Abstract:
Disclosed are a method and a device for determining a cell identifier. A method for determining a cell identifier can comprise the steps of: a network control unit generating an Nth neighbor cell list which comprises information about a neighboring cell of a search cell; the network control unit receiving, from a search cell, information about a global cell identifier and a first PCI of an unknown cell; the network control unit determining whether or not the unknown cell is a moving cell on the basis of the global cell identifier; and, if the unknown cell is a moving cell and the first PCI and a second PCI of a neighbor cell that is comprised in the Nth neighbor cell list, the network control unit transmitting to the moving cell a PCI change signal that requests changing of the first PCI.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the National Stage filing under 35 U.S.C. 371 of International Application No. PCT/KR2014/008696, filed on Sep. 18, 2014, the contents of which are hereby incorporated by reference herein in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Field of the invention 
         [0003]    The present invention relates to wireless communications, and more particularly, to a method and a device for determining a cell identifier. 
       Related Art 
       [0004]    After a radio resource control (RRC) connection is established, a user equipment (UE) may receive a measurement report request from a connected base station. The UE may continuously report a measurement to the base station according to an indication in the measurement report request in an RRC-connected mode. 
         [0005]    The UE may report all detected physical cell identifiers (PCIs) according to a measurement report criterion indicated by the connected base station in the RRC-connected mode. A PCI is a cell identifier, which may be a reusable value assigned in a local area. There is a possibility that a specific cell and another cell have the same PCI. 
         [0006]    When a measurement report received from the UE includes identification information on an unknown cell, automatic neighbor relation (ANR) of the connected base station may start a procedure for identifying the unknown cell for a possible handover to the unknown cell. 
         [0007]    When the unknown cell is not registered as a neighbor cell in the connected base station, ANR of the connected base station may request the UE to detect the E-UTRAN cell global identifier (ECGI) of the unknown cell in order to identify the unknown cell. ECGI information is the unique identifier of a cell. A PCI value may be reusable, while ECGI is the unique value of each base station. 
         [0008]    The UE requested to detect the ECGI of the unknown cell may receive ECGI information transmitted from the unknown cell and may report the ECGI information to the connected base station. When the ECGI information on the unknown cell is reported, the connected base station may obtain the Internet Protocol (IP) address of a neighbor base station by the aid of a mobility management entity (MME) as one component of an evolved packet core (EPC). 
       SUMMARY OF THE INVENTION 
       [0009]    An object of the present invention is to provide a method for determining a cell identifier. 
         [0010]    Another object of the present invention is to provide a device for determining a cell identifier. To achieve the foregoing object of the present invention, an aspect of the present invention provides a method for determining a cell identifier, the method including: generating, by a network controller, an Nth neighbor cell list including information on a neighbor cell of a detecting cell; receiving, by the network controller, information on a first physical cell identifier (PCI) and a global cell identifier of an unknown cell from the detecting cell, the first PCI being identification information in a local area that is also available for a cell other than the unknown cell and the global cell identifier being unique identification information allocated only to the unknown cell; determining, by the network controller, whether the unknown cell is a moving cell based on the global cell identifier; and transmitting, by the network controller, a PCI change signal that requests a change of the first PCI to the moving cell when the unknown cell is the moving cell and the first PCI is the same as a second PCI of the neighbor cell included in the Nth neighbor cell list, wherein the neighbor cell is listed in the Nth neighbor cell list based on a possibility of interference by movement of the moving cell on the basis of the detecting cell. 
         [0011]    To achieve the foregoing object of the present invention, an aspect of the present invention provides a network controller for determining a cell identifier, the network controller including: a communication unit configured to transmit or receive cable and radio signals; and a processor operatively connected to the communication unit, wherein the processor is configured to: generate an Nth neighbor cell list including information on a neighbor cell of a detecting cell; receive information on a first physical cell identifier (PCI) and a global cell identifier of an unknown cell from the detecting cell, the first PCI being identification information in a local area that is also available for a cell other than the unknown cell and the global cell identifier being unique identification information allocated only to the unknown cell; determine whether the unknown cell is a moving cell based on the global cell identifier; and transmit a PCI change signal that requests a change of the first PCI to the moving cell when the unknown cell is the moving cell and the first PCI is the same as a second PCI of the neighbor cell included in the Nth neighbor cell list, and the neighbor cell is listed in the Nth neighbor cell list based on a possibility of interference by movement of the moving cell on the basis of the detecting cell. 
         [0012]    According to the present invention, a collision between the PCIs of cells due to a moving cell is predicted in advance, thereby preventing a PCI collision caused by a moving cell. Further, when the PCI of a moving cell is changed, battery consumption due to a handover of a UE served by the moving cell may be reduced and a delay caused by a link setup failure may be prevented. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a conceptual view illustrating a collision between the physical cell identifiers (PCIs) of cells due to the movement of a moving cell. 
           [0014]      FIG. 2  is a conceptual view illustrating a method of predicting a PCI collision caused by a moving cell according to an embodiment of the present invention. 
           [0015]      FIG. 3  is a conceptual view illustrating a method of generating an Nth neighbor cell list according to an embodiment of the present invention. 
           [0016]      FIG. 4  discloses an operation of a UE according to a change in the PCI of a moving cell according to an embodiment of the present invention. 
           [0017]      FIG. 5  is a flowchart illustrating a handover method according to an embodiment of the present invention. 
           [0018]      FIG. 6  is a conceptual view illustrating an operation of changing the PCI of a moving cell according to an embodiment of the present invention. 
           [0019]      FIG. 7  is a conceptual view illustrating an operation of changing the PCI of a moving cell according to an embodiment of the present invention. 
           [0020]      FIG. 8  is a block diagram illustrating a wireless communication system according to an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0021]      FIG. 1  is a conceptual view illustrating a collision between the physical cell identifiers (PCIs) of cells due to the movement of a moving cell. 
         [0022]    Hereinafter, in an embodiment of the present invention, a moving cell may refer to a moving base station, and a fixed cell may refer to a stationary base station. A moving cell may be replaced with a moving base station, and a fixed cell may be replaced with a stationary base station. 
         [0023]    A PCI may be used as an identifier of a cell. A PCI is a value allocated to a cell in a local area and may be a reusable value. Therefore, there is a possibility that the PCI of a specific cell is the same as the PCI of another cell. E-UTRAN cell global identifier (ECGI) information may be used as an identifier of a cell. ECGI information is the unique identifier of a specific cell. A PCI is a reusable value, while an ECGI is the unique value of each base station. ECGI may be referred to as a global cell ID. 
         [0024]    Referring to  FIG. 1 , three fixed cells  110 ,  120 , and  130  and one moving cell  100  may be located in local area  1 . The PCI of the one moving cell  100  may be 1, and the PCIs of the three fixed cells  110 ,  120 , and  130  may be 0, 2, and 3, respectively. As described above, a PCI is a reusable value assigned in a local area. Therefore, there is a possibility that the PCI of a specific cell is the same as the PCI of another cell. 
         [0025]    When the moving cell  100  moves from local area  1  to local area  2 , the PCI ( 1 ) of the moving cell  100  may be the same as the PCI ( 1 ) of a fixed cell  140  located in local area  2 . When a collision between the PCIs of cells occurs, synchronization signals transmitted from the respective cells may collide. Therefore, a user equipment (UE) may fail to receive a synchronization signal and may fail to connect to a network. 
         [0026]    It may be assumed that the PCI of the moving cell  100  is forcibly changed by a network controller in order to avoid a collision between the PCIs of the cells. In this case, when UEs served by the moving cell  100  are not handed over to the moving cell  100  corresponding to a newly changed PCI, the UEs may fail in link connection. In the case of a fixed cell, when a handover to a target cell fails, a UE may establish a connection back to the existing cell. However, in the case of the moving cell  100 , since the PCI is changed, a UE may not connect back to the existing cell. 
         [0027]    The current cellular network supports  503  PCIs from 0 to 502. Based on buses running in Seoul, a large number of moving cells  100 , for example, 2000 moving cells, may be present. Therefore, there is a high possibility of a collision between the moving cells  100  and fixed cells. 
         [0028]    Hereinafter, an embodiment of the present invention discloses a specific method for preventing a collision between cell PCIs due to a moving cell  100  and for handing over a UE served by the moving cell  100  when the PCI of the moving cell  100  is changed. 
         [0029]    According to the embodiment of the present invention, a network controller, such as a mobility management entity (MME), may predict a collision between cell PCIs and may change the PCI of a moving cell  100  if needed in order to prevent a PCI collision due to the movement of the moving cell  100 . 
         [0030]      FIG. 2  is a conceptual view illustrating a method of predicting a PCI collision caused by a moving cell according to an embodiment of the present invention. 
         [0031]      FIG. 2  discloses a method of predicting a PCI collision based on an Nth neighbor cell list. 
         [0032]    Referring to  FIG. 2 , a specific cell  200  may detect the movement of a moving cell. The cell that detects the movement of the moving cell is referred to as a detecting cell  200 . For example, a UE served by the detecting cell  200  may receive a synchronization signal or a cell detection signal transmitted by the moving cell  250  and may detect whether the moving cell  250  is a new cell. The UE may report, to the detecting cell  200 , identification information (for example, PCI and ECGI) on the moving cell  250  detected by the movement of the moving cell  250 . 
         [0033]    Specifically, the UE connected to the detecting cell  200  may report identification information on all cells detected according to a measurement report criterion indicated by the detecting cell (connected base station)  200  in an RRC connected mode. When a measurement report received from the UE includes an unknown cell, the detecting cell  200  may perform a procedure for identifying the unknown cell for a potential handover to the unknown cell. 
         [0034]    According to the embodiment of the present invention, when the detecting cell  200  identifies the moving cell  250 , the detecting cell  200  may generate an Nth neighbor cell list based on the detecting cell  200 . The Nth neighbor cell list may include information on a possibility of interference by the moving cell with neighbor cells. In the Nth neighbor cell list, a neighbor cell with a relatively higher possibility of interference by the moving cell may be allocated a lower order. 
         [0035]    A neighbor cell adjacent to the detecting cell  200  may be set as a first neighbor cell  120 . Next, a neighbor cell adjacent to the first neighbor cell  120  may be set as a second neighbor cell  220  and  225 . Likewise, a neighbor cell adjacent to the second neighbor cell  220  may be set as a third neighbor cell  230 . That is, the Nth neighbor cell list includes the first neighbor cell and the Nth neighbor cell (here, N is a natural number of 2 or greater), the first neighbor cell indicates a cell adjacent to the detecting cell, and the Nth neighbor cell may be a cell adjacent to an (N- 1 )th neighbor cell. A neighbor cell included in the Nth neighbor cell list may be the first neighbor cell or the Nth neighbor cell in the Nth neighbor cell list. 
         [0036]    In this manner, the Nth neighbor cell list, which includes at least one neighbor cell based on the detecting cell, may be generated. That is, a neighbor cell more adjacent to the detecting cell  200  is more likely to receive interference by the moving cell  250  and may have a relatively higher order in the Nth neighbor cell list. 
         [0037]    According to the embodiment of the present invention, a network controller may predict a possibility of a collision between the PCI of a neighbor cell (or neighbor fixed cell) and the PCI of the moving cell based on the Nth neighbor cell list generated by the detecting cell  200 . As a result of prediction, when there is a possibility of a collision between the PCIs of the neighbor cell and the moving cell, the network controller may request the moving cell  250  to change the PCI. 
         [0038]    Specifically, the network controller may generate an Nth neighbor cell list based on the detecting cell  200 . When the moving cell approaches the detecting cell  200 , the detecting cell  200  may transmit the identification information (for example, PCI and ECGI) on the moving cell  250  to the network controller (for example, an MME). The network controller may predict a possibility of a collision between the PCIs of the cells caused by the movement of the moving cell  250  based on the Nth neighbor cell list for the detecting cell  200  and the identification information on the moving cell  250 . For example, the network controller may determine whether the PCIs of the neighbor cells  210 ,  220 ,  225 , and  230  included in the Nth neighbor cell list for the detecting cell  200  are the same as the PCI of the moving cell  250 . 
         [0039]    If the same PCI as the PCI of the moving cell  250  is present among the PCIs of the neighbor cells  210 ,  220 ,  225 , and  230  included in the Nth neighbor cell list, the network controller may request the moving cell  250  to change the PCI. On the contrary, when the PCIs of neighbor cells  210 ,  220 ,  225 , and  230  included in the Nth neighbor cell list are not the same as the PCI of the moving cell  250 , the network controller may not request the moving cell  250  to change the PCI. 
         [0040]    The network controller may generate an Nth neighbor list for all controlled cells but may generate an Nth neighbor list only for a specific cell to decrease complexity. For example, a specific cell for which an Nth neighbor list is generated may be a cell located on the edge of the network controller (for example, MME edge). 
         [0041]    The network controller may determine a PCI change timing of the moving cell  250  in view of the distance between the moving cell and a neighbor cell having the same PCI as the PCI of the moving cell  250  based on location information on the moving cell  250  and location information on the neighbor cell. Although the PCIs of the neighbor cells included in the Nth neighbor cell list are the same as the PCI of the moving cell  250 , interference by a PCI collision between the neighbor cells and the moving cell may not occur according to a moving direction of the moving cell (or the distance between the neighbor cells and the moving cell). 
         [0042]    Therefore, even though the PCI of a neighbor cell included in the Nth neighbor cell list is the same as the PCI of the moving cell  250 , the network controller may request the moving cell  250  to change the PCI only when the neighbor cell having the same PCI as the moving cell  250  becomes close to the moving cell  250  by a certain threshold distance or shorter. 
         [0043]    According to another embodiment of the present invention, the network controller may change the PCI of the moving cell based on a map on a relationship between cells located within a certain area. For example, the network controller may generate an inter-cell relationship map including information on relationships between a plurality of cells managed by the network controller. The inter-cell relationship map may include information on relationships between a plurality of cells, such as information on the location of each of the plurality of cells, information on a cell adjacent to each of the plurality of cells, and information on the coverage of each of the plurality of cells. 
         [0044]    When the presence of the moving cell is reported to the network controller, the network controller may determine the necessity to change the PCI of the moving cell using the inter-cell relationship map. Likewise, the network controller may determine a PCI change timing of the moving cell in view of the current location of the moving cell and location information on a cell having the same PCI as the PCI of the moving cell. 
         [0045]    According to still another embodiment of the present invention, the network controller may utilize a local cell list, instead of creating a separate Nth neighbor cell list for the detecting cell. The local cell list may be a list including PCI information on all cells managed by the network controller. When the PIC of the moving cell is the same as the PCI of a specific cell included in the local cell list, the network controller may request the moving cell to change the PCI. 
         [0046]      FIG. 3  is a conceptual view illustrating a method of generating an Nth neighbor cell list according to an embodiment of the present invention. 
         [0047]      FIG. 3  discloses a method of generating an Nth neighbor cell list for a specific cell (for example, fixed cell  0 ) based on an inter-neighbor cell automatic detection process. 
         [0048]    Referring to  FIG. 3 , fixed cell  1  may transmit a synchronization signal or a cell detection signal (operation S 300 ). 
         [0049]    A UE served by fixed cell  0  may receive the synchronization signal or the cell detection signal from fixed cell  1  and may transmit identification information (for example, PCI) on fixed cell  1  to fixed cell  0  (operation S 305 ). 
         [0050]    When a UE detects a new neighbor cell, the UE may transmit identification information on the new neighbor cell to a serving cell. 
         [0051]    Fixed cell  0  may determine whether fixed cell  1  is an unknown cell based on the identification information on fixed cell  1  (operation S 310 ). 
         [0052]    When fixed cell  1  is an unknown cell, fixed cell  0  may request the UE to identify a network identifier (ID, for example, ECGI) of fixed cell  1  (operation S 315 ). 
         [0053]    The UE may receive system information on fixed cell  1  (operation S 320 ). 
         [0054]    The UE may receive the system information from fixed cell  1  through a data channel and a control channel, such as a physical downlink control channel (PDCCH) and a physical downlink shared channel (PDSCH) (operation S 325 ) 
         [0055]    The system information may include network identification information on fixed cell  1 . 
         [0056]    The UE may transmit the information on the network ID of fixed cell  1  to fixed cell  0  (operation S 330 ). 
         [0057]    Fixed cell  0  may update neighbor cell list  0  based on the information on the network ID of fixed cell  1  (operation S 335 ). 
         [0058]    Neighbor cell list  0  may be a neighbor cell list of fixed cell  0 . 
         [0059]    Fixed cell  0  may make a request to set up an X2 interface with fixed cell  1  based on the network ID of fixed cell  1  (operation S 340 ). 
         [0060]    Fixed cell  0  may request the network controller to set up the X2 interface. Further, fixed cell  0  may transmit the updated neighbor cell list to the network controller. 
         [0061]    The network controller may receive the neighbor cell list from fixed cell  0  (operation S 345 ). 
         [0062]    The network controller may request fixed cell  1  to set up the X2 interface with fixed cell  0 . 
         [0063]    Further, the network controller may transmit the neighbor cell list of fixed cell  0  to fixed cell  1  (operation S 350 ). 
         [0064]    Fixed cell  1  may update neighbor cell list  1  of fixed cell  1  based on received neighbor cell list  0  of fixed cell  0 . 
         [0065]    Fixed cell  1  may notify the network controller that the X2 interface has been set up (operation S 355 ). 
         [0066]    Further, fixed cell  1  may transmit neighbor cell list  1  of fixed cell  1  to the network controller. 
         [0067]    The network controller may receive neighbor cell list  1  from fixed cell  1  (operation S 360 ). 
         [0068]    The network controller may transmit an X2 interface setup completion message from fixed cell  0  (operation S 365 ). 
         [0069]    The network controller may generate an Nth neighbor cell list for fixed cell  0  based on neighbor cell list  0  of fixed cell  0  and neighbor cell list  1  of fixed cell  1  (operation S 370 ). 
         [0070]    The network controller may generate an Nth neighbor cell list including information on a possibility of interference by a moving cell with neighbor cells. 
         [0071]    For example, the network controller may generate the Nth neighbor cell list by determining the relative distance between neighbor cells based on fixed cell  0  in view of location information on neighbor cells included in each of the neighbor cell lists. The Nth neighbor cell list may be generated by determining the relative locations of neighbor cells radially located from a detecting cell and/or the coverages of the neighbor cells. 
         [0072]    Alternatively, the network controller may determine, as a first neighbor cell, at least one neighbor cell adjacent to the detecting cell based on whether cells are adjacent and may determine, as a second neighbor cell, at least one adjacent neighbor cell based on the first neighbor cell, thereby generating the Nth neighbor cell list. A plurality of neighbor cells with the same rank may be included in the Nth neighbor cell list. 
         [0073]    Alternatively, the network controller may generate the Nth neighbor cell list further considering the moving direction of the moving cell based on fixed cell  0 . When the moving directions of a plurality of moving cells are considered, an Nth neighbor cell list corresponding to the moving directions of the plurality of moving cells may be generated for one fixed cell. 
         [0074]      FIG. 4  discloses an operation of a UE according to a change in the PCI of a moving cell according to an embodiment of the present invention. 
         [0075]      FIG. 4  illustrates a problem that may occur to a UE when the PCI of a moving cell is changed without notice. 
         [0076]    Referring to  FIG. 4 , when the PCI of a moving cell is changed without notice, a UE served by the moving cell may fail in connection and may not be served by the moving cell for a long time. Specifically, it may be assumed that a network controller forcibly changes the PCI of the moving cell without notice in order to prevent a collision between the PCIs of cells. In this case, when a handover of the UE fails, a change in the PCI of the moving cell may not allow the UE to establish a connection back to the previous moving cell corresponding to the PCI before the change. 
         [0077]    A PCI before a change may be referred to as an old PCI, and a PCI after a change may be referred to as a new PCI. Further, a moving cell corresponding to a PCI before a change may be referred to as an old PCI moving cell, and the moving cell corresponding to a PCI after a change may be referred to as a new PCI moving cell. 
         [0078]    Referring to  FIG. 4 , when the UE does not recognize a PCI change of the moving cell, the UE may receive a synchronization signal (primary synchronization signal (PSS)/secondary synchronization signal (SSS)) and/or a reference signal (for example, a cell-specific reference signal (CRS)) generated based on the new PCI. Due to the PCI change, the UE is unable to detect the old PCI moving cell corresponding to the old PCI. Thus, the UE may attempt to reconnect to the old PCI moving cell during T RLF . Since the PCI of the moving cell is changed, the UE may recognize a radio link failure (RLF). The UE may attempt to connect to the new PCI moving cell after T RACH . The UE may be connected to the new PCI moving cell through an RACH. 
         [0079]    When the UE is unable to detect the PCI change of the moving cell, the UE may detect the new PCI moving cell through the foregoing procedure and may lose a connection to a link for a long time. 
         [0080]    Hereinafter, an embodiment of the present invention discloses a method of handing over a UE when the PCI of a moving cell is changed. 
         [0081]    According to the embodiment of the present invention, a handover of the UE may be induced before the PCI of the moving cell is changed. For example, the moving cell may first transmit a handover request message to a UE served by the moving cell. When the handover request message is received, the UE may be temporarily handed over to a cell other than the moving dell. Alternatively, the power of the moving cell may be gradually reduced for a moment to induce the UE served by the moving cell to be handed over to another cell. 
         [0082]    Subsequently, the moving cell may change the PCI and then may transmit a new PCI-based synchronization signal to the UE. The UE may receive the new PCI-based synchronization signal and may connect to the new PCI moving cell. 
         [0083]    According to another embodiment of the present invention, the moving cell may forcibly hand over the UE. The moving cell may command a handover of the UE before the UE requests a handover. In the embodiment of the present invention, a higher-layer signal for forcing a handover by the moving cell may be defined. The higher-layer signal for a moving cell-triggered handover may include information on a handover instruction, a new PCI value, a handover completion report, and a PCI change completion report. 
         [0084]    Hereinafter, a method of a handover forced by a moving cell is disclosed in detail. 
         [0085]      FIG. 5  is a flowchart illustrating a handover method according to an embodiment of the present invention. 
         [0086]    In  FIG. 5 , a moving cell corresponding to a PCI before a change may be referred to as an old PCI moving cell, and the moving cell corresponding to a PCI after a change may be referred to as a new PCI moving cell. 
         [0087]    Referring to  FIG. 5 , for a moving cell-triggered handover, an old PCI moving cell may receive a PCI change request message from a higher layer (operation S 500 ). 
         [0088]    The PCI change request message may be a message to request a change in the PCI of the moving cell. The PCI change request message may include information on a new PCI value. 
         [0089]    When the PCI change request message is received, the old PCI moving cell may transmit an RRC connection reconfiguration message to a UE (operation S 510 ). 
         [0090]    The RRC connection reconfiguration message may include information to indicate a change of the old PCI moving cell, which currently provides a service, to a new PCI moving cell and/or information on a new PCI. When the RRC connection reconfiguration message is received, the UE may perform a handover from the old PCI moving cell to the new PCI moving cell. 
         [0091]    When the RRC connection reconfiguration message is received, the UE may transmit an acknowledgement (ACK) to the moving cell in response to the RRC connection reconfiguration message (operation S 520 ). 
         [0092]    When the ACK is received, the old PCI moving cell changes an old PCI to the new PCI, thereby changing to the new PCI moving cell (operation S 530 ). 
         [0093]    The new PCI moving cell may transmit a new PCI confirmation request message to the UE (operation S 540 ). 
         [0094]    The new PCI confirmation request message may be a message for confirming whether the UE is handed over from the old PCI moving cell to the new PCI moving cell and for synchronizing the UE and the new PCI moving cell. 
         [0095]    When the new PCI confirmation request message is received, the UE may synchronize with the new PCI moving cell (operation S 550 ). 
         [0096]    Further, the UE may transmit a new PCI confirmation message for confirming that the UE has been handed over to the new PCI moving cell to the new PCI moving cell. 
         [0097]    The UE may transmit an RRC connection reconfiguration completion message to the new PCI moving cell (operation S 560 ). 
         [0098]    Further, the new moving cell may transmit a PCI change completion message indicating that a change from the old PCI to the new PCI is completed to a higher layer. 
         [0099]      FIG. 6  is a conceptual view illustrating an operation of changing the PCI of a moving cell according to an embodiment of the present invention. 
         [0100]      FIG. 6  discloses an operation of changing the PCI of a moving cell by a network controller when a fixed cell does not recognize whether an unknown cell is a fixed cell or a moving cell. 
         [0101]    Referring to  FIG. 6 , a moving cell may transmit a synchronization signal or a cell detection signal (operation S 600 ). 
         [0102]    A UE of a fixed cell may receive the synchronization signal or the cell detection signal from the moving cell and may detect that the moving cell is a new neighbor cell (operation S 605 ). 
         [0103]    The UE may transmit identification information (for example, PCI) on the moving cell to the fixed cell. 
         [0104]    The fixed cell may determine whether the moving cell is an unknown cell based on the received identification information on the moving cell (operation S 610 ). 
         [0105]    When the moving cell is an unknown cell, the fixed cell may request the UE to identify a network ID (for example, ECGI) of the moving cell (operation S 615 ). 
         [0106]    The UE may receive system information on the moving cell (operation S 620 ). 
         [0107]    The moving cell may transmit the system information including ECGI information to the UE (operation S 625 ). 
         [0108]    The UE may receive the system information through a control channel and a data channel of the moving cell (operation S 630 ). 
         [0109]    The system information may include information on the network ID of the moving cell. The UE may transmit the information on the network ID of the moving cell to the fixed cell. 
         [0110]    The fixed cell may update a neighbor cell list based on the received information on the network ID of the moving cell (operation S 635 ). 
         [0111]    The fixed cell may request a network controller to set up an X2 interface with the moving cell (operation S 640 ). 
         [0112]    Further, the fixed cell may transmit the updated neighbor cell list to the network controller. 
         [0113]    The network controller may receive ID information a new cell (that is, ID information on the moving cell) from the fixed cell (operation S 645 ). 
         [0114]    The network controller may transmit an X2 interface setup request message to the moving cell based on the received ID information the new cell (that is, ID information on the moving cell). 
         [0115]    Further, the network controller may transmit the neighbor cell list of the fixed cell to the moving cell (operation S 650 ). 
         [0116]    The network controller may recognize the approach of the moving cell based on the identification information on the moving cell included in the updated neighbor cell list (operation S 655 ). 
         [0117]    For example, when new ECGI information included in the neighbor cell list is not registered in the network controller, the network controller may recognize the approach of the moving cell to the fixed cell. 
         [0118]    The network controller may detect an N neighbor cell list of the fixed cell and may determine whether the PCI of a specific neighbor cell included in the Nth neighbor cell list is the same as the PCI of the moving cell (operation S 660 ). 
         [0119]    When the PCI of the specific neighbor cell included in the Nth neighbor cell list is the same as the PCI of the moving cell, the network controller may request the moving cell to change the PCI through a higher layer (operation S 665 ). 
         [0120]    The moving cell may receive information on a new PCI from the network controller (operation S 670 ). The moving cell requested to change the PCI may forcibly hand over the UE using the handover method of the UE illustrated in  FIG. 5  and may change the PCI (operation S 675 ). 
         [0121]    When the PCI has been changed, the moving cell may transmit a PCI change completion message, which indicates that the PCI has been changed to the new PCI, to the network controller (operation S 680 ). 
         [0122]      FIG. 7  is a conceptual view illustrating an operation of changing the PCI of a moving cell according to an embodiment of the present invention. 
         [0123]      FIG. 7  discloses an operation of changing the PCI of a moving cell by a network controller when a fixed cell recognizes whether an unknown cell is a fixed cell or a moving cell. 
         [0124]    Referring to  FIG. 7 , a moving cell may transmit a synchronization signal or a cell detection signal (operation S 700 ). 
         [0125]    A UE of a fixed cell may receive the synchronization signal or the cell detection signal from the moving cell and may detect that the moving cell is a new neighbor cell (operation S 705 ). 
         [0126]    The UE of the fixed cell may transmit identification information (for example, PCI) on the moving cell to the fixed cell. 
         [0127]    When the fixed cell knows the identification information on the moving cell, the fixed cell may recognize the approach of the moving cell based on the received identification information on the moving cell (operation S 710 ). 
         [0128]    For example, the fixed cell may already have information on ID information (for example, PCI and ECGI) on the moving cell that is likely to approach the fixed cell. When a detected cell is the moving cell, the fixed cell may not update a neighbor cell list. 
         [0129]    When the approach of the moving cell is recognized, the fixed cell may request a network controller to set up an X2 interface with the moving cell (operation S 715 ). 
         [0130]    Further, the fixed cell may transmit the identification information on the moving cell that approaches the network controller to the network controller. 
         [0131]    Further, the network controller may receive the ID information on the moving cell from the fixed cell (operation S 725 ). 
         [0132]    The network controller may transmit an X2 interface setup request message to the moving cell. 
         [0133]    Also, the network controller may transmit the neighbor cell list of the fixed cell to the moving cell (operation S 730 ). 
         [0134]    The network controller may receive the identification information on the approaching moving cell and may determine whether the PCI of a neighbor cell included in an Nth neighbor cell list is the same as the PCI of the moving cell (operation S 735 ). 
         [0135]    When the PCI of the specific neighbor cell included in the Nth neighbor cell list is the same as the PCI of the moving cell, the network controller may request the moving cell to change the PCI through a higher layer (operation S 740 ). 
         [0136]    The moving cell may receive information on a new PCI (operation S 745 ). The moving cell requested to change the PCI may forcibly hand over the UE using the foregoing handover method of the UE and may change the PCI (operation S 750 ). 
         [0137]    When the PCI has been changed, the moving cell may transmit a PCI change completion message, which indicates that the PCI has been changed to the new PCI, to the network controller (operation S 755 ). 
         [0138]      FIG. 8  is a block diagram illustrating a wireless communication system according to an embodiment of the present invention. 
         [0139]    Referring to  FIG. 8 , a base station  800  includes a processor  810 , a memory  820 , and a radio frequency (RF) unit  830 . The memory  820  is connected to the processor  810  and stores various pieces of information to operate the processor  810 . The RF  820  is connected to the processor  810  and transmits and/or receives a radio signal. The processor  810  implements proposed functions, processs and/or methods. The operations of the base station illustrated in the above embodiments may be implemented by the processor  810 . 
         [0140]    For example, the processor  810  may be configured to receive a PCI change signal from a network controller and to change a PCI based on the PCI change signal. Further, before a PCI is changed, the processor  810  may transmit an RRC reconfiguration signal to a UE to transmit information on a PCI to be changed. 
         [0141]    A wireless device  850  includes a processor  860 , a memory  870 , and an RF unit  880 . The memory  870  is connected to the processor  860  and stores various pieces of information to operate the processor  860 . The RF  880  is connected to the processor  860  and transmits and/or receives a radio signal. The processor  860  implements proposed functions, processs and/or methods. The operations of the wireless device illustrated in the above embodiments may be implemented by the processor  860 . 
         [0142]    For example, the processor  860  may receive an RRC reconfiguration signal from a base station and may perform a handover to a cell corresponding to a PCI indicated based on the RRC reconfiguration signal. 
         [0143]    The network controller according to the embodiment of the present invention may include a communication unit configured to transmit or receive cable and radio signals and a processor operatively connected to the communication unit. 
         [0144]    The processor of the network controller may be configured to generate an Nth neighbor cell list including information on a neighbor cell of a detecting cell and to receive information on a first PCI and a global cell ID of an unknown cell from the detectingcell. Here, the first PCI may be identification information in a local area that is also available for a cell other than the unknown cell, and the global cell identifier may be unique identification information allocated only to the unknown cell. Further, the processor of the network controller may be configured to determine whether the unknown cell is a moving cell based on the global cell identifier and to transmit a PCI change signal that requests a change of the first PCI to the moving cell when the unknown cell is the moving cell and the first PCI is the same as a second PCI of the neighbor cell included in the Nth neighbor cell list. 
         [0145]    The neighbor cell may be listed in the Nth neighbor cell list based on a possibility of interference by movement of the moving cell on the basis of the detecting cell. 
         [0146]    The processors may include Application-specific Integrated Circuits (ASICs), other chipsets, logic circuits, and/or data processors. The memories may include Read-Only Memory (ROM), Random Access Memory (RAM), flash memory, memory cards, storage media, and/or other storage devices. The RF units may include a baseband circuit for processing a radio signal. When the above-described embodiments are implemented in software, the above-described schemes may be implemented using a module (process or function) which performs the above function. The module may be stored in the memories and executed by the processors. The memories may be disposed to the processors internally or externally and connected to the processors using a variety of well-known means. 
         [0147]    In the above exemplary systems, although the methods have been described on the basis of the flowcharts using a series of the steps or blocks, the present invention is not limited to the sequence of the steps, and some of the steps may be performed at different sequences from the remaining steps or may be performed simultaneously with the remaining steps. Furthermore, those skilled in the art will understand that the steps shown in the flowcharts are not exclusive and may include other steps or one or more steps of the flowcharts may be deleted without affecting the scope of the present invention.