Patent Publication Number: US-9907000-B2

Title: Method and apparatus for identifying microcells in macrocells in wireless communication systems, and handover method and system using same

Description:
PRIORITY 
     This application is a National Phase Entry of PCT International Application No. PCT/KR2013/001048, which was filed on Feb. 8, 2013, and claims a priority to Korean Patent Application No. 10-2012-0013937, which was filed on Feb. 10, 2012, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a method and apparatus for identifying microcells in a wireless communication system, and handover method and system using the same. 
     BACKGROUND ART 
     Wireless networks have been developed into a form of a heterogeneous network (HetNet) to improve general wireless network performance and meet user demands, as well as to efficiently correspond to a relatively higher user&#39;s traffic demand in a particular area. The HetNet may be understood as a wireless network in a form where a macrocell and microcells like pico cells, femto cells, Closed Subscriber Group (CSG) cells, etc., are overlapped. For example, the CGS cell refers to a cell that provides a service only to a subscriber group authorized to use the service. 
     The HetNet environment is affected largely by a huge difference in transmit power between the macrocell and the microcell and interference due to the CSG cell. In the case of the CSG cell among the microcells, only authorized users are serviced and thus the CSG cell interferes with the user of the macrocell. In the case of the pico cells and femto cells among the microcells, the macrocell having relatively large transmit power affects the user of the microcell. Such an effect of interference makes it difficult to determine an optimal time for handover between the macrocell and the microcell. 
     In the following specification, explanation will be focused on interference between a macrocell and a CSG cell. Since the number of Physical Cell IDs (PCIs) for identification of the CSG cells is limited, PCI confusion may occur between the CSG cells. 
     Specifically, in the case of CSG cells, a PCI region only for the CSG cells is set and CSG cells located within an area of a single macrocell may use the same PCI. In this case, during handover from the macrocell to a CSG cell, PCI overlapping may occur, in which case a target cell may not be determined only with a PCI of the CSG cell. The PCI overlapping becomes a factor that makes it difficult to correctly identify a CSG cell during handover from the macrocell to the CSG cell. 
     With rapid advancements in communication services, it is expected that (e.g., thousands to tens of thousands) CSG cells and microcells greater in number than CSG cells (having the maximum number of about 500) distinguishable only with the PCI operate in the service area of a macrocell. 
     However, it is difficult to check proximity to a microcell, such as a CSG cell or check separation from a microcell in a macrocell, and to correctly identify a microcell, and thus a scheme to address the difficulty is required. 
     DISCLOSURE 
     Technical Problem 
     The present disclosure provides a method and apparatus for easily identifying a microcell within a macrocell in a wireless communication system. 
     The present disclosure also provides a method for identifying a Closed Subscriber Group (CSG) cell, by which proximity to or separation from the CSG cell may be easily determined. 
     The present disclosure also provides a method and system for handing over to a CSG cell in a wireless communication system. 
     Technical Solution 
     In accordance with an aspect of the present disclosure, a method for identifying a microcell within a macrocell in a wireless communication system is provided. The method includes a User Equipment (UE) obtaining a physical identity of the microcell by performing cell search on an adjoining microcell; and the UE identifying the microcell by using the physical identity and a particular pattern of a subframe sent by the microcell. 
     In accordance with another aspect of the present disclosure, a User Equipment (UE) for identifying a microcell within a macrocell in a wireless communication system is provided. The UE includes a transceiver for transmitting and receiving wireless signals; and a controller for obtaining a physical identity of the microcell by performing cell search on an adjoining microcell; and identifying the microcell by using the physical identity and a particular pattern of a subframe sent by the microcell. 
     In accordance with another aspect of the present disclosure, a handover method from a macrocell to a microcell in a wireless communication system is provided. The method includes a User Equipment (UE) obtaining a physical identity of the microcell by performing cell search on an adjoining microcell; the UE identifying the microcell by using the physical identity and a particular pattern of a subframe sent by the microcell; a source eNB of the macrocell receiving system information resulting from identification of the microcell from the UE; and a target eNB of the microcell responding to a handover request from the source eNB and the source eNB sending a handover command to the UE. 
     In accordance with another aspect of the present disclosure, a wireless communication system that supports handover from a macrocell to a microcell is provided. The wireless communication system includes a User Equipment (UE) for obtaining a physical identity of an adjoining microcell by performing cell search on the microcell; and identifying the microcell by using the physical identity and a particular pattern of a subframe sent by the microcell; a source eNB of the macrocell for receiving system information resulting from identification of the microcell from the UE and send a handover command to the UE according to a handover procedure; and a target eNB of the microcell for responding to a handover request from the source eNB. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a flow chart illustrating a method for identifying a Closed Subscriber Group (CSG) cell in a wireless communication system of a related art; 
         FIG. 2  is a diagram for explaining a method for determining whether a User Equipment (UE) approaches a CSG cell, according to an embodiment of the present disclosure; 
         FIG. 3  is a diagram for explaining a method for identifying a CSG cell, according to an embodiment of the present disclosure; 
         FIG. 4  is a flow chart illustrating a method for identifying a CSG cell for handover, according to an embodiment of the present disclosure; and 
         FIG. 5  is a flow chart illustrating a handover method employing the method for identifying a CSG cell, according to an embodiment of the present disclosure. 
     
    
    
     BEST MODE 
     Embodiments of the present disclosure will be described in conjunction with accompanying drawings. In the description of the present disclosure, if it is determined that a detailed description of commonly-used technologies or structures related to the disclosure may unnecessarily obscure the subject matter of the invention, the detailed description will be omitted. 
     While the embodiments of the present disclosure will be described assuming Closed Subscriber Group cells among microcells for convenience, it should be noted that the present disclosure may be applied to identification for many different kinds of microcells that may affect the user of a macrocell in terms of interference and handover procedures between the macrocell and the microcell. To facilitate understanding of the following embodiments of the present disclosure, an existing method for identifying a CSG cell will be described first and then followed by description of a method for identifying a CSG cell in accordance with the embodiments of the present disclosure. 
       FIG. 1  is a flow chart illustrating a method for identifying a CSG cell in a wireless communication system of a related art, which illustrates a process in which a User Equipment (UE)  110  identifies a CSG cell to hand over from a source eNB  130  to a target eNB  150 . 
     In  FIG. 1 , assuming that the source eNB  130  is an eNB of a macrocell while the target eNB  150  is an eNB of a CSG cell, and although not shown, there are many CSG cells within the macrocell. 
     In operation  101 , the UE  110  receives a proximity configuration message from the source eNB  130 , the message instructing proximity indication. In operation  103 , the UE  110  performs autonomous search proposed in an existing standard to indicate proximity to a CSG cell. The autonomous search is to notify a network that the UE  110  has entered a CSG cell on a white list when the entrance has occurred, which is described in 3GPP TS 36.300 v10.3.0. 
     However, considering e.g., the limited number of Physical Cell IDs (PCIs), the existing standards have not yet proposed any specific and efficient scheme for the autonomous search. 
     If there is no measurement configuration for measuring a frequency/wireless access technology of the CSG cell, the UE  110  receives a measurement configuration message from the source eNB  130  about cell search for the frequency/wireless access technology used by an adjoining CSG cell, in operation  105 . In operation  107 , the UE  110  performs cell search for the frequency/wireless access technology and obtains measurements as a result of the cell search including a PCI of the adjoining CSG cell. A measurement report including the PCI is sent to the source eNB  130 . In operation  109 , the source eNB  130  sends the UE  110  a request for System Information (SI) regarding the CSG cell of the PCI for handover, and in operation  111 , the UE  110  performs measurement using an autonomous gap to obtain the SI. 
     With the autonomous gap, the UE  110  obtains the SI on a broadcasting channel (BCCH) transmitted by the target eNB  150  of the CSG cell to be handed over. The SI includes different kinds of information for handover, such as an ID of a CSG cell to be handed over (CSG ID), a Cell Group ID (CGI), a Tracking Area Identification (TAT), etc. The autonomous gap is described in the 3GPP TS 36.300 v10.3.0. 
     A procedure of the autonomous gap proposed by the existing standard may cause significant time delay during handover, which negatively affects normal operations of the UE  110 . 
     Given that the method for identifying a CSG cell, which is proposed by the related art as shown in  FIG. 1 , does not suggest any specific and efficient scheme for proximity indication with the autonomous search and causes time delay in obtaining the SI using the autonomous gap to identify a CSG cell, the embodiments of the present disclosure proposes an improved method for identifying a CSG cell that may easily perform the proximity indication and easily obtain SI, such as e.g., a CSG ID without using the autonomous gap procedure. 
     In the embodiments of the present disclosure, a UE performs proximity indication by using a particular pattern of a subframe used by each CSG cell. The particular pattern of a subframe indicates, for example, a transmission interval (location) in which Almost Blank Subframe (ABS) is sent from a CSG cell for signal quality measurement of a serving cell. The particular pattern of a subframe may use, for example, an ABS pattern. Specifically, since a CSG cell causes interference with the user of the macrocell, the CSG cell sends a particular subframe indicated by the ABS pattern, for example, in the almost blank form, such as a null signal. Hereinafter, the subframe of a CSG cell sent in the almost blank form is referred to as the ABS. 
     Referring to existing standards 3GPP TS 36.331 v10.1.0; TS 36.331 v10.2.0, the ABS pattern is defined as “measSubframePatternServ”, “measSubframePatternPCell”, which is used for signal quality measurement of a serving cell (or primary cell). Based on a bit value indicated by the ABS pattern, signal quality of a serving cell, i.e., a macrocell, is measured. The ABS pattern is also defined as a serving cell measurement restriction pattern in the existing standard. 
     The ABS pattern is a term defined in the position of a cell that causes interference (e.g., a CSG cell) while the serving cell measurement restriction pattern is a term defined in the position of a cell that is interfered (e.g., a macrocell). Hereinafter, the two terms will be collectively called an ‘ABS pattern’. 
     In the existing standard, the ABS pattern is defined to be given to a UE through “measSubframePattern-Serv”, “measSubframePatternPCell” of RRC signaling, i.e., from a macrocell, which is a serving cell, and it is defined to use one ABS pattern. 
     However, in the embodiments of the present disclosure, a new method for identifying a CSG cell, by which the number of ABS patterns is extended to the multiple number and CSG cells are identified by mapping a combination of the ABS pattern and a PCI of a CSG cell to the CSG cell. Hence each CSG cell is mapped with a pair of (PCI, ABS pattern) in the embodiments of the present disclosure. On the other hand, transmission interval of the ABS among multiple subframes sent by a CSG cell is a blank interval that appears as if signals of the CSG cell are not be sent. Thus, the UE may better receive the signal of the macrocell without being interfered by the CSG cell in the ABS transmission interval. 
     The reception quality of the subframe sent by the macrocell in the ABS transmission interval is better than that of the subframe sent by the macrocell in typical subframe transmission intervals, because it is barely interfered by the CSG cell. If the ABS pattern is used to measure the reception quality of the macrocell in a situation where the UE connected to a macrocell approaches a CSG cell in the macrocell, the reception quality of the macrocell is better measured and the UE may maintain good quality of a service provided by the macrocell without being affected by interference from the CSG cell. 
     A method for a CSG cell to measure reception quality of a macrocell with ABS patterns will now be described in detail in accordance with embodiments of the present disclosure. 
     Assume that measurement of reception quality of the macrocell is performed on a subframe basis and different ABS patterns are used by multiple CSG cells. The UE measures reception quality of a subframe sent by the macrocell in a transmission interval in which the CSG cell sends the ABS, according to the ABS pattern. At the same time when the UE measures reception quality of the macrocell according to the ABS pattern, the UE also measures reception quality of the macrocell according to a reference pattern (hereinafter, referred to as ‘pattern 0’) used by the macrocell. 
     That is, in the embodiment of the present disclosure, reception quality measurement of the macrocell is performed using both the ABS pattern of the CSG cell and the pattern 0 of the macrocell. The ABS pattern and pattern 0 are each comprised of a bitstream of “0s” and “1s”, and the bitsreams are different because the ABS pattern and pattern 0 are different from each other. 
     Each bit of the bitstream of the ABS pattern or the pattern 0 corresponds to a single subframe. For example, a bit value of “1” in the bitstream indicates that reception quality of the corresponding subframe of the macrocell is measured while a bit value of “0” indicates that reception quality of the corresponding subframe of the macrocell is not measured. 
     In the embodiment of the present disclosure, average reception quality of a macrocell, i.e., a serving cell, measured according to the ABS pattern is compared with average reception quality of the serving cell measured according to the pattern 0, and it is determined that the UE is adjacent to a CSG cell that uses an ABS pattern having the greatest difference in the reception quality, i.e., having the best reception quality of the serving cell. Furthermore, it is also possible to use accumulated reception quality instead of the average reception quality in measuring reception quality of the serving cell. 
     Specifically, the average reception quality of the serving cell, E n [M], may be measured using the following equations 1 and 2. 
     
       
         
           
             
               
                 
                   
                     
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     Attributes in the equations 1 and 2 are defined as follows: 
     M i : reception quality in an i th  subframe according to a measurement subframe pattern (i.e., ABS pattern having e.g., 40 bits in case of FDD) 
     P n : n th  ABS pattern of multiple ABS patterns. 
     P n (i): bit corresponding to i th  subframe in the n th  ABS pattern, P n    
     S n [M]: accumulated reception quality of a serving cell measured according to an ABS pattern P n    
     E n [M]: average reception quality of a serving cell measured according to an ABS pattern P n    
     If a bit value of P n (i), a corresponding bit in the bitstreram, is “1”, the UE measures reception quality of the serving cell in the i th  subframe; or if the bit value is “0”, the UE does not measure reception quality of the serving cell in the i th  subframe. 
     Equation 2 is defined as accumulated reception quality S n [M] of the serving cell measured according to the nth ABS pattern being divided by a sum of counts of “1” of a bit value corresponding to the subframe indicated by the nth ABS pattern, i.e., the number of subframes on which measurement is performed, which means average reception quality of a serving cell per subframe according to the n th  ABS pattern. 
     Provided that average reception quality of a serving cell measured according to the aforementioned method without taking into account any ABS pattern (i.e., with the pattern 0) is E 0 [M], the UE determines that it is adjacent to a CSG cell that uses an ABS pattern having the greatest difference between the average reception quality E N [M] of the serving cell measured according to the ABS pattern and average reception quality E 0 [M] of the serving cell measured according to the reference pattern, which is the pattern 0. 
     
       
         
           
             
               
                 
                   
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     In the embodiment of the present disclosure, the pattern 0 may be created in various methods, such as, for example, a method for creating it to be different from the ABS pattern so that a target subframe of the serving cell for measurement is different from that according to the ABS pattern, a method for randomly selecting a target subframe of the serving cell for measurement, or a method for selecting all the subframes of the serving cell to be target subframes for measurement. 
     The pattern 0 may have a different form than multiple ABS patterns allocated to multiple CSG cells. For example, among N patterns having the same hamming distance, N−1 patterns may be used as ABS patterns while the remaining one may be used as the pattern 0. In another example, among N patterns having no correlation or weak correlations, N−1 patterns may be used as ABS patterns while the remaining one may be used as the pattern 0. 
     With the multiple ABS patterns, proximity indication and CSG cell identification may be performed simultaneously. Specifically, with combinations of the PCIs and ABS patterns in the CSG cell identification, the number of identifiers available for CSG cell identification may be steeply increased, and the CSG cell identification is possible only by utilizing both of the results of measurement for a macrocell which is the serving cell or of routine neighbor cell measurement. 
     For convenience, assuming a network situation where there are two CSG cells within a macrocell, the method for identifying a CSG cell in accordance with an embodiment of the present disclosure will now be described in more detail. 
     Assume an example where a UE uses three serving cell measurement restriction patterns as represented in Table 1 in a network where the macrocell is a serving cell and includes two CSG cells. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Pattern Type 
                 Bitstream of Pattern 
                 Target Cell 
               
               
                   
                   
               
             
            
               
                   
                 Pattern 0 
                 100100100 . . . 
                 Macrocell 
               
               
                   
                 Pattern 1 
                 010010010 . . . 
                 First CSG Cell 
               
               
                   
                 Pattern 2 
                 001001001 . . . 
                 Second CSG Cell 
               
               
                   
                   
               
            
           
         
       
     
     In Table 1, pattern 0 refers to a reference pattern for the macrocell; pattern 1 refers to an ABS pattern used by the first CSG cell; and pattern 2 is an ABS pattern used by the second CSG cell. The three different patterns illustrated in Table 1 are patterns all used in reception quality measurement of a serving cell, and respective bit values of bitstream of each pattern indicate whether to perform measurement in the corresponding subframes of the macrocell. In each pattern, if the bit value is “1”, reception quality of a corresponding subframe sent by the macrocell is measured; and if the bit value is “0”, reception quality of the corresponding subframe is not measured. 
     Furthermore, each CSG cell sends the ABS if the bit value of a corresponding ABS pattern is “1”, and sends a typical subframe including e.g., transmit data of the CSG cell if the bit value is “0”. 
       FIG. 2  is a diagram for explaining a method for determining whether a UE approaches a CSG cell, according to an embodiment of the present disclosure, which facilitates multiple CSG cells to perform proximity indication and identification of CSG cells with different ABS patterns. 
     In  FIG. 2 , a UE  210  is connected to a macrocell in connection state and only a part of a service area  230   a  of an eNB  230  of the macrocell is illustrated for convenience of explanation. In the service area  230   a  of the macrocell, there are eNBs  250  and  270  of first and second CSG cells, respectively, and service areas  250   a  and  270   a  of the first and second CSG cells overlap the service area  230   a  of the macrocell. Thus, typical subframes sent by the first and second CSG cells interfere with the user of the macrocell. 
     In the network condition of  FIG. 2 , it is assumed that the UE  210  has moved from point A less interfered by the first and second CSG cells to point B adjacent to the first CSG cell in the macrocell. At point A, subframes C 1  of the first CSG cell barely interfere with subframes M 1  of the macrocell. However, at point B, since the UE  210  approaches the first CSG cell, subframes C 2  of the first CSG cell causes relatively large interference to subframes M 2  of the macrocell that the UE  210  receives. 
     In  FIG. 2 , the subframes C 2  sent by the first CSG cell includes typical subframes S 1  that interfere with the macrocell and ABSs B 1  based on an ABS pattern. The UE  210  may receive better quality of signals sent by the macrocell in the transmission intervals of the ABSs B 1 . 
     When the UE  210  operates in the aforementioned condition, results of measuring reception quality of the macrocell with the serving cell measurement restriction pattern shown in Table 1 at points A and B are represented in the following Table 2. Here, point A is far distant from the first and second CSG cells, where there is almost no interference by the two CSG cells. Point B is adjacent to the first CSG cell and far from the second CSG cell, and is thus strongly interfered by the first CSG cell. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 A 
                 B 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Pattern 0 
                 Good 
                 Bad 
               
               
                   
                 Pattern 1 
                 Good 
                 Good 
               
               
                   
                 Pattern 2 
                 Good 
                 Bad 
               
               
                   
                   
               
            
           
         
       
     
     The UE  210  may determine whether it approaches a particular CSG cell by comparing results of measuring reception quality of the macrocell according to respective measurement restriction patterns (patterns 1 and 2 in Table 1) corresponding to ABS patterns allocated differently to respective CSG cells and a result of measuring reception quality of the macrocell according to the reference measurement restriction pattern (pattern 0 in Table 1). According to the embodiment of the present disclosure, proximity indication and identification of CSG cells may be performed by performing routine measurement on a serving cell. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 A 
                 B 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Pattern 1-0 
                 small 
                 Big 
               
               
                   
                 Pattern 2-0 
                 small 
                 small 
               
               
                   
                   
               
            
           
         
       
     
     For example, in Table 3, pattern 1-0 indicates a difference between results of reception quality measurement of the macrocell measured using equation 3 for pattern 1 and pattern 0 of Table 1 at points A and B, respectively. Since the difference is small at point A of “pattern 1-0”, the UE  210  determines that it is not in a state of approaching the first CSG cell. On the other hand, since the difference is big at point B of “pattern 1-0”, the UE  210  determines that it is in a state of approaching the first CSG cell. 
     In Table 3, pattern 2-0 indicates a difference between results of reception quality measurement of the macrocell measured using equation 3 for pattern 2 and pattern 0 of Table 1 at points A and B, respectively. Since the difference is small at both of points A and B of “pattern 2-0”, the UE  210  determines that it is not in a state of approaching the second CSG cell. 
     Accordingly, the UE  210  may measure reception quality of the macrocell with ABS patterns differently allocated to multiple CSG cells in the aforementioned method, and use the measurement results to easily perform proximity indication that indicates whether the UE approaches the CSG cell. 
     Various modifications of the embodiment of the present disclosure are possible by applying the embodiment to a Long Term Evolution (LTE) network under a HetNet environment that uses multiple CSG cells. 
     (1) Method for Identifying a CSG Cell with an ABS Pattern
         allocating different ABS patterns to CSG cells;   allocating different pairs of (PCI, ABS pattern) to CSG cells, where CSG cells allocated different PCIs may be allocated the same ABS pattern;   using a pair of (PCI, ABS pattern) to identify a CSG cell.       

     (2) Method for Obtaining a CSG ID After Identification of a CSG Cell
         configuring an ABS pattern to include a CSG ID (having e.g., 27 bits);   storing mapping information between the ABS pattern and the CSG ID in the eNB, and obtaining a CSG ID by informing the eNB of an ABS pattern selected by the UE; or   providing mapping information between the ABS pattern and the CSG ID to the UE in advance, and identifying a CSG ID mapped to an ABS pattern selected by the UE.       

     (3) Method for Measuring Reception Quality of a Macrocell on a Subframe Basis
         measuring reception quality of a data signal included in a subframe;   measuring reception quality of a Reference Signal (RS) included in a subframe. In case of measuring the reception quality of the RS, measurement is performed by separating a case where the arrangement of the RS in the subframe is identical between the macrocell and the CSG cell from a case where the arrangement is different between them.       

     (4) Method for Proximity Indication and CSG Cell Identification
         according to reception quality of a macrocell on a subframe basis, determining to be “1” if reception quality is good and to be “0” if it is not good, comparing them with ABS pattern of a CSG cell, and determining that it approaches a corresponding CSG cell if the bitstreams match;   determining whether it approaches a CSG cell by measuring reception quality of the macrocell according to ABS patterns, accumulating or averaging the reception quality, and comparing the accumulated reception quality or average reception quality with a predetermined reference value;   comparing average reception quality or accumulated reception quality measured according to ABS patterns with average reception quality or accumulated reception quality measured according to a reference pattern of the macrocell, determining to be far distant from a CSG cell of a corresponding ABS pattern if the difference is small and determining to be adjacent to a CSG cell of a corresponding ABS pattern if the difference is big.       

     Furthermore, in a case the embodiment of the present disclosure is applied to the LTE network after e.g., 3GPP release 10, the following embodiment is proposed. 
     However, the following embodiment is not necessarily limited to the LTE network after 3GPP release 10 but may also be applied to various wireless communication networks where there are multiple CSG cells within a macrocell. 
     (1) UE performs cell search on an adjoining cell (i.e., CSG cell) and obtains a PCI of the CSG cell through the cell search. 
     (2) a CSG cell is identified using a pair of (PCI, ABS pattern) allocated to the CSG cell. Since a combination of a PCI and an ABS pattern is used in identification of a CSG cell, the number of PCIs and ABS patterns required for the CSG cell identification may be reduced. 
     Information about the pair of (PCI, ABS pattern) may be provided by at least one of the CSG cell and macrocell to the UE. 
     In the embodiment, different PCIs and identical ABS pattern are allocated to adjoining CSG cells. This is to distinguish sets (groups) of CSG cells according to ABS patterns. 
     Furthermore, the current 3GPP RRC (release 10, March, 2011) standard regulates that only one serving cell measurement restriction pattern be used, but in the embodiment of the present disclosure, multiple serving cell measurement restriction patterns are used. Accordingly, a single pattern 0 and multiple ABS patterns are used to measure reception quality of a serving cell (macrocell). In addition, accessible CSG cells may be identified from a white list of CSG cells, and a method for sending only a corresponding measurement restriction pattern may reduce the RRC signaling overhead. 
       FIG. 3  is a diagram for explaining a method for identifying a CSG cell, according to an embodiment of the present disclosure, which indicates an example of a method for identifying a CSG cell, by which a pair of (PCI, ABS pattern) is allocated to each CSG cell. 
     Referring to  FIG. 3 , for convenience of explanation, a part of service area  300   a  of an eNB  300  of a macrocell is shown. In the service area  300   a  of the macrocell, adjoining CSG cells that use the same ABS pattern are divided into two groups. Assuming that the two CSG cell groups are a first CSG cell group  310  and a second CSG cell group  330 , first adjoining CSG cells included in the first CSG cell group  310  use the same ABS pattern but are allocated different PCIs. Second adjoining CSG cells included in the second CSG cell group  330  use the same ABS pattern, which is different from the ABS pattern used by the first CSG cells, but are allocated different PCIs. Furthermore, in  FIG. 3 , reference numerals  301  and  303  indicate an example where CSG cells of the first and second CSG cell groups  310  and  330  send typical subframes S 1  and ABSs B 1  according to different ABS patterns. 
     Although it is assumed in  FIG. 3  that PCIs allocated to the first CSG cell group  310  are reallocated to the second CSG cell group  330  in the same way, it is possible that PCIS are differently allocated to the two groups  310  and  330 . In the example of  FIG. 3 , PCI confusion occurs because the same PCI is used between the two groups  310  and  330 , but there is no problem with the PCI confusion in identifying CSG cells because a CSG cell is identified using a pair of (PCI, ABS pattern). 
       FIG. 4  is a flow chart illustrating a method for identifying CSG cells for handover, according to an embodiment of the present disclosure, which represents a procedure of identifying an adjoining CSG cell to hand over to a target eNB (not shown) of the adjoining CSG cell from a source eNB  430  of a macrocell in the LTE network. 
     In operation  401 , a UE  410  receives a proximity configuration message from the source eNB  430 , the message instructing proximity indication. In operation  403 , the UE  410  performs proximity indication that indicates whether it approaches an adjoining CSG cell by measuring reception quality of the macrocell with ABS patterns differently allocated to multiple CSG cells in accordance with the embodiment of  FIG. 2 , and sends the proximity indication information to a source eNB  430 . 
     In operation  405 , the UE  410  receives a measurement configuration message instructing the UE  410  to perform cell search on adjoining CSG cells from the source eNB  430 . In operation  407 , the UE  410  performs cell search on at least one adjoining CSG cell and accordingly obtains PCI of the at least one adjoining CSG cell. A measurement report including the PCI is sent to the source eNB  430 . The UE  410  may also perform reception quality measurement on the macrocell with ABS patterns of CSG cells, including the adjoining CSG cells, during the cell search. (Such reception quality measurement using the at least one adjoining CSG cell may be selectively performed, and during the measurement, the UE  410  may measure reception quality of the macrocell e.g., in typical subframes S 1  exclusive of blank subframes B 1  or the UE  410  may measure reception quality of the macrocell for the entire subframes including the blank subframes B 1  and typical subframes S 1 .) 
     Subsequently, in operation  409 , the source eNB  430  requests the UE  410  to obtain SI for handover, and in operation  411 , the UE  410  identifies ID of an adjoining CSG cell (CSG ID) through the method for identifying a CSG cell with the ABS pattern. That is, the UE  410  may identify a CSG ID of an adjoining CSG cell to be handed over with a pair of (PCI, ABS pattern) allocated to each CSG cell. For example, the CSG ID may be identified by including the CSG ID in the ABS pattern or sending information about the pair of (PCI, ABS pattern) to the source eNB  430 . In another embodiment, it is also possible for the UE  410  to provide only the information about the pair of (PCI, ABS pattern) to the source eNB  430  without directly identifying the CSG ID. 
     Furthermore, in operation  413 , the UE  410  sends the SI obtained in operation  411  to the source eNB  430 . The SI includes at least one of CSG ID of a CSG cell to be handed over, CGI, and Tracking Area ID (TAI). In addition, if the SI sent by the UE  410  to the source eNB  430  includes only a CSG ID or information about a pair of (PCI, ABS pattern) (or the index information), the source eNB  430  may obtain corresponding SI (CGI, TAI, etc.) of a CSG cell corresponding to the CSG ID or the information about the pair of the (PCI, ABS pattern) by referring to a pre-stored database or over a network. 
     On the other hand, once the UE  410  has ever been in a CSG cell on the white list, a corresponding CGI, CSG ID, etc. of the CSG cell are already stored and thus the UE  410  may obtain the SI in a simpler procedure when the UE  410  revisits the CSG cell. The SI may also be informed to the UE together with a resource restriction pattern by extending content of the existing RRC signaling message. 
     According to the method of the present disclosure, operation  111  of  FIG. 1  may be omitted. For example, handover delay may be drastically reduced by omitting the process of obtaining SI with the autonomous gap from the existing procedure to hand over to a CSG cell from a macrocell in RRC connection (RRC Connected) mode. 
     The following matters may be additionally taken into account in the cell identification method of  FIG. 4 . 
     In Discontinuous Receive (DRX) mode, if ON interval is not long enough for reception quality to be sufficiently measured, the UE may decide to use the existing procedure as shown in  FIG. 1  instead of the method shown in  FIG. 4 . If the number of pairs of (PCI, ABS pattern) is not enough and thus there occurs overlapping of the pairs of (PCI, ABS pattern), the UE may decide to use the existing procedure of  FIG. 1 . Furthermore, it is also possible to apply the method of  FIG. 1  only if a particular ABS pattern is determined among the multiple ABS patterns, allowing the overlapping of pairs of (PCI, ABS pattern) having the particular ABS pattern and “identifying that it is a corresponding ABS pattern allowed to overlap”, and to apply the method of  FIG. 4  otherwise. 
       FIG. 5  is a flow chart illustrating a handover method employing the method for identifying a CSG cell, according to an embodiment of the present disclosure. In  FIG. 5 , UE  510  and source eNB  530  perform the same operations as corresponding elements of  FIG. 4 ; Mobility Management Entity (MME)  550  is a network entity that manages mobility of the UE  510 , such as handover; gateway  570  is a gateway of a home network that supports handover between the source eNB  530  of a macrocell and a target eNB  590  of a CSG cell; and the target eNB  590  is an eNB of the CSG cell to be handed over by the UE  510 . 
     Description of operations  501  to  513  will be omitted herein because the operations  501  to  513  correspond to operations  401  to  413  described in  FIG. 4  in that SI for handover is sent to a source eNB  530  in accordance with the method for identifying a CSG cell as described in  FIG. 4 . Operations  515  to  531  of  FIG. 5  represent a procedure of handover from a source eNB  530  of a macrocell to a target eNB  590  of a CSG cell, which may use a procedure known to the LIE network. 
     In  FIG. 5 , the source eNB  530  that received the SI of operation  513  sends an MME  550  a message notifying that the UE  530  is in need of handover to a CSG cell, in operation  515 . The message that notifies the need for handover includes a CSG ID of the CSG cell. The MME  550  performs access control for handover to the CSG cell based on the reported CSG ID, in operation  517 , and sends a handover request message including the CSG ID to the gateway  570  of the home network to which the CSG cell belong, in operation  519 . The target eNB  590  that received the handover request message from the gateway  570  in operation  521  compares the CSG ID received through the handover request message with its own CSG ID, and sends the handover request confirmation message to the gateway  570  in operation  525  if the CSG IDs are the same. In operation  527 , the MME  550  that received the handover request confirmation message from the gateway  570  in operation  527  sends the UE&#39;s  510  handover command message to the source eNB  530  of the macrocell in operation  529 , and the source eNB  530  forwards the handover command message to the UE  510  for the UE  510  to complete a procedure of handover to the CSG cell in operation  531 . 
     Although not shown in the embodiments of the present disclosure, UE and eNBs that perform CSG cell identification and handover-related operations of  FIGS. 2 to 5  may be implemented with transceivers for transmit and receive wireless signals and controllers for controlling overall operations of  FIGS. 2 to 5 .