Patent Publication Number: US-2017367097-A1

Title: Method and apparatus for handling radio link failure in mobile communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to and the benefit of Korean Patent Application Nos. 10-2016-0076043 and 10-2016-0091645, filed in the Korean Intellectual Property Office on Jun. 17, 2016 and Jul. 19, 2016, respectively, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for handling a radio link failure in a mobile communication system and, more particularly, to a method and apparatus for handling a radio link failure that frequently occurs due to movement of a terminal in a mobile communication system of a cloud radio access network (C-RAN) to which a millimeter wave-based remote radio head (RRH) is applied. 
     2. Description of Related Art 
     Research into the use of a millimeter wave (mmWave) band to secure an effective bandwidth of 1 GHz or greater, rather than an existing cellular band, has been actively conducted to enhance spatial reuse of a frequency and a data rate in a mobile communication system. 
     In addition, in order to enhance quality of a mobile communication service, as well as satisfying recently increased wireless data traffic demand, a size of cells is decreased, the number of cells is increased, and wireless access technologies tend to be more elaborate and complicated to increase spatial reuse efficiency of a frequency. The increase in cells and progress in network incur high cost for cell installation and operation, laying a considerable burden on communication providers. As a solution, C-RAN technologies using an RRH, one of methods of providing a high speed wireless data service, while minimizing cost for advanced communication network, has been developed. A method for configuring a user-centric virtual cell, capable of simplifying an unnecessary handover procedure, while maintaining a user experienced data to rate in consideration of enhancement of performance in a cell boundary regarding a user which is located in the cell boundary or has high mobility, a problem which remains unsolved in an existing cellular system, has also be studied. 
     However, in the case of the C-RAN structure using millimeter wave-based RRH, an area in charge of each RRH is limited due to constraints of pathloss due to the use of a high frequency, poor penetration, and in particular, guaranteeing a line of sight (LOS), unlike transmission using an existing cellular band. In addition, since LOS is not secured due to movement of a user equipment (UE), a radio link established between the UE and a base station (BS) is frequently cut off. 
     Therefore, a method for rapidly handling a failure of a radio link which frequently occurs due to movement of a UE which is located in a cell boundary or which has high mobility in a millimeter wave-based mobile communication system of a C-RAN environment to which an RRH reducing installation cost of a BS and facilitating management, compared with the related art BS system, is required. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in an effort to provide a method and apparatus for handling a failure of a radio link in a mobile communication system having advantages of rapidly handling a failure of a radio link established between a base station (BS) and a user equipment (UE) due to a movement of the UE in the mobile communication system of a millimeter based-based RRH-applied C-RAN environment. 
     An exemplary embodiment of the present invention provides a method for handling a radio link failure in a base station (BS) of a mobile communication system. The method for handling a radio link failure may include: receiving, from a user equipment (UE), a candidate list including a candidate remote radio head (RRH) adjacent to the UE, excluding at least one serving RRH connected to the UE, among a plurality of RRHs; allocating a random access code index to at least one candidate RRH included in the candidate list; and transmitting, to the UE, the random access code index allocated to the at least one candidate RRH. 
     The method for handling a radio link failure may further include: when a failure in a radio link established between the at least one serving RRH and the UE is detected, processing a random access between a target candidate RRH with strongest signal strength received from the UE on the candidate list and the UE; and establishing a radio link between the target candidate RRH and the UE. 
     The processing of random access may include: receiving a random access preamble transmitted from the UE using the random access code index allocated to the target candidate RRH; and transmitting a random response message regarding the random access preamble. 
     The process of random access may further include: transmitting the random access code index allocated to the target candidate RRH to the target candidate RRH. 
     The method for handling a radio link failure may further include: transmitting a synchronization signal including a physical layer cell ID and a to reference signal including a unique identifier of a corresponding RRH through a plurality of RRHs, before the candidate list is received from the UE, wherein the candidate list includes a unique identifier of the candidate RRH. 
     The candidate list may further include a relative reference time difference of a signal received from the candidate RRH with respect to a reference signal of a signal received from the serving RRH, and the relative reference time difference is used as an uplink timing adjustment value for random accessing the candidate RRH. 
     When signal strength of a synchronization signal received by the UE continuously exceeds a preset first threshold by a predetermined first number of times and a unique identifier obtained through a reference signal is the same for the first number of times, an RRH which has transmitted the corresponding synchronization signal and the reference signal may be added as the candidate RRH to the candidate list, and when signal strength of a synchronization signal received by the UE does not continuously exceed a preset second threshold by a predetermined second number of times, a candidate RRH which has transmitted the corresponding synchronization signal and the reference signal may be deleted from the candidate list. 
     The allocating may include: allocating a random access code index to the candidate RRH when the candidate RRH is first reported through the candidate list; and maintaining the random access code index allocated to the candidate RRH until the candidate RRH is deleted from the candidate list. 
     Another exemplary embodiment of the present invention provides a method for handling a radio link failure in a user equipment (UE) of a mobile to communication system. The method for handling a radio link failure may include: configuring a candidate list including at least one candidate remote radio head (RRH) adjacent to the UE, excluding at least one serving RRH to which the UE is connected, among a plurality of RRHs; receiving a random access code index of the at least one candidate RRH allocated by a base station (BS), through the serving RRH; and adding the random access code index of the at least one candidate RRH to the candidate list. 
     The method for handling a radio link failure may further include: when a failure that occurs in a radio link established between the at least one serving RRH and the UE is detected, performing random access with a target candidate RRH using a random access code index of the target candidate RRH with strongest signal strength received from the UE on the candidate list; and establishing a radio link with the target candidate RRH. 
     The configuring may include: receiving synchronization signals including a physical layer cell ID and reference signals including a unique identifier of a corresponding RRH from the plurality of RRHs; selecting the serving RRH and the candidate RRH from among the plurality of RRHs using the synchronizations and the reference signals; and generating a candidate list including the candidate RRH and transmitting the generated candidate list to the BS through the serving RRH. 
     The selecting may include: selecting an RRH which has transmitted a synchronization signal with strongest signal strength among signal strengths of synchronization signals received from the plurality of RRHs, as the serving RRH; and when signal strength of a synchronization signal among to synchronization signals and reference signals received from the plurality of RRHs continuously exceeds a preset first threshold by a predetermined number of times and a unique identifier transmitted through a received reference signal is the same for the first number of times, selecting an RRH which has transmitted the corresponding synchronization signal and the reference signal as the candidate RRH. 
     The selecting may further include: when signal strength of a synchronization signal of the candidate RRH does not continuously exceed a preset second threshold by a predetermined second number of times, deleting the corresponding candidate RRH from the candidate list. 
     The transmitting may include: calculating a relative reference time difference of a synchronization signal received from the candidate RRH with respect to a reference time of a synchronization signal received from the at least one serving RRH; and generating a candidate list including a unique identifier of the candidate RRH obtained through the reference signal received from the candidate RRH and the relative reference time difference of the candidate RRH. 
     The performing of random access may include: adjusting an uplink timing based on a RRH with the fastest transmission time using relative reference time differences calculated with respect to the serving RRH and the target candidate RRH. 
     Yet another exemplary embodiment of the present invention provides an apparatus for handling a radio link failure in a user equipment (UE) of a mobile communication system. The apparatus for handling a radio link failure may to include: a candidate list configuring unit, a random access processing unit, and a radio link connection unit. The candidate list configuring unit may select a candidate remote radio head (RRH) adjacent to the UE, excluding a serving RRH to which the UE is connected, among a plurality of RRHs connected to a single baseband unit (BBU) pool, configure a candidate list including the candidate RRH, add a random access code index allocated to the candidate RRH to the candidate list, and manage the candidate list. The random access processing unit may perform random access with a target adjacent RRH using a random access code index allocated to the target adjacent RRH with strongest signal strength received from the UE, on the candidate list, when a failure occurs in a radio link established between the serving RRH and the UE. The radio link connection unit may connect the target candidate RRH and a radio link, when the random access is completed. 
     The apparatus for handling a radio link failure may further include: a transceiver unit receiving synchronization signals each including a physical layer cell ID and reference signals including a unique identifier of a corresponding RRH from the plurality of RRHs, wherein the candidate list configuring unit may select an RRH which has transmitted a synchronization signal with strongest signal strength among signal strengths of synchronization signals received from the plurality of RRHs, as the serving RRH, and when a signal strength of a received synchronization signal continuously exceeds a preset first threshold by a predetermined first number of times and a unique identifier transmitted through the received reference signal is the same for the first number of times, the candidate list configuring unit may select an RRH to which has transmitted the corresponding synchronization signal and the reference signal, as the candidate RRH. 
     The radio link connection unit may perform a radio resource control (RRC) connection re-establishment procedure to connect the target candidate RRH and a radio link. 
     The candidate list configuring unit may calculate a relative reference time difference of a synchronization signal received from the candidate RRH with respect to a reference signal of a synchronization signal received from the serving RRH, and the candidate list may include a unique identifier of the candidate RRH obtained through a reference signal received from the candidate RRH and a relative reference time difference of the candidate RRH. 
     The random access processing unit may adjust an uplink timing based on a RRH with the fastest transmission time using the relative reference time differences calculated with respect to the serving RRH and the target candidate RRH. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a remote radio head (RRH)-applied millimeter wave-based mobile communication system according to an exemplary embodiment of the present invention. 
         FIG. 2  is a view illustrating a communication environment between a plurality of RRHs and a UE according to an exemplary embodiment of the present invention. 
         FIG. 3  is a view illustrating a relative reference time difference included in a candidate list in the communication environment of  FIG. 2 . 
         FIG. 4  is a view illustrating a communication environment between a plurality of RRHs and a UE according to another exemplary embodiment of the present invention. 
         FIG. 5  is a view illustrating a relative reference time difference included in a candidate list in the communication environment of  FIG. 4 . 
         FIG. 6  is a flow chart illustrating an operation of a base station (BS) for managing an RRH according to an exemplary embodiment of the present invention. 
         FIG. 7  is a flow chart illustrating an operation of a user equipment (UE) for managing an RRH according to an exemplary embodiment of the present invention. 
         FIGS. 8 to 11  are views illustrating a procedure for handling a radio link failure according to an exemplary embodiment of the present invention. 
         FIG. 12  is a view illustrating an apparatus for handling a radio link failure in a BS according to an exemplary embodiment of the present invention. 
         FIG. 13  is a view illustrating an apparatus for handling a radio link failure in a UE according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the following detailed description, only certain exemplary embodiments of the present invention have been shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings to and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
     Throughout the specification and claims, unless explicitly described to the contrary, the word “comprise” and variations such as “comprises” or “comprising”, will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. 
     Throughout the specification, a terminal may refer to a mobile terminal (MT), a mobile station (MS), an advanced mobile station (AMS), a high reliability mobile station (HR-MS), a subscriber station (SS), a portable subscriber station (PSS), an access terminal (AT), or a user equipment (UE), and may include the entirety or a portion of functions of the MT, MS, AMS, HR-MS, SS, PSS, AT, or UE. 
     Also, a base station (BS) may refer to an advanced base station (ABS), a high reliability base station (HR-BS), a node B, an evolved node B (eNodeB), an access point (AP), a radio access station (RAS), a base transceiver station (BTS), a mobile multihop relay (MMR)-BS, a relay station (RS) serving as a base station, a relay node (RN) serving as a base station, an advanced relay station (ARS) serving as a base station, a high reliability relay station (HR-RS) serving as a base station, small base stations (BSs) (e.g., a femto base station (BS), a home node B (HNB), a home eNodeB (HeNB), a pico BS, a metro BS, a micro BS, etc.), and the like, and may include the entirety or a portion of functions of an ABS, a node B, an eNodeB, an AP, an RAS, a BTS, an MMR-BS, an RS, an RN, an ARS, an HR-RS, a small BS, and the like. 
     Hereinafter, a method and apparatus for managing a remote radio head to (RRH) in a mobile communication system according to an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a view illustrating a remote radio head (RRH)-applied millimeter wave-based mobile communication system according to an exemplary embodiment of the present invention 
     Referring to  FIG. 1 , a base station (BS) of an RRH-applied millimeter wave-based mobile communication system separately operates a plurality of RRHs  200 ,  210 , . . . ,  260  amplifying a radio frequency (RF) signal and radiating the amplified RF signal to an antenna within a service area and a baseband unit (BBU) pool  100  responsible for controlling and signal processing. 
     The plurality of RRHs  200 ,  210 , . . . ,  260  are distributed within a cell managed by the BS and connected to the BBU pool  100  through an optical cable, or the like. Each of the plurality of RRHs  200 ,  210 , . . . ,  260  may use a millimeter wave frequency band of 10 GHz or higher as a carrier frequency, and use a bandwidth from hundreds of MHz to  1  GHz or higher for data transmission. 
     The plurality of RRHs  200 ,  210 , . . . ,  260  connected to the same BBU pool  100  simultaneously transmit a synchronization signal including the same physical layer cell ID (PCID) to user equipments (UEs)  300 ,  310 , and  320  using the same radio resource. During this process, the plurality of RRHs  200 ,  210 , . . . ,  260  interfere with each other because they transmit different data using the same radio resource. Thus, the plurality of RRHs  200 ,  210 , . . . ,  260  transmit a unique identifier (ID) identifying each RRH to the UEs  300 ,  310 , and  320  using to a reference signal for alleviating interference of signals transmitted by neighboring RRHs. Here, radio resource refers to a resource element of a time and frequency space defined in 3GPP LTE/LTE-A (Advanced) specification, and it is assumed that radio frames transmitted from the plurality of RRHs  200 ,  210 , . . . ,  260 , subframes forming the radio frames, and symbols are in synchronization. 
       FIG. 2  is a view illustrating a communication environment between a plurality of RRHs and a UE according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 2 , the plurality of RRHs  200 ,  210 , and  220  for which the BBU pool  110  is responsible transmit a synchronization signal and a reference signal to the UE  300  at the same time. Here, since relative position of the UE  300  with respect to the RRHs  200 ,  210 , and  220  is different, time delays δ 0 , δ 1 , and δ 2  occur depending on the relative position of the UE  300 . 
     The UE  300  belonging to an area of the plurality of RRHs  200 ,  210 , and  220  receives the synchronization signals and the reference signals from the plurality of RRHs  200 ,  210 , and  220  after the time delays δ 0 , δ 1 , and δ 2 . 
     The UE  300  sets a reference time using a synchronization signal of an RRH (e.g.,  220 ) having the largest signal strength among the synchronization signals received from the plurality of RRHs  200 ,  210 , and  220 , and determines the RRH  220  as a serving RRH  220  using a unique identifier included in the reference signal of the RRH  220 . 
     When the serving RRH  220  is determined using the synchronization signal and the reference signal, the UE  300  establishes a radio resource control (RRC) connection with the serving RRH  220 . 
     When the UE  300  is switched from an RRC idle (RRC_IDLE) state to an RRC-connected (RRC_CONNECTED) state, the UE  300  continuously searches whether another RRH excluding the serving RRH  220  is present in the vicinity of the UE  300  from synchronization signals received from the plurality of RRHs  200 ,  210 , and  220 . The UE  300  starts monitoring to add RRHs  200  and  210 , from which synchronization signals received by the UE  300  exceed a preset threshold TH 1 , to a candidate list. Here, the RRHs  200  and  210  transmitting synchronization signals whose strength exceeds the preset threshold TH 1  will be referred to as adjacent RRH_ 1   200  and an adjacent RRH_ 2   210 . When reception signal strength of the synchronization signals respectively received from the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210  after monitoring starts continuously exceeds a preset threshold TH 2  a predetermined number of times N 1  and unique identifiers transmitted through the reference signals respectively received from the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210  are the same the predetermined number of times N 1 , the UE  300  adds the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210 , as candidate RRHs, to a candidate list. 
     When the candidate list is created, the UE  300  transmits the candidate list to the BBU pool  100  through the serving RRH  220  at a predetermined period. 
     The candidate list transmitted to the BBU pool  100  may include unique identifiers of the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210 . Also, the candidate list may further include information regarding a relative reference time difference of the synchronization signals received from the candidate RRHs  200  to and  210  with respect to a reference time set on the basis of a synchronization signal from the serving RRH  220 . The relative reference time different will be described in detail with reference to  FIG. 3 . 
     The BBU pool  100  allocates contention-free-based random access code indices RA a  and RA b  which may be used in a radio link failure or during a handover process with respect to the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210  included in the candidate list received from the UE  300  and subsequently transmit the random access code indices RA a  and RA b  to the UE  300  using the serving RRH  220 . 
     The UE  300  adds the random access code indices RA a  and RA b  allocated to the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210  to the candidate list. The candidate list may further include the contention-free-based random access code indices RA a  and RA b  regarding the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210 . 
     When receive strengths of the synchronization signals from the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210  do not continuously exceed a preset threshold TH 3  a predetermined number of times N 2 , the UE  300  may delete the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210  included in the candidate list, from the candidate list. 
     When the candidate list is updated due to deletion of the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210  from the candidate list, the UE  300  may transmit the updated candidate list to the BBU pool  100  using the serving RRH  220 . 
     Allocation of the random access code indices RA a  and RA b  may be to performed when the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210  in the vicinity of the UE  300  are first reported through the candidate list, and values of the random access code indices RA a  and RA b  may be maintained until the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210  are deleted from the candidate list. 
       FIG. 3  is a view illustrating a relative reference time difference included in a candidate list in the communication environment of  FIG. 2 . 
     Referring to  FIG. 3 , the serving RRH  220  and the candidate RRHs  200  and  210  positioned around the UE  300  are in synchronization, the serving RRH  220  and the candidate RRHs  200  and  210  transmit a synchronization signal and a reference signal through a downlink subframe #n to the UE  300  at the same downlink transmission time T. However, since relative position of the UE  300  with respect to each of the RRHs  200 ,  210 , and  220  is different, time delays δ 0 , δ 1 , and δ 2  occur. 
     As described above, the UE  300  sets a reference time of a reception signal using the synchronization signal received from the serving RRH  220 . 
     The UE  300  calculates relative reference time differences (d 1 =δ 1 −δ 0 , d 2 =δ 2 −δ 0 ) of the synchronization signals received from the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210  with respect to the set reference time. Here, the values of the calculated reference time differences d 1  and d 2  are used as timing adjustment values for random-accessing to a high ranking candidate RRH_ 1   200  on the candidate list when a radio link between the UE  300  and the serving RRH  220  is cut off. 
     Table 1 shows an example of a candidate list configured by the UE  300  according to  FIGS. 2 and 3 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Relative reference time difference 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Serving 
                   
                 random 
               
               
                 List 
                 RRH 
                 RRH    terminal 
                   
                 access code 
               
               
                 number 
                 identifier 
                 (or UE) 
                 — 
                 index 
               
               
                   
               
               
                 1 
                 Adjacent 
                 d 1   
                 — 
                 RA a   
               
               
                   
                 RRH_1 
               
               
                 2 
                 Adjacent 
                 d 2   
                 — 
                 RA b   
               
               
                   
                 RRH_2 
               
               
                   
               
            
           
         
       
     
     In the case of  FIG. 2 , since only one serving RRH  220  connected to the UE  300  by a radio link is present, relative reference time differences regarding the candidate RRHs  200  and  210  are respectively present on the candidate list, and in cases where the number of contention-free-based random access code indices which may be allocated by the BBU pool  100  is limited, the BBU pool  100  may allocate the random access code indices only to M number of RRH in a high ranking on the candidate list and transmit the same to the UE  300 . Here, order of the candidate RRHs on the candidate list may be determined signal strength of synchronization signals received from the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210 . For example, the UE  300  may align values obtained by averaging signal strengths of synchronization signals respectively received from the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210  the predetermined number of times N 1 , starting from a largest value, in descending to order. 
       FIG. 4  is a view illustrating a communication environment between a plurality of RRHs and a UE according to another exemplary embodiment of the present invention. 
     Referring to  FIG. 4 , a plurality of RRHs  240 ,  250 , and  260  for which the BBU pool  100  is responsible transmit a synchronization signal and a reference signal to a UE  320  at the same time. Here, since a relative position of the UE  320  with respect to the RRHs  240 ,  250 , and  260  is different, time delays δ 3 , δ 4 , and δ 5  occur depending on a relative position of the UE  320 . 
     The UE  320  included in an area of the plurality of RRHs  240 ,  250 , and  260  receives the synchronization signals and reference signals from the plurality of RRHs  240 ,  250 , and  260  after time delays 
     Here, the UE  320  included in the area of the plurality of RRHs  240 ,  250 , and  260  may be connected to a plurality of serving RRHs  240  and  250 . In a state in which the UE  320  is not connected to an RRH therearound, an RRH (e.g.,  240 ) having strength of a synchronization signal is greatest is determined as the serving RRH  240  and connection is established between the UE  320  and the serving RRH  240 . Also, a candidate list is created using the RRH around the UE  320 . After this process is completed, when an additional connection is required, an RRH (e.g.,  250 ) whose signal strength is strong on the candidate list may be determined as the serving RRH  250  in a state in which the existing serving RRH  240  is maintained, and connection may be additionally established between the UE  320  and the serving RRH  250 . 
     In an RRC-connected state in which the UE  320  is connected to the to plurality of serving RRHs  240  and  250 , the UE  320  continuously searches whether another RRH excluding the serving RRHs  240  and  250  is present around the UE  320  on the basis of synchronization signals received from the plurality of RRHs  240 ,  250 , and  260 . Here, for the purposes of description, the two serving RRHs  240  and  250  will be referred to as a serving RRH_ 1   240  and a serving RRH_ 2   250 , respectively. Among the serving RRH_ 1   240  and the serving RRH_ 2   250 , it is assumed that the serving RRH_ 1   240  is a main serving RRH having priority of every connection and the serving RRH_ 2   250  is an auxiliary serving RRH, and distinguishment of the main serving RRH and the auxiliary serving RRH may be determined according to priority connected to the UE  320 . 
     When signal strength of the synchronization signal received from the adjacent RRH  260 , excluding the serving RRH_ 1   240  and the serving RRH_ 2   250 , exceeds the preset threshold TH 1 , the UE  320  starts monitoring to add the adjacent RRH  260  to the candidate list. 
     When signal strength of the synchronization signal received from the adjacent RRH  260  continuously exceeds the preset threshold TH 2  the predetermined number of times N 1  and a unique identifier transmitted through the reference signal of the adjacent RRH  260  is the same the predetermined number of times N 1 , the UE  320  adds the adjacent RRH  260  as monitored, as a candidate RRH  260  to the candidate list. 
     When the candidate list is created, the UE  320  transmits the candidate list to the BBU pool  100  through the serving RRH_ 1   240  and the serving RRH_ 2   250  at a predetermined period, but the BBU pool  100  allocates a to contention-free-based random access code index (RA c ) which may be used in the occurrence of a radio link failure or during a handover process to the adjacent RRH  260  included in the candidate list received from the UE  320  through the serving RRH_ 1   240  as a main serving RRH. Also, the BBU pool  100  transmits the contention-free-based random access code index (RA c ) allocated to the adjacent RRH  260  to the UE  320  using the serving RRH_ 1   240  as a main serving RRH. 
     The candidate list may include information regarding a relative reference time difference of a synchronization signal received from the adjacent RRH  260  to a reference time of synchronization signals respectively received from the serving RRH_ 1   240  and the serving RRH_ 2   250  and the contention-free-based random access code index (RA c ) allocated to the adjacent RRH  260 , as well as the unique identifier of the candidate RRH  260 . 
     When a receive strength of the synchronization signal from the adjacent RRH  260  added to the candidate list does not continuously exceed the preset threshold TH 3  the predetermined number of times N 2 , the UE  320  deletes the candidate RRH  260  from the candidate list. 
     When the candidate list is updated due to deletion of the adjacent RRH  260  from the candidate list, the UE  320  transmits the updated candidate list to the BBU pool  100  using the serving RRH_ 1   240 . 
     Allocation of the random access code index (RA c ) may be performed when the adjacent RRH  260  near the UE  320  is first reported through the candidate list, and the value of the random access code index (RA c ) may be maintained until the adjacent RRH  260  is deleted from the candidate list. 
       FIG. 5  is a view illustrating a relative reference time difference included in a candidate list in the communication environment of  FIG. 4 . 
     Referring to  FIG. 5 , since signals from the serving RRH_ 1   240 , the serving RRH_ 2   250 , and the candidate RRH  260  positioned around the UE  320  are in synchronization, the serving RRHs  240  and  250  and the candidate RRH  260  transmit a synchronization signal and a reference signal through a downlink subframe #n to the UE  320  at the same downlink transmission time T. However, since relative position of the UE  320  with respect to each of the RRHs  240 ,  250 , and  260  is different, time delays δ 3 , δ 4 , and δ 5  occur. 
     As described above, the UE  320  sets a reference time of a reception signal using the synchronization signals respectively received from the serving RRH_ 1   240  and the serving RRH_ 2   250 . 
     The UE  320  may calculate relative reference times (d 3 =δ 5 −δ 3 , d 4 =δ 5 −δ 4 ) with respect to a reference time of the synchronization signal received from the adjacent RRH  260  from a reference time of the synchronization signals respectively received from the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210 . 
     Here, the values of the calculated reference time differences d 3  and d 4  are used as timing adjustment values for random-accessing to a high ranking candidate RRH  260  on the candidate list when a radio link between the UE  320  and the serving RRHs  240  and  250  is cut off. 
     Table 2 shows an example of a candidate list configured by the UE  320  in accordance with  FIGS. 4 and 5 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Relative reference time difference 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Serving 
                 Serving 
                 random 
               
               
                 List 
                 RRH 
                 RRH_1    termi- 
                 RRH_2    termi- 
                 access code 
               
               
                 number 
                 identifier 
                 nal 
                 nal 
                 index 
               
               
                   
               
               
                 1 
                 Adjacent 
                 d 3   
                 d 4   
                 RA c   
               
               
                   
                 RRH 
               
               
                   
               
            
           
         
       
     
     A relative reference time difference of the candidate RRH  260  on the candidate list to the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210  connected to the UE  320  by a radio link is respectively present, and in cases where the number of contention-free-based random access code indices which may be allocated by the BBU pool  100  is limited, the BBU pool  100  may allocate the random access code indices only to M number of RRH in a high ranking on the candidate list and transmit the same to the UE  320 . Here, candidate RRHs on the candidate list may be aligned in descending order, starting from a largest one of values obtained by averaging signal strengths of synchronization signals from the candidate RRH  260  received by the UE  320  the predetermined number of times N 1 . 
       FIG. 6  is a flow chart illustrating an operation of a base station (BS) for managing an RRH according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 6 , the BBU pool  100  transmits a synchronization signal and a reference signal to a UE through a plurality of RRHs (S 610 ). 
     The BBU pool  100  receives a candidate list regarding neighbor RRHs to from the UE (S 620 ). 
     The BBU pool  100  allocates a random access code index to each of the RRHs included in the received candidate list (S 630 ) and subsequently transmits the allocated random access code index to the UE (S 640 ). The BBU pool  100  may allocate the random access code index when a candidate RRH is first reported through the candidate list transmitted from the UE. Also, the allocated random access code index may be maintained until the corresponding candidate RRH is deleted from the candidate list. When the number of contention-free-based random access code indices which may be allocated is limited, the BBU pool  100  may allocate the random access code indices only to M number of RRH in a high ranking on the candidate list. 
       FIG. 7  is a flow chart illustrating an operation of a user equipment (UE) for managing an RRH according to an exemplary embodiment of the present invention. In  FIG. 7 , for the purposes of description, the UE  300  will be described for reference, but the other UEs  310  and  320  may also operate in the same or similar manner. 
     Referring to  FIG. 7 , the UE  300  receives synchronization signals and reference signals transmitted from a plurality of RRHs (S 710 ). 
     The UE  300  selects a serving RRH using the received synchronization signals and reference signals, and when the UE  300  is connected to the serving 
     RRH in an RRC-connected state, the UE  300  performs a process of searching for and selecting a candidate RRH (S 720 ). 
     When a candidate RRH is selected, the UE  300  calculates a relative reference time difference of a synchronization signal received from the to candidate RRH to a reference time of a synchronization signal received from the serving RRH (S 730 ). 
     The UE  300  configures a candidate list using a unique identifier of the candidate RHH and the calculated reference time difference (S 740 ). 
     The UE  300  transmits the configured candidate list to the plurality of RRHs (S 750 ). 
     Thereafter, when a random access code index regarding the candidate RRH included in the candidate list from the serving RRH is received (S 760 ), the UE  300  updates the candidate list including the received random access code index (S 770 ). 
       FIG. 8  is a view illustrating a procedure for handling a radio link failure according to an exemplary embodiment of the present invention. A procedure for processing a radio link failure on the basis of a communication environment illustrated in  FIG. 2  will be described with reference to  FIG. 8 . 
     Referring to  FIG. 8 , when the serving RRH  200  and the UE  300  are set in an RRC-connected state (S 802 ), the UE  300  receives synchronization signals and reference signals transmitted from the plurality of RRHs  200 ,  210 , and  220  (S 804 ). 
     The UE  300  performs a process of searching for and selecting a candidate RRH using the received synchronization signals and reference signals. The process (step S 806  to S 810 ) of searching for and selecting the candidate RRH and configuring and managing the candidate list are the same as the contents described above with reference to  FIGS. 2 and 7 . That is, when receive strengths of synchronization signals from the adjacent RRH_ 1  to  200  and the adjacent RRH_ 2   210 , excluding the serving RRH  220 , continuously exceed the preset threshold TH 2  the predetermined number of times N  1  and unique identifiers obtained through the reference signals respectively received from the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210  are the same the predetermined number of times N  1 , the UE  300  registers the adjacent RRH_ 1   200  and the adjacent RRH_ 2   210  as candidate RRHs and configures a candidate list including the candidate RRHs (S 806 ). The UE  300  periodically reports the candidate list to the BBU pool  100  through the serving RRH  220  (S 808 ). The BBU pool  100 , which has received the candidate list from the UE  300 , allocates random access code indices for contention-free-based random access which may be used by M number of candidate RRHs in a high ranking on the candidate list, and transmits information regarding the random access code indices allocated to the M number of candidate RRHs to the UE  300  through the serving RRH  220  (S 810 ). The UE  300  adds the random access code indices regarding the M number of RRHs in a high ranking received from the serving RRH  220  to the candidate list and manages the indices. 
     Meanwhile, when a failure regarding a radio link connected between the serving RRH  220  and the UE  300  occurs (S 812 ), the serving RRH  220  transmits radio link failure occurrence information to the BBU pool  100 . 
     Upon receiving the wireless link failure occurrence information, the BBU pool  100  transfers information regarding the random access code index allocated to the adjacent RRH_ 1   200  to the candidate RRH positioned in a highest ranking on the candidate list, e.g., the adjacent RRH_ 1   200  (S 814 ). 
     The UE  300  adjusts an uplink timing using the reference time difference to d 1  stored to correspond to the adjacent RRH_ 1   200  positioned in the highest ranking on the candidate list (S 816 ). Thereafter, the UE  300  generates a random access preamble using the random access code index allocated to the adjacent RRH_ 1   200  and subsequently transmits the random access preamble to the adjacent RRH_ 1   200  (S 818 ). 
     When the random access preamble is detected, the adjacent RRH_ 1   200  transmits a detection result to the BBU pool  100 . The BBU pool  100  generates a random access response on the basis of the detection result transmitted from the adjacent RRH_ 1   200  and subsequently transmits the random access response to the adjacent RRH_ 1   200 . The adjacent RRH_ 1   200  transmits the random access response with respect to the random access preamble to the UE  300  (S 820 ). 
     As the UE  300  receives the random access response, the random access procedure is completed and a radio link is established between the UE  300  and the adjacent RRH_ 1   200  through an RRC connection re-establishment process between the UE  300  and the adjacent RRH_ 1   200  (S 822 ). 
       FIG. 9  is a view illustrating a procedure for handling a radio link failure according to another exemplary embodiment of the present invention. A procedure for processing a radio link failure on the basis of a communication environment illustrated in  FIG. 4  will be described with reference to  FIG. 9 . Here, it is assumed that the serving RRH_ 1   240  is a main serving RRH with priority of every connection and the serving RRH_ 2   250  is an auxiliary serving RRH. 
     Referring to  FIG. 9 , when the UE  320  is connected to the serving to RRH_ 1   240  and the serving RRH_ 2   250  in an RRC-connected state (S 902 ), the UE  320  receives synchronization signals and reference signals from the plurality of RRHs  240 ,  250 , and  260  (S 904 ). 
     The UE  320  performs a process of searching for and selecting a candidate RRH on the basis of the received synchronization signals and reference signals. The process (step S 906  to S 910 ) of searching for and selecting the candidate RRH and configuring and managing the candidate list are the same as the contents described above with reference to  FIGS. 4 and 7 . That is, when receive strength of a synchronization signal received from the adjacent RRH  260 , excluding the serving RRH_ 1   240  and the serving RRH_ 2   250 , continuously exceeds the preset threshold TH 2  the predetermined number of times N 1  and a unique identifier obtained through a reference signal received from the adjacent RRH  260  is the same the predetermined number of times N 1 , the UE  300  registers the adjacent RRH  260  as a candidate RRH and configures a candidate list including the candidate RRH (S 906 ). The UE  320  periodically transmits the configured candidate list to the BBU pool  100  through the serving RRH_ 1   240  and the serving RRH_ 2   250  (S 908 ). Upon simultaneously receiving the candidate list through the serving RRH_ 1   240  and the serving RRH_ 2   250 , the BBU pool  100  allocates a random access code index for a contention-free-based random access which may be used by M number of candidate RRHs in a high ranking on the candidate list received from the serving RRH_ 1   240  corresponding to a main serving RRH. The BBU pool  100  transmits information regarding random access code indices allocated to the M number of candidate RRHs in a high ranking to the UE  320  through the serving to RRH_ 1   240  as a main serving RRH (S 910 ). The UE  320  adds the random access code indices regarding the M number of RRHs in a high ranking received from the serving RRH_ 1   240  to the candidate list and manages the indices. 
     Meanwhile, when a failure regarding a radio link connected between the serving RRH_ 1   240  and the UE  320  occurs (S 912 ), the serving RRH_ 1   240  transmits radio link failure occurrence information to the BBU pool  100 . 
     Upon receiving the wireless link failure occurrence information, the BBU pool  100  transfers information regarding the random access code index allocated to the adjacent RRH  260  to the adjacent RRH  260  positioned in a highest ranking on the candidate list (S 914 ). 
     The UE  320  adjusts an uplink timing regarding the adjacent RRH  260  positioned 
     The UE adjusts the uplink timing based on the RRH (for example, serving RRH_ 2   250 ) with the fastest transmission time using the reference time differences d 3  and d 4  stored to correspond to the serving RRH_ 2   250  and the adjacent RRH  260  positioned in the highest ranking on the candidate list (S 916 ). 
     The UE  320  generates a random access preamble using the random access code index allocated to the adjacent RRH  260  and subsequently transmits the random access preamble to the adjacent RRH  260  (S 918 ). 
     When the random access preamble is detected considering the relative reference time difference d 4  of the signal transmitted from the serving RRH_ 2   250 , the adjacent RRH  260  transmits a detection result to the BBU pool  100 . The BBU pool  100  generates a random access response on the basis of the to detection result transmitted from the adjacent RRH  260  and subsequently transmits the random access response to the adjacent RRH  260 . The adjacent RRH  260  transmits the random access response to the UE  320  (S 920 ). 
     As the UE  320  receives the random access response, the random access procedure is completed and a radio link is established between the UE  320 , the serving RRH_ 2   250 , and the adjacent RRH  260  through an RRC connection re-establishment process between the UE  320  and the adjacent RRH  260  (S 922 ). 
       FIG. 10  is a view illustrating a procedure for handling a radio link failure according to another exemplary embodiment of the present invention. A procedure for processing a radio link failure on the basis of a communication environment illustrated in  FIG. 4  will be described with reference to  FIG. 10 , like  FIG. 9 . 
     Referring to  FIG. 10 , when the UE  320  is connected to the serving RRH_ 1   240  and the serving RRH_ 2   250  in an RRC-connected state (S 1002 ), the UE  320  receives synchronization signals and reference signals from the plurality of RRHs  240 ,  250 , and  260  (S 1004 ). 
     The UE  320  performs a process of searching for and selecting a candidate RRH on the basis of the received synchronization signals and reference signals. The process (step S 1006  to S 1010 ) of searching for and selecting the candidate RRH and configuring and managing the candidate list are the same as the process (S 906  to S 910 ) illustrated in  FIG. 9 , and thus, a detailed description thereof will be omitted. 
     Meanwhile, when a failure occurs in the radio link established between to the serving RRH_ 2   250  corresponding to an auxiliary serving RRH and the UE  320  (S 1012 ), the serving RRH_ 2   250  transmits radio link failure occurrence information to the serving RRH_ 1   240  corresponding to the main serving RRH through the BBU pool  100  (S 1014 ). 
     The serving RRH_ 1   240  transfers information regarding the random access code index allocated to the adjacent RRH  260  to the adjacent RRH  260  positioned in a highest ranking on the candidate list through the BBU pool  100  (S 1016 ). 
     Since the radio link failure of the serving RRH_ 1  (  240 ) corresponding to the main serving RRH did not occur, the UE  320  does not perform the step of adjusting the uplink timing. 
     The UE  320  generates a random access preamble using the random access code index allocated to the adjacent RRH  260  and subsequently transmits the random access preamble to the adjacent RRH  260  (S 1018 ). 
     When the random access preamble is detected, the adjacent RRH  260  transmits a detection result to the BBU pool  100 . The BBU pool  100  generates a random access response on the basis of the detection result transmitted from the adjacent RRH  260  and subsequently transmits the random access response to the adjacent RRH  260 . The adjacent RRH  260  transmits the random access response to the UE  320  (S 1020 ). 
     As the UE  320  receives the random access response, the random access procedure is completed and a radio link is established between the UE  320  and the adjacent RRH  260  through an RRC connection re-establishment process between the UE  320 , the serving RRH_ 1   240  and the adjacent RRH to  260  (S 1022 ). 
       FIG. 11  is a view illustrating a procedure for handling a radio link failure according to another exemplary embodiment of the present invention. A procedure for processing a radio link failure on the basis of a communication environment illustrated in  FIG. 4  will be described with reference to  FIG. 11 , like  FIGS. 9 and 10 . 
     Referring to  FIG. 11 , when the UE  320  is connected to the serving RRH_ 1   240  and the serving RRH_ 2   250  in an RRC-connected state (S 1102 ), the UE  320  receives synchronization signals and reference signals from the plurality of RRHs  240 ,  250 , and  260  (S 1104 ). 
     The UE  320  performs a process of searching for and selecting a candidate RRH on the basis of the received synchronization signals and reference signals. The process (step S 1106  to S 1110 ) of searching for and selecting the candidate RRH and configuring and managing the candidate list are the same as the process (S 906  to S 910 , and S 1006  to S 1010 ) illustrated in  FIGS. 9 and 10 , and thus, a detailed description thereof will be omitted. 
     Meanwhile, when failures occur at the same time in the radio link established between the serving RRH_ 1   240  corresponding to a main serving RRH and the UE  320  and the radio link established between the serving RRH_ 2   250  corresponding to an auxiliary serving RRH and the UE  320  (S 1112 ), the serving RRH_ 1   240  and the serving RRH_ 2   250  transmit radio link failure occurrence information to the BBU pool  100 , respectively (S 1114 ). 
     The BBU pool  100  transfers information regarding the random access code index allocated to the adjacent RRH  260  to the adjacent RRH  260  to positioned in a highest ranking on the candidate list (S 1116 ). 
     The UE  320  adjusts an uplink timing regarding the adjacent RRH  260  positioned in the highest ranking on the candidate list using the reference time difference d 4  stored to correspond to the adjacent RRH  260  positioned in the highest ranking on the candidate list (S 1118 ). 
     The UE  320  generates a random access preamble using the random access code index allocated to the adjacent RRH  260  and transmits the random access preamble to the adjacent RRH  260  (S 1120 ). 
     When the random access preamble is detected, the adjacent RRH  260  transmits a detection result to the BBU pool  100 . The BBU pool  100  generates a random access response on the basis of the detection result transmitted from the adjacent RRH  260  and subsequently transmits the random access response to the adjacent RRH  260 . The adjacent RRH  260  transmits the random access response to the UE  320  (S 1122 ). 
     As the UE  320  receives the random access response, the random access procedure is completed and a radio link is established between the UE  320  and the adjacent RRH  260  through an RRC connection re-establishment process between the UE  320  and the adjacent RRH  260  (S 1124 ). 
       FIG. 12  is a view illustrating an apparatus for handling a radio link failure in a BS according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 12 , an apparatus  1200  for handling a radio link failure includes an allocation unit  1210 , a random access processing unit  1220 , a radio link connection unit  1230 , and a transceiver unit  1240 . The allocation unit  1210 , the random access processing unit  1220 , the radio link connection unit to  1230 , and the transceiver unit  1240  are executed according to an instruction from at least one processor to perform a corresponding function. Instructions to be performed in the processor may be stored in a memory or a storage, and the processor executes an instruction stored in the memory or the storage. 
     The allocation unit  1210  may be implemented in a BBU pool, and the random access processing unit  1220 , the radio link connection unit  1230 , and the transceiver unit  1240  may be implemented in an RRH. 
     The allocation unit  1210  may perform the function of the BBU pool  100  described above with reference to  FIG. 6 . When the allocation unit  1210  receives a candidate list regarding neighbor RRHs from a UE, the allocation unit  1210  manages the candidate list received from the UE and allocates a random access code index to at least one RRH included in the candidate list on the basis of the candidate list. The allocated random access code index is transmitted to the UE through the transceiver unit  1240 . 
     When a failure occurs in a radio link established between a serving RRH and the UE, the random access processing unit  1220  detects a random access preamble using a random access code index allocated to an adjacent RRH positioned in a highest ranking on the candidate list and transmits a random access response with respect to the random access preamble to the UE through the transceiver unit  1240 . The random access response is transmitted to the UE through the transceiver unit  1240 . 
     When the failure occurs in the radio link established between the serving RRH and the UE, the radio link connection unit  1230  performs a process of RRC connection re-establishment between the adjacent RRH positioned in the to highest ranking on the candidate list to establish a radio link between the adjacent RRH and the UE. 
     The transceiver unit  1240  may include a plurality of RRHs and may be connected to the allocation unit  1210 , the random access processing unit  1220 , and the radio link connection unit  1230  to transmit and receive a radio signal to and from the UE. 
       FIG. 13  is a view illustrating an apparatus for handling a radio link failure in a UE according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 13 , an apparatus  1300  for handling a radio link failure includes a candidate list configuring unit  1310 , a random access processing unit  1320 , a radio link connection unit  1330 , and a transceiver unit  1340 . The candidate list configuring unit  1310 , the random access processing unit  1320 , and the radio link connection unit  1330  are executed according to an instruction from at least one processor to perform a corresponding function. Instructions to be performed in the processor may be stored in a memory or a storage, and the processor executes an instruction stored in the memory or the storage. 
     The candidate list configuring unit  1310  may perform a function of the UE  300  described above with reference to  FIG. 7 . The candidate list configuring unit  1310  may select a serving RRH using synchronization signals and reference signals received from a plurality of RRHs, searches for a candidate RRH, and selects the candidate RRH. When the candidate RRH is selected, a relative reference time difference of a signal received from the candidate RRH with respect to a reference time of a signal received from the serving RRH, and subsequently configures a candidate list including a unique to identifier of the candidate RRH and the calculated reference time difference. 
     The candidate list is transmitted to a BS through the transceiver unit  1340  and through the serving RRH. Also, when the candidate list configuring unit  1310  receives a random access code index regarding the RRH included in the candidate list through the transceiver unit  1340  from the BS, the candidate list configuring unit  1310  updates the candidate list using the received random access code index. 
     When a failure occurs in a wireless link established between the serving RRH and the UE, the random access processing unit  1320  performs a random access procedure with an adjacent RRH positioned in a highest ranking on the candidate list. The random access processing unit  1320  generates a random access preamble using the random access code index allocated to the adjacent RRH positioned in the highest ranking on the candidate list, and receives a random access response from the BS. The random access preamble is transmitted through the transceiver unit  1340 , and the random access response is received from the BS through the transceiver unit  1340 . 
     When a failure occurs in the radio link established between the serving RRH and the UE, the radio link connection unit  1330  performs a process of RRC connection re-establishment with the adjacent RRH positioned in the highest ranking on the candidate list to establish a radio link between the adjacent RRH and the UE. 
     The transceiver unit  1340  is connected to the candidate list configuring unit  1310 , the random access processing unit  1320 , and the radio link connection unit  1330  to transmit and receive a radio signal to and from the BS. According to an exemplary embodiment of the present invention, when a failure of a radio link established between a BS and a UE is detected due to a frequent movement of the UE which is located in a cell boundary or has high mobility, the radio link failure is rapidly handled using candidate list information regarding an adjacent RRH managed by the UE in advance, whereby a user experience data rate in consideration of enhancement of performance in a cell boundary may be maintained. 
     The exemplary embodiments of the present invention may not necessarily be implemented only through the foregoing devices and/or methods but may also be implemented through a program for realizing functions corresponding to the configurations of the embodiments of the present invention, a recording medium including the program, or the like. Such an implementation may be easily conducted by a person skilled in the art to which the present invention pertains from the foregoing description of embodiments. 
     The exemplary embodiments of the present invention have been described in detail, but the scope of the present invention is not limited thereto and various variants and modifications by a person skilled in the art using a basic concept of the present invention defined in claims also belong to the scope of the present invention.