantages, and salient features of the disclosure will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses various embodiments of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The above and other aspects, features, and advantages of certain embodiments of the present disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
           [0012]      FIG. 1  is a diagram illustrating the architecture of a Long Term Evolution (LTE) system to which the present invention is applied; 
           [0013]      FIG. 2  is a diagram illustrating a radio link failure detection and recovery mechanism adopted in a 3rd Generation Partnership Project (3GPP) LTE system; 
           [0014]      FIG. 3  is a diagram illustrating an exemplary situation of carrier aggregation in the LTE system to which the present invention is applied; 
           [0015]      FIG. 4  is a diagram illustrating a radio link failure detection and recovery method of a Carrier Aggregation (CA)-enabled User Equipment (UE) according to an embodiment of the present invention; 
           [0016]      FIG. 5  is a flowchart illustrating a UE procedure according to an embodiment of the present invention; and 
           [0017]      FIG. 6  is a block diagram illustrating a configuration of a UE according to an embodiment of the present invention. 
       
    
    
       [0018]    Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures. 
       DETAILED DESCRIPTION 
       [0019]    The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the present disclosure as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the various embodiments described herein can be made without departing from the scope and spirit of the present disclosure. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness. 
         [0020]    The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the present disclosure. Accordingly, it should be apparent to those skilled in the art that the following description of various embodiments of the present disclosure is provided for illustration purpose only and not for the purpose of limiting the present disclosure as defined by the appended claims and their equivalents. 
         [0021]    It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
         [0022]    In the following description, the term “primary carrier” denotes a carrier referenced for security and mobility among the carriers aggregated for use by a terminal and is interchangeably used with the term “first carrier.” The term “secondary carrier” denotes at least one carrier other than the primary carrier. 
         [0023]      FIG. 1  is a diagram illustrating the architecture of a Long Term Evolution (LTE) system to which the present invention is applied. 
         [0024]    Referring to  FIG. 1 , the radio access network of the mobile communication system includes evolved Node Bs (eNBs)  105 ,  110 ,  115 , and  120 , a Mobility Management Entity (MME)  125 , and a Serving-Gateway (S-GW)  130 . The User Equipment (UE)  135  connects to an external network via eNBs  105 ,  110 ,  115 , and  120  and the S-GW  130 . In  FIG. 1 , the eNBs  105 ,  110 ,  115 , and  120  correspond to legacy node Bs of Universal Mobile Communications System (UMTS). The eNBs  105 ,  110 ,  115 , and  120  allow the UE to establish a radio link and are responsible for complicated functions as compared to the legacy node B. In the LTE system, all the user traffic including real time services such as Voice over Internet Protocol (VoIP) are provided through a shared channel and thus there is a need of a device which is located in the eNB to schedule data based on the state information such as UE buffer conditions, power headroom state, and channel state. Typically, one eNB controls a plurality of cells. In order to secure the data rate of up to 100 Mbps, the LTE system adopts Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology. Also, the LTE system adopts Adaptive Modulation and Coding (AMC) to determine the modulation scheme and channel coding rate in adaptation to the channel condition of the UE. The S-GW  130  is an entity to provide data bearers so as to establish and release data bearers under the control of the MME  125 . MME  125  is responsible for various control functions and connected to a plurality of eNBs  105 ,  110 ,  115 , and  120 . 
         [0025]      FIG. 2  is a diagram illustrating a radio link failure detection and recovery mechanism adopted in a 3rd Generation Partnership Project (3GPP) LTE system. 
         [0026]    Referring to  FIG. 2 , the radio link failure detection is performed in such a way the UE monitors radio channel state to determine whether the connection to the eNB is broken, and the radio link recovery is performed in such a way that, when the radio link failure is detected, the UE waits for radio link recovery to the serving cell in a predetermined time rather than transitions to the idle mode immediately and, if the radio link recovery fails in the predetermined time, discovers a suitable cell for receiving the normal service among the neighbor cell to perform re-establishment procedure with the new cell. The normal service and the suitable cell for receiving the normal service follows the term ‘normal service’ and ‘suitable cell’ defined in the 3GPP standard 36.304, and the suitable cell discovery and re-establishment procedure for receiving the normal service after the radio link failure and multiple recovery failures follows ‘RRC connection re-establishment procedure’ defined in the 3GPP standard 36.331. 
         [0027]    In case that a single downlink carrier is assigned to the UE, Qin or Qout information is signaled from the Physical layer to the Radio Resource Control (RRC) layer through Common Reference Signal (CRS) of the carrier. Qin is signaled when the CRS measurement result value on the carrier is greater than a predetermined reference value_ 1 , and Qout is signaled when the CRS measurement result value on the carrier is less than a predetermined reference value_ 2 . The CRS is the channel for measurement and channel estimation as defined in the 3GPP standard 36.211, and the RRC layer is the layer for controlling radio resource follows the Radio Link Monitoring procedure specified in the 3GPP standard 36.133. 
         [0028]    That is, if the N310 Qout signals are sent from the physical layer to the RRC layer in sequence, the UE starts a T310 timer. If N311 Q1 signals are received from the physical layer in sequence during the T310 timer period, the UE regards that the connection to the current serving cell is recovered. The CRS managed by the UE during the T310 timer period is the signal transmitted in the serving cell to which the UE has established the RRC connection. If it fails to receive N311 Qin signals in sequence from the physical layer before the expiry of the T310 timer, the UE starts the t 311  timer and search neighbor cells (including the serving cell) for a suitable cell for receiving the normal service during the T311 timer period. If a suitable cell for receiving the normal service is discovered during the T311 timer period, the UE performs RRC connection re-establishment to the found cell. If the RRC connection re-establishment is successful, the UE can continue communication of control signal/data with the reestablished target cell without necessity of transition to idle mode. If it fails to discover a suitable cell during the T11 timer period, the UE transitions to the idle mode upon expiry of the T311 timer. The values of T310, N 310, T311, and N311 are signaled to the UE and determined in the ranges specified in the 3GPP standard 36.311, and the detail of the RRC connection re-establishment procedure follows the definition of the 3GPP standard 36.311 (RRC Connection Re-establishment). 
         [0029]      FIG. 3  is a diagram illustrating an exemplary situation of carrier aggregation in the LTE system to which the present invention is applied. 
         [0030]    Referring to  FIG. 3 , typically an eNB can use multiple carriers transmitted and receive in different frequency bands. For example, the eNB  305  can be configured to use the carrier  315  with center frequency f 1  and the carrier  310  with center frequency f 3 . If carrier aggregation is not supported, the UE  330  has to transmit/receive data unit one of the carriers  310  and  315 . However, the UE  330  having the carrier aggregation capability can transmit/receive data using both the carriers  310  and  315 . The eNB can increase the amount of the resource to be allocated to the UE having the carrier aggregation capability in adaptation to the channel condition of the UE so as to improve the data rate of the UE. In case that a cell is configured with one downlink carrier and one uplink carrier as a conventional concept, the carrier aggregation can be understood as if the UE communicates data via multiple cells. With the use of carrier aggregation, the maximum data rate increases in proportion to the number of aggregated carriers. 
         [0031]    If the radio link failure detection and recovery procedure designed for the UE having single downlink carrier capability is applied to the UE  330  having multiple downlink carrier aggregation capability, the unnecessarily frequent radio link failure detection and recovery procedure is likely to cause inefficiency and thus there is a need of defining efficient radio link failure detection and recovery procedure for the UE  330  having the multiple downlink carrier aggregation capability. 
         [0032]    The present invention proposes an efficient radio link failure detection and recovery method of a UE for receiving the data transmitted by the eNB through a plurality downlink carriers aggregated. In the present invention, the procedure begins in the state where multiple carriers are activated among the aggregated carriers and the UE has established the radio link on the respective carriers. 
         [0033]    If radio link failure is detected on one of multiple secondary downlink carriers excluding the primary carrier representing a specific cell referenced for security and mobility, the UE waits for the recovery of the radio link with the serving cell on the secondary carrier for the timer_ 1  duration. At this time, the primary and secondary carriers represent the serving cell and neighbor cells of the serving cell. If radio link failure is detected in a serving cell of a different carrier, the UE may detect the radio link failure on the corresponding carrier. If the radio link failure is detected, the UE starts the timer_ 1  to determine whether the radio link is recovered. If the radio link is recovered before the expiry of the timer_ 1 , the UE stops the timer_ 1 . 
         [0034]    If it is failed to recovery the radio link with the serving cell of other carrier before the expiry of the timer_ 1 , the UE checks the radio quality of the primary carrier. If the radio quality of the first carrier does not satisfy a predetermined level, the terminal starts the timer_ 2  to discover the neighbor cells around the serving cell. That is, if the radio quality of a specific/serving cell of the primary carrier to cause radio link failure or has caused the radio link failure already, the UE starts the timer_ 2  on the carrier, where the timer_ 1  has expired, to perform a measurement operation for discovery of the suitable cell for receiving the normal service among the neighbor cells (including the serving cell. Here, if the signal indicating that the CRS measurement value on the first carrier is less than a predetermined threshold value, i.e. Qout, is detected a predetermined number of times in sequence, if the timer_ 1  and timer_ 2  start on the primary carrier, or if the radio quality of the primary carrier is less than a predetermined threshold value, the UE determines that the radio quality of the first carrier does not fulfill the radio level. 
         [0035]    If the radio quality of the first carrier is recovered before the expiry of the timer_ 2 , the UE stops the timer_ 2  and deactivates the other carrier among the plural carriers. 
         [0036]    At this time, the timer_ 2  may be ended, when the radio quality of the specific/serving cell of the primary carrier becomes good enough not to cause radio link failure, if the timer_ 1  starts on the primary carrier and the radio link is recovered in the specific/serving cell before the expiry of the timer_I, or if the timer_ 2  starts on the primary carrier and a suitable cell for receiving the normal service is discovered among the neighbor cells (including the serving cell). If the timer_ 2  is stopped, the UE reports the radio quality information on the neighbor cells (including serving cell) of the secondary carrier to the eNB (when the neighbor cell having good radio quality is discovered on the secondary carriers) or deactivates the secondary carrier and regards that the carrier is not one of the aggregated ones (when no neighbor cell having good radio quality is detected on the carrier). 
         [0037]    Meanwhile, if the radio quality of the primary carrier fulfills a predetermined radio quality at the time point when the timer_ 1  has expired on a secondary carrier but not the primary carrier, the UE deactivates the secondary carrier among the multiple carriers. That is, if the radio quality of the specific/serving cell of the primary carrier is good enough so as not to cause radio link failure, the UE deactivates the secondary carrier on which the timer_ 1  is terminated and regards the carriers as one of the aggregated carriers. 
         [0038]    The timer_ 1  and timer_ 2  may be new timers set to the values different from those of the timers T310 and T311 described with reference to  FIG. 2  without exclusion of reuse of the timers T310 and T311. 
         [0039]    The above-described operation may be applied when the secondary carrier on which radio link failure is detected is a solitary carrier with the exception of the primary carrier in the aggregation. If multiple secondary carriers exist in the aggregation with the exception of the primary carrier and if radio link failure is detected on one of the secondary carriers, the UE waits for the radio link recovery with the serving cell of the secondary carrier before the expiry of the timer_ 1  and, if the timer_ 1  expires without radio link recovery, the UE deactivates the secondary carrier and excludes the carrier from the aggregation. 
         [0040]    According to the proposed method, if radio link failure is detected on a secondary carrier but not the primary carrier and if the radio link is not recovered in the serving cell of the secondary carrier before the expiry of the timer_ 1 , the UE skips performing measurement operation for discovering a suitable cell to receive the normal service, thereby minimizing power waste of the UE. In case that the probability of radio link failure is high on the primary carrier, the UE performs measurement operation to discover a suitable cell for receiving the normal service on the secondary carrier in advance so as to reduce the delay for the RRC connection re-establishment when the radio link failure is detected on the primary carrier afterward. 
         [0041]      FIG. 4  is a diagram illustrating a radio link failure detection and recovery method of the Carrier Aggregation (CA)-enabled UE according to an embodiment of the present invention. 
         [0042]    Referring to  FIG. 4 , it is assumed that the downlink carrier_ 1   401  is the primary carrier including a specific cell referenced for security and mobility. The downlink carrier_ 2  is a secondary carrier aggregated for use by the UE. In this embodiment, it is assumed that the carrier_ 1   401  and carrier_ 2   403  are aggregated for use by the UE. 
         [0043]    In this embodiment, if N310 Qout signals are issued in sequence from the physical layer to the RRC layer for the carrier_ 2   403 , the UE starts timer_ 1  at step  421 . Before the expiry of the timer_ 1 , the UE waits for radio link recovery with the serving cell on the carrier_ 2   403  at step  421 . The radio link recovery may be performed in the same way as the radio link recovery procedure with T310 as described with reference to  FIG. 2 . 
         [0044]    If the timer_ 1  expires (i.e. if radio link recovery with the serving cell fails on the carrier_ 2   403  before the expiry of the timer_ 1 ), the UE detects the radio link recovery failure at step  441  and checks the radio quality of the carrier_ 1   401  including the specific cell responsible for the security and mobility at step  443 . If the radio quality of the carrier_ 1   401  is bad to cause radio link failure afterward, the UE starts the timer_ 2 . Whether the radio quality of the carrier_ 1   401  is bad to cause radio link failure afterward may be determined depending on the following conditions: 
         [0045]    i) if N_ 1  consecutive Qout signals issued from the physical layer to the RRC layer for the carrier _ 1   401 , 
         [0046]    ii) if the timer_ 1  or timer_ 2  starts on the carrier_ 1   401  (timer_ 1  starts when N310 consecutive Qout signals are received from the physical layer on the carrier _ 1   401 , and the timer_ 2  starts when the timer_ 1  has started but the radio link to the specific/serving cell is not recovered on the carrier_ 1   401  before the expiry of the timer_ 1   401 ), and 
         [0047]    iii) the radio measurement result on the carrier_ 1   401  is less than a predetermined threshold (THRESHOLD_ 1 ). That is, if the above conditions are fulfilled, the UE starts the timer_ 2  on the carrier_ 2   403 . 
         [0048]    The UE searches for a suitable cell for receiving the normal service (perform measurement and check accessibility to the cell with the receipt of system information) before the expiry of the timer_ 2  at step  451 . 
         [0049]    If the radio quality of the carrier_ 1   401  is improved such that no radio link failure is detected before the expiry of the timer_ 2  on the carrier_ 2   403  at step  451  or if radio link failure is detected on the carrier_ 1   401  but recovered soon, the UE detects this at step  463  and terminates the timer_ 2  at step  461 . At this time, the recovery of the radio quality of the carrier_ 1   401  may be determined when the following conditions are fulfilled: 
         [0050]    i) N_ 2  consecutive Qin signals are issued from the physical layer to the RRC layer, 
         [0051]    ii) the timer_ 1  or timer_ 2  running on the carrier_ 1   401  is terminated (timer_ 1  termination occurs when the radio link is recovered in the specific/serving cell on the carrier_ 1   401  before the expiry of the timer_ 1  (the radio link is recovered when the N311 consecutive Qin signals are received from the physical layer as described with reference to  FIG. 2 ), and the timer_ 2  determination occurs when a suitable cell for receiving the normal service is discovered before the expiry of the timer_ 2 ), and 
         [0052]    iii) the radio measurement result on the carrier_ 1   401  is greater than a predetermined threshold_ 2 . 
         [0053]    If the timer_ 2  is terminated at step  461  or if the timer expires at step  463 , the UE reports the radio quality information of the neighbor cells (including serving cell) to the eNB (e.g. when a neighbor cell having good radio quality on the carrier_ 2 ) or deactivates the carrier_ 2  and excludes the carrier from the aggregation (e.g. when no neighbor cell having the radio quality good enough on the carrier_ 2 ). 
         [0054]    At this time, the timer_ 1  and timer_ 2  may be defined as new timers apart from the T310 and T311 as shown in  FIG. 2  but it is not ruled out to reuse the T310 and  311 . Although the description is directed to the embodiment in which two downlink carriers are aggregated, the present invention is not limited thereto. That is, the present invention is applicable as follows: 
         [0055]    1) when a plurality of secondary carriers exists among the aggregated carriers and the radio link failure is detected on one of the secondary carriers other than the primary carrier. 
         [0056]    2) when the carrier on which the radio link failure is detected is one secondary carrier other than the primary carrier (when there are multiple secondary carriers and the radio link failure is detected on the secondary carrier other than the primary carrier, the UE waits for the radio link recovery to the serving cell of the secondary carrier before the expiry of the timer_ 1 ; and if the radio link is not recovered before the expiry of the timer_ 1  with the serving cell of the secondary carrier, the UE deactivates the secondary carrier and excludes the secondary carrier from the aggregation). 
         [0057]      FIG. 5  is a flowchart illustrating the UE procedure according to an embodiment of the present invention. 
         [0058]    Referring to  FIG. 5 , the UE detects radio link failure on a secondary carrier other than the primary carrier among a plurality of aggregated carriers at step  501 . At this time, if N310 consecutive Qout signals are issued from the physical layer, the UE can detect the radio link failure. Afterward, the UE starts the timer_ 1  of the secondary carrier on which the radio link failure is detected at step  505 . At this time, the UE waits for the radio link recovery in the serving cell of the secondary carrier on which the radio link failure is detected. If N311 consecutive Qin signals are issued from the physical layer, the UE determines that the radio link is recovered in the serving cell of the secondary carrier. If the timer_ 1  of the secondary carrier expires (i.e. if the radio link recovery fails in the serving cell of the secondary carrier before the expiry of timer_ 1 ), the UE detects this at step  511  and checks the radio quality of the primary carrier at step  521 . If the expiry of the timer_ 1  of the secondary carrier on which the radio link failure is detected is not detected at step  511  but the radio link is recovered in the serving cell of the secondary carrier before the expiry of the timer_ 1 , the UE terminates the timer_ 1  of the secondary carrier at step  515 . 
         [0059]    Next, the UE determines whether the radio quality of the primary carrier is bad enough to cause the radio link failure after ward or has caused the radio link failure already at step  525  according to the radio link quality check result on the primary carrier at step  521 . At this time, the UE may compare the radio quality of the primary carrier with a threshold value. For example, if the radio quality of the primary carrier is less than the threshold value, the UE may determine that the radio quality of the primary carrier is bad enough to cause radio link failure afterward or has caused radio link failure on the primary carrier. If it is determined that the radio quality of the primary carrier is bad enough to cause radio link failure afterward or has caused radio link failure on the primary carrier at step  525 , the UE starts the timer_ 2  of the secondary carrier, at step  535 , on which the radio link failure has been detected. The UE searches for a suitable cell for receiving the normal service before the expiry of the timer_ 2 . This operation includes measurement and cell access check with the receipt of the system information. Since the determination conditions for use at step  525  have been described already with reference to  FIG. 4 , detailed description thereon is omitted herein. If the radio quality of the primary carrier does not satisfy the conditions at step  525 , the UE deactivates the secondary carrier on which the radio link failure has been detected and excludes the deactivated carrier from the aggregation. 
         [0060]    If the radio quality of the primary goes better or if the radio link failure has been detected but recovered soon, the UE detects this at step  541  and terminates the timer_ 2  of the secondary carrier on which the radio link failure has been detected at step  545 . Next, the UE deactivates the secondary carrier and/or reports the measurement result on the neighbor cells of the secondary carrier (including serving cell) to the eNB at step  549 . Since the exemplary determination conditions available at step  541  have been described with reference to  FIG. 4 , detailed description thereon is omitted herein. 
         [0061]    The UE determines whether the timer_ 1  of the primary carrier has expired and a suitable cell for receiving the normal service is discovered during the operation of step  535  at step  551 . If a suitable cell is discovered at step  551 , the UE performs RRC connection re-establishment to the suitable cell at step  555  or reports the result of step  535  (e.g. the information on the suitable cell discovered at step  535  or intra-frequency/inter-frequency/inter-Radio Access Technology neighbor cell measurement result) to the eNB at step  555 . If no suitable cell is discovered at step  551  and the timer_ 2  of the secondary carrier on which the radio link failure has been detected expires, the UE detects this at step  559  and deactivates the secondary carrier or reports the neighbor cell (including serving cell) measurement result on the secondary carrier to the eNB at step  549 . 
         [0062]      FIG. 6  is a block diagram illustrating a configuration of the UE according to an embodiment of the present invention. 
         [0063]    Referring to  FIG. 6 , the UE according to this embodiment includes a transceiver  601 , a failure detector  611 , a radio quality checker  621 , a timer  631 , a recovery executor  641 , and search executer  651 . 
         [0064]    The transceiver  601  configures the carrier aggregation of a plurality carriers for communication with the eNB and radio links in the mobile communication and communicates data with the eNB. For this purpose, the transceiver  601  measures CRS and transmits and receives control information and data. The failure detector  611  uses the CRS measurement value as the input information for detecting radio link failure and performs radio link failure detection per carrier in carrier aggregation mode. If the failure detector  611  detects radio link failure on a secondary carrier other than the primary carrier, the timer  631  starts timer_ 1 . The recovery executer  641  waits for radio link recovery on the secondary carrier where the radio link failure has been detected before the expiry of timer_ 1 . If the radio link is recovered in the serving cell of the secondary carrier before the expiry of the timer_ 1 , the timer  631  terminates the timer_ 1 . If the radio link is not recovered in the serving cell of the secondary carrier before the expiry of the timer_ 1  of the timer  631 , the radio quality checker  621  checks the radio quality of the primary carrier. If the radio quality of the primary carrier is greater than a threshold value, the recovery executor  641  deactivates the secondary carrier in the aggregation. 
         [0065]    Otherwise, if the radio quality of the primary carrier is less than the threshold value, the timer  631  starts timer_ 2  on a secondary carrier other than the primary carrier where the radio link failure has been detected or the recovery executor  641  deactivates the secondary carrier immediately without start of timer_ 2  and excludes the secondary carrier from the aggregation. If the timer_ 2  starts on the secondary carrier, the search executor  651  takes an action for discovery a suitable cell to receive the normal service. If the radio quality checker  621  detects that the radio quality of the primary carrier goes better to be greater than the threshold value or the radio link is recovered from the radio link failure on the secondary, the timer  631  terminates the timer_ 2 . If the timer is terminated, the search executor  651  may terminate the suitable cell discovery operation for receiving the normal service. If the timer_ 2  starts on the primary carrier while discovering a cell suitable for the normal service before the expiry of the timer_ 2 , the recovery executor  641  performs RRC connection re-establishment to the suitable cell immediately or reports the measurement result of the suitable cell discovered before the expiry of the timer_ 2  or the neighbor cell to the eNB. 
         [0066]    That is, the transceiver  601  configures a radio link in one of serving cells on a plurality of carriers aggregated. If the failure detector  611  detects radio link failure, the radio quality checker  621  checks the radio quality of the primary carrier. If radio link failure is detected, the timer  631  starts timer_ 1 , and the radio quality checker  621  may determine whether the radio link is recovered. If the radio link is recovered before the expiry of the timer_ 1 , the timer  631  terminates the timer_ 1  and, otherwise, if the radio link is not discovered, the radio link checker  621  may check the radio quality of the primary carrier. 
         [0067]    If the radio link checker  621  determines that the radio quality of the primary carrier does not fulfill a predetermined radio condition, the timer  631  starts timer_ 2 , and the search executor  641  searches neighbor cells around the serving cell. If the radio quality of the primary carrier is recovered before the expiry of the timer_ 2 , the timer  631  terminates the timer_ 2 , and the recovery executor  641  controls to deactivate one of the aggregated carriers. If the radio quality of the primary carrier is not recovered before the expiry of the timer_ 2 , the recovery executor  641  controls to configure the radio link in the found cell. Otherwise, if the radio link checker  621  determines that the radio quality of the primary carrier fulfills the radio condition, the recovery executor  641  controls to deactivate one of a plurality of carriers. 
         [0068]    According to the present invention, the UE is capable of performing radio link failure discovery and recovery efficiently in carrier aggregation mode. That is, if radio link failure is detected on a secondary carrier other than the primary carrier among aggregated carriers and if the radio link is not recovered on the secondary carrier before expiry of a timer, the UE does not perform measurement to discover a cell suitable for receiving the normal service, resulting in reduction of UE&#39;s power waste. If the probability of the radio link failure is high on the primary carrier, the UE performs measurement to discover the cell suitable for receiving the normal service on a secondary carrier so as to reduce the delay for radio link connection re-establishment when the radio link failure is detected on the primary carrier afterward. 
         [0069]    While the present invention has been shown and described with reference to various embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure as defined by the appended claims and their equivalents.