Abstract:
A communication system, femto cell, and clustering and handover method of the same are provided. The method includes storing identity information and customer information for a plurality of femto base stations, generating, when location information of the corresponding femto base stations are received, at least one cluster including at least one of the femto base stations according to the customer information and location information, sharing the identity information of the corresponding femto base station in the cluster. The method is capable of supporting handover between femto base stations so as to secure reliable communication service connectivity for the terminal on the move.

Description:
PRIORITY 
     This application claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed on Jun. 13, 2011 in the Korean Intellectual Property Office and assigned Serial No. 10-2011-0056713, the entire disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a communication system and method. More particularly, the present invention relates to a communication system, femto cell, and clustering and handover method of the same. 
     2. Description of the Related Art 
     Typically, a mobile communication system is composed of a plurality of macro cells each of which is managed by a macro base station (e.g., evolved Node B (eNB)). The eNB provides the communication terminals (User Equipment (UE)) with communication service. In a communication system, the UE experiences change of signal environment in a specific cell, however the macro eNB serves the UE without consideration of the UE-specific signal environment. This is likely to cause communication service quality degradation and delay. 
     This problem can be mitigated by installing an intermediate device, such as for example, a repeater and a remote unit, between the macro eNB and UE. The macro eNB and intermediate device are installed by the service provider and result in an increase of service cost. Furthermore, the introduction of the extra device derives interoperability issue between the macro eNB and the intermediate device. 
     In order to solve these problems, it is proposed to deploy femto eNB(s) within the macro cell managed by the macro eNB. The femto eNB is an indoor base station installed at an area with weak macro eNB signal or shadow such as inside of a house or a building. Typically, the femto eNB is configured to provide the communication service to the UEs that have been registered with the communication system in advance. 
     This means that the registered UE is capable of being served by both the macro eNB and the femto eNB. In order to accomplish this, it is necessary for the communication system to support handover between macro and femto eNBs and between femto eNBs as well as between macro eNBs. Because the registered UE is capable of being served by both the macro eNB and the femto eNB, if a communication supported handovers between femto eNBs and between macro eNBs, it can be expected to improve the communication service quality in the communication system. 
     However, the communication system according to the related art does not support the inter-femto eNB handover of the UE. Accordingly, when the UE enters a weak signal area of the macro eNB or a shadow area, it is necessary to perform the handover from the femto eNB to the macro eNB and then the handover from the macro eNB to another femto eNB. This causes the degradation of seamless connectivity of the communication service to the UE. 
     There is therefore a need of a method for securing robust communication service connectivity of a UE in a communication system. 
     The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention are to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a method for securing robust communication service connectivity of a UE in a communication system. 
     In accordance with an aspect of the present invention, a method for clustering femto base stations in a communication system is provided. The method includes storing identity information and customer information for a plurality of femto base stations, generating, when location information of the corresponding femto base stations are received, at least one cluster including at least one of the femto base stations according to the customer information and location information, and sharing the identity information of the corresponding femto base station in the cluster. 
     In accordance with another aspect of the present invention, a method for a femto base station to support handover in a communication system is provided. The method includes determining, when a terminal connected to the femto base station requests for handover, whether the handover is directed to another femto base station in the same cluster, and performing, when the handover is directed to the another femto base station in the same cluster, the handover to the another femto base station. 
     In accordance with another aspect of the present invention, a communication system is provided. The system includes a plurality of femto base stations that transmit respective location information at power-on, and a femto manager which stores identity information for the femto base stations and customer information, generates, when receiving location information of the respective femto base stations, at least one cluster including a least one of the femto base station according to the corresponding customer information and location information, and shares the identity information of the corresponding femto base stations in the cluster. 
     In accordance with another aspect of the present invention, a handover execution apparatus of a femto base station in a communication system is provided. The apparatus includes a memory which stores identity information of another femto base station in a cluster to which the current femto base station belongs, a determination unit which determines, when a handover request is received from a terminal connected to the femto base station, whether the handover is directed to the another femto base station, and a controller which controls, when the handover is directed to the another femto base station, the handover to the another femto base station. 
     In accordance with another aspect of the present invention, a method for a femto base station to support handover in a communication system is provided. The method includes determining, when a terminal connected to the femto base station requests for handover, whether the handover is directed to another femto base station, and performing, when the handover is directed to the another femto base station, the handover to the another femto base station. 
     Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an exemplary environment of a communication system according to an exemplary embodiment of the present invention; 
         FIG. 2  is a diagram illustrating a configuration of a communication system for supporting an operation of femto eNBs according to an exemplary embodiment of the present invention; 
         FIG. 3  is a signaling diagram illustrating a clustering procedure of a communication system according to an exemplary embodiment of the present invention; 
         FIG. 4  is a diagram illustrating a configuration of clusters generated according to a clustering procedure according to an exemplary embodiment of the present invention such as, for example, the procedure of  FIG. 3 ; 
         FIG. 5  is a diagram illustrating a principle of sharing information on clusters generated according to a clustering procedure according to an exemplary embodiment of the present invention such as, for example, the procedure of  FIG. 3 ; 
         FIG. 6  is a diagram illustrating a frequency allocation in a communication system according to an exemplary embodiment of the present invention; 
         FIG. 7  is a signaling diagram illustrating a handover procedure between femto cells in a communication system according to an exemplary embodiment of the present invention; 
         FIG. 8  is a diagram illustrating a principle of an inter-femto eNB handover according to an exemplary embodiment of the present invention such as, for example, the handover procedure of  FIG. 7 ; 
         FIG. 9  is a block diagram illustrating a configuration of a femto manager of a communication system according to an exemplary embodiment of the present invention; 
         FIG. 10  is a flowchart illustrating an eNB clustering procedure of a femto manager in a communication system according to an exemplary embodiment of the present invention; 
         FIG. 11  is a block diagram illustrating a configuration of a femto eNB according to an exemplary embodiment of the present invention; 
         FIG. 12  is a flowchart illustrating a handover procedure of a femto eNB according to an exemplary embodiment of the present invention; 
         FIG. 13  is a diagram illustrating a format of a handover request message for use in a handover according to an exemplary embodiment of the present invention; 
         FIG. 14  is a diagram illustrating a format of a handover request message for use in a handover according to another exemplary embodiment of the present invention; 
         FIG. 15  is a block diagram illustrating a configuration of a femto switch according to an exemplary embodiment of the present invention; and 
         FIG. 16  is a flowchart illustrating a femto switch procedure for handover between femto eNBs according to an exemplary embodiment of the present invention. 
     
    
    
     Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, description of well-known functions and constructions may be omitted for clarity and conciseness. 
     The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention is provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
     It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
       FIG. 1  is a diagram illustrating an exemplary environment of a communication system according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , the communication system includes a plurality of macro cells. In the communication system, at least one of the macro cells includes at least one femto cell. The communication system includes UEs  10 , macro eNBs  20 , and femto eNBs  30 . 
     The UE  10  may roam across a macro cell boundary. The macro eNB  20  manages its macro cell and provides the UE with the communication service within the corresponding macro cell. The femto eNB  30  manages its femto cell deployed in the macro cell and provides the UE with the communication service within the femto cell. When entering a specific macro cell, the UE  10  connects to a the corresponding macro eNB  20 . Also, when the UE  10  moves to a femto cell within the macro cell, the UE  10  connects to the corresponding femto eNB  30  within the femto cell. As an example, the UE  10  might be pre-registered for receiving communication service from the femto eNB  30 . The UE  10  may be served by the macro eNB  20  or the femto eNB  30 . 
     The communication system according to an exemplary embodiment of the present invention supports handover of the UE  10  between macro eNBs  20 , between macro eNB  20  and femto eNB  30 , and between femto eNBs  30 . 
     Although the description is directed to a handover of the UE  10  between femto eNBs  30 , the present invention is not limited thereto. That is, the communication system according to exemplary embodiments of the present invention supports handover between the macro eNBs  20  and between the macro eNB  20  and femto eNB  30 , as well as between femto eNBs  30 . 
       FIG. 2  is a diagram illustrating a configuration of a communication system for supporting an operation of femto eNBs according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 2 , the communication system further includes a UE  10 , a femto eNB  30 , a security gateway  150 , a femto manager  160 , and a femto switch  170 . The UE  10  may perform radio access with the femto eNB  30 , and the femto eNB  30  connects to the security gateway through Internet  140  managed by an Internet Service Provider (ISP). 
     The security gateway  150  connects to the femto manager  160  and the femto switch  170  separately. The security gateway  150  relays the communication between the femto eNB  30  and the femto manager  160  and between the femto eNB  30  and the femto switch  170 . That is, the security gateway  150  provides the security service between the femto eNB  30  and the femto manager  160  and between the femto eNB  30  and the femto switch  170 . In order to accomplish this, the security gateway  150  may be provided with a firewall function and an Internet Protocol (IP) security function. 
     The femto eNB  160  manages the femto eNBs  30  remotely. That is, the femto manager  160  stores and provides MAC address, frequency allocation, Pseudo Noise offset, and authentication information, for auto self-installation and configuration of the femto eNB  30 . The femto manager  160  also stores the management information on the femto eNBs  30  according to an exemplary embodiment of the present invention. As an example, the management information includes a femto identifier, customer information, and location information. For example, the femto identifier corresponds to information for identifying the femto eNBs  30 , the customer information corresponds to customer identity information, and the location information corresponds to information indicating the location of the femto eNB. The femto manager  160  clusters the femto eNBs  30  based on the customer information and the location information so as to generate at least one cluster according to an exemplary embodiment of the present invention. The femto manager  160  controls the femto eNBs  30  such that the femto eNBs  30  share the identity information within the cluster according to an exemplary embodiment of the present invention. 
     The femto switch  170  connects to the core network  180 . The femto switch  170  supports packet communication between the femto eNB  30  and the core network  180 . The femto switch  170  also supports handover of the UE  10 . According to an exemplary embodiment of the present invention, the femto switch  170  supports the handover between macro eNBs  20 , between macro eNB  20  and femto eNB  30 , and between femto eNBs  30 . The femto switch  170  includes a voice switch  171  such as, for example, a Wireless Soft Switch (WSS), Wireless Gate-Way (WGW), and the like, and a data switch  173  such as, for example, a Packet Control Function (PCF) or the like. The core network  180  may be divided into a voice core network  181  and a data core network  183 . For example, the voice switch  171  is responsible for voice packet switching between the femto eNB  30  and the voice core network  181 , and the data switch  173  is responsible for data packet switching between the femto eNB  30  and the data core network  183 . 
       FIG. 3  is a signaling diagram illustrating a clustering procedure of a communication system according to an exemplary embodiment of the present invention.  FIG. 4  is a diagram illustrating a configuration of clusters generated according to a clustering procedure according to an exemplary embodiment of the present invention such as, for example, the procedure of  FIG. 3 .  FIG. 5  is a diagram illustrating a principle of sharing information on clusters generated according to a clustering procedure according to an exemplary embodiment of the present invention such as, for example, the procedure of  FIG. 3 . 
     Referring to  FIG. 3 , the clustering procedure of the communication system according to an exemplary embodiment of the present invention begins such that the femto manager  160  receives the femto eNB identity information and the customer information from the provider server  190  at step  211 . That is, the provider server  190  registers the identity information of the femto eNB  30  and the customer information in response to the user request through the femto eNB  30 . The owner of the femto eNB  30  can register the identity information of the femto eNB  30  and the user information with the femto eNB  30 . The provider server  190  sends the identity information of the femto eNB  30  and the customer information to the femto manager  160 . Afterward, the femto manager  160  saves the identity information on the femto eNB  30  and the customer information at step  213 . The femto eNB  160  may sort out a plurality of femto eNBs  30  by user to facilitate management. 
     If the femto eNB  30  powers on, the femto eNB  30  detects the power-on at step  215 . The femto eNB  30  sends its identity information and its location to the femto manager  160  at step  217 . As an example, the location information includes latitude information and longitude information. If the femto manager  160  receives the identity information and the location information of the femto eNB  30 , then at step  219 , the femto manager  160  saves the location information along with the customer information stored previously in association with the identity information of the femto eNB  30 . The femto manager  160  can manage the femto eNBs  30  with the management information thereof. 
     Next, the femto manager  160  clusters the femto eNBs  30  at step  221 . At this time, the femto manager  160  clusters the femto eNBs  30  based on the customer information and the location information per femto eNB  30 . The femto manager  160  may generate the cluster with at least one femto eNB  30  to have the same customer information. The femto manage  160  also may generate at least two clusters each having the femto eNBs  30  located within a range of a predetermined distance. The femto manager  160  also may generate the clusters such that each cluster has a predetermined number of femto eNBs  30 . In such a way, the femto manager  160  may generate a plurality of clusters (e.g. first cluster and second cluster as shown in  FIG. 4 ). The femto manager  160  also generates clustering information on each cluster. As an example, the clustering information includes the identifiers of the femto eNBs  30  belonging to each respective cluster and a pseudo noise code per femto eNB  30 . The femto eNBs  30  of each cluster are configured to use different pseudo noise codes. 
     Next, the femto manager  160  sends the clustering information to the femto eNBs  30  belonging to the respective clusters at step  223  (e.g., as illustrated in  FIG. 5 ). That is, the femto manager  160  controls such that the femto eNBs  30  belonged to the same cluster share the identity information. If the femto eNB  30  receives the clustering information from the femto manager  160 , the femto eNB  30  saves the clustering information at step  225 . That is, the femto eNB  30  checks the identity information and the pseudo noise codes of the femto eNBs  30  belonging to the same cluster based on the clustering information and saves the identity information and pseudo noise codes. 
       FIG. 6  is a diagram illustrating a frequency allocation in a communication system according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 6 , the macro eNBs  20  and femto eNBs  30  use the same Frequency Area (FA). For example, the macro eNB  20  uses four FAs, while the femto eNB 3   30  use the same FAs as their Beacon FAs. At this time, the femto eNBs  30  use the same pseudo noise code (e.g. 0), on the beacon FAs. Through this, it is possible to perform the handover from the macro eNB  20  to the femto eNB  30  (i.e. in-bound handover). When moving to the femto cell within the macro cell, the UE  10  may detects the pseudo noise code 0 in one of the beacon FAs. In this manner, it is possible to perform handover of the UE  10  from the macro eNB  20  to the femto eNB  30 . 
     Meanwhile, the femto eNBs  30  uses the FAs that are not used by the macro eNB  20  as their operation FAs according to an exemplary embodiment of the present invention. Here, the femto eNBs  30  may check the operation FA by referencing the message transmitted in the beacon FA. At this time, the femto eNBs  30  use different pseudo noise codes that are not used by the macro eNB  20  in correspondence to the operation FA. For example, one of the femto eNBs  30  uses a pseudo noise code of 110 corresponding to the operation FA while another of the femto eNBs  30  uses a pseudo noise code of 120. In this manner, it is possible to perform handover between femto eNBs  30  on the operation FAs. When moving between femto cells, the UE  10  may detect the pseudo noise code of 120 on the operation RA. Accordingly, it is possible to perform the handover of the UE  10  between the femto eNBs  30 . 
       FIG. 7  is a signaling diagram illustrating a handover procedure between femto cells in a communication system according to an exemplary embodiment of the present invention.  FIG. 8  is a diagram illustrating a principle of an inter-femto eNB handover according to an exemplary embodiment of the present invention such as, for example, the handover procedure of  FIG. 7 . 
     Referring to  FIGS. 7 and 8 , the handover procedure in the communication system according to an exemplary embodiment of the present invention begins such that the UE  10  requests a handover from the currently connected femto eNB  31  at step  311 . At this time, the UE  10  sends a pseudo noise code corresponding to another femto eNB  33  to the femto eNB  31  to request for the handover to the other femto eNB  33 . That is, when moving between the femto cells, the UE  10  may detect the pseudo noise code of 120 on the operation FA. The UE may send the pseudo noise code of 120 to the femto eNB  31 . 
     If the UE  10  requests handover, the femto eNB  31  requests the femto switch  170  for handover at step  313 . That is, the femto eNB  31  checks another femto eNB  33  according to the pseudo noise coded received from the UE  10  and requests the femto switch  170  for the handover to the femto eNB  33 . At this time the femto eNB  31  may check whether the femto eNB  33  corresponding to the pseudo noise code received from the UE  10  exists in the same cluster as the femto eNB  31 , based on the clustering information. If the femto eNB  33  exists, the femto eNB  31  acquires the identity information on the femto eNB  33  from the clustering information and sends the identity information to the femto switch  170 . That is, the femto eNB  31  may acquires the identity information of the femto eNB  33  (i.e., femto id#2), in correspondence with the pseudo noise code of 120. The femto eNB  31  may send the identity information (i.e., femto id#2) of the femto eNB  33  to the femto switch  170 . 
     If the handover request is received from the femto eNB  31 , the femto switch  170  requests the other femto eNB  33  for the handover at step  315 . That is, the femto switch  170  checks the femto eNB  33  according to the identity information received from the femto eNB  31  and requests the femto eNB  33  for handover. Upon receipt of the handover request from the femto switch  170 , the femto eNB  33  sends a handover response to the femto switch  170  at step  317 . At this time, the femto eNB  33  determines whether to accept or reject the handover request, and sends the determination result to the femto switch  170 . 
     If the handover response is received form the femto eNB  33 , the femto switch  170  forwards the handover response to the femto eNB  31  at step  319 . That is, the femto switch  170  sends the femto eNB  31  the handover response indicating whether the handover is accepted or rejected. Upon receipt of the handover response, the femto eNB  31  forwards the handover response to the UE  10  at step  321 . That is, the femto eNB  31  sends the UE  10  the handover response indicating whether the handover is accepted or rejected. If the femto eNB  33  has accepted the handover request, the femto eNB  31  instructs the UE  10  to perform handover. The handover of the UE  10  from the femto eNB  31  to the femto eNB  33  is performed in this way. 
       FIG. 9  is a block diagram illustrating a configuration of a femto manager of a communication system according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 9 , the femto manager  160  includes a management interface  161 , a management memory  163 , and a management controller  165 . 
     The management interface  161  is responsible for interfacing the femto manager  160  with another node. At this time, the management interface  161  is responsible for interfacing the management gateway  150  for communication with the femto eNB  30 . 
     According to exemplary embodiments of the present invention, the management memory  163  stores a MAC address, a frequency allocation, a pseudo noise offset, and authentication information for auto self-installation and configuration of the femto eNB  30 . The management memory  163  also stores the programs for clustering the femto eNBs  30  according to an exemplary embodiment of the present invention. The management memory  163  stores management information on the femto eNBs  30  according to an exemplary embodiment of the present invention. The management information includes the identity information, customer information, and the location information per femto cell. Here, the identity information corresponds to the information discriminating among the femto eNBs  30 , the customer information corresponds to the information indicating the user of the femto eNB  30 , and the location information corresponds to the information indicating the installation location of the femto eNB  30  (e.g., latitude and longitude information). 
     The management controller  165  is responsible for controlling overall operations of the femto manager  160 . For example, the management controller  165  clusters the femto eNBs  30  according to an exemplary embodiment of the present invention. In order to accomplish this, the management controller  165  is provided with a clustering unit  167 . The clustering unit  167  clusters the femto eNBs  30  according to the customer information and location information, and generates at least one cluster. The management controller  165  controls such that the femto eNBs  30  share the identity information in the cluster. 
       FIG. 10  is a flowchart illustrating an eNB clustering procedure of a femto manager in a communication system according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 10 , the clustering procedure of the femto manager  160  according to an exemplary embodiment of the present invention begins with the management controller  165  receiving the identity information of the femto eNB and the customer information from the provider server  190  at step  411 . The provider server  190  registers the identity information of the femto eNB  30  and the customer information according to the user request from the femto eNB  30 . When the user purchases a new femto eNB  30 , the provider server  190  may register the corresponding identity information of the femto eNB  30  and the customer information. The provider server sends the identity information of the femto eNB  30  and customer information to the femto manager  160 . Next, the management controller  165  saves the identity information of the femto eNB  30  and the customer information at step  413 . 
     If the identity information of the femto eNB  30  and the location information are received from the femto eNB  30 , the management controller  165  detects reception of the identity information and the location information at step  415 . The management controller  165  saves the location information along with the customer information in match with the identity information of the femto eNB  30  at step  417 . For example, the location information includes the latitude information and the longitude information of the installation location of the femto eNB  30 . That is, the management controller  165  configures and saves the management information of the femto eNB  30 . 
     Next, the management controller  165  determines whether there is the same customer information stored previously as the femto eNB  30  at step  419 . That is, the management controller  165  determines whether there exists a cluster created previously in association with the same customer information as the femto eNB  30 . 
     If it is determined at step  419  that there is the same customer information stored previously, the management controller  165  determines whether another femto eNB exists within a predetermined range around the femto eNB  30  at step  421 . That is, when there is the cluster created already in correspondence with the same customer information of the femto eNB  30 , the management controller  165  determines whether the distance between the femto eNB  30  and another femto eNB included in the previously created cluster is appropriate for supporting handover of the UE  10 . 
     If there is at least one other femto eNB  30  at step  421 , the management controller  165  determines whether the number of other femto eNBs found within the range around the femto eNB  30  is less than a predetermined value at step  423 . That is, the management controller  165  determines that the distance between the two femto eNBs are appropriate for supporting handover of the UE  10 . The management controller  165  determines whether the number of other femto eNBs belonging to the previously created cluster is less than a predetermined value. 
     If it is determined at step  423  that the number of other femto eNBs is less than a predetermined value, the management controller  165  adds the femto eNB  30  to the previously created cluster in association with the same customer information as the femto eNB  30  at step  425 . That is, so long as the number of other femto eNBs is not greater than a predetermined value, the management controller  165  adds the femto eNB  30  to the previously created cluster. 
     Otherwise, if it is determined at step  419  that there is no same customer information as the femto eNB  30 , the management controller  165  creates a new cluster corresponding to the customer information of the femto eNB  30  at step  427 . That is, when there is no cluster created previously in correspondence to the same customer information as the femto eNB  30 , the management controller  165  configures a new cluster with the femto eNB  30 . 
     If it is determined at step  421  that there is no other femto eNB within a predetermined range around the femto eNB  30 , the management controller  165  creates a new cluster corresponding to the customer information of the femto eNB  30 . That is, although there is any cluster created previously in association with the customer information of the femto eNB  30 , the management controller  165  generates a new cluster in separation from the previously created cluster. The management controller  165  judges that the distance between the current femto eNB and the other femto eNB is not appropriate for supporting handover of the UE  10  so as to configure a new cluster with the femto eNB  30 . 
     If it is determined at step  423  that the number of other femto eNBs within a predetermined range around the current femto eNB  30  is equal to or greater than the predetermined value, the management controller  165  creates a new cluster corresponding to the user information of the femto eNB  30  at step  427 . That is, the management controller  165  configures a new cluster with the femto eNB  30  such that the number of other femto eNBs of the previously generated cluster is maintained to be equal to or less than the predetermined value. 
     Finally, the management controller  165  generates and transmits the clustering information corresponding to the cluster at step  429 . If the femto eNB  30  is added to the previously generated cluster, the management controller  165  generates the clustering information associated with current femto eNB and the other femto eNBs corresponding to the previously created cluster. Here, the management controller  165  may generate the clustering information with the identity information of the current femto eNB  30  and other femto eNBs  30  and pseudo noise codes for use in the operation FAs of the current femto eNB and the other femto eNBs. The management controller  165  sends the clustering information to the current femto eNB and the other femto eNBs. If the new cluster is created with the femto eNB  30 , the management controller  165  may generate the clustering information with the identity information of the current eNB and the pseudo noise code for use in the operation FA of the current femto eNB  30 . The management controller  165  sends the clustering information to the femto eNB  30 . 
       FIG. 11  is a block diagram illustrating a configuration of a femto eNB according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 11 , the femto eNB  30  according to an exemplary embodiment of the present invention includes a radio communication unit  31 , a femto interface unit  33 , a femto memory  35 , and a femto controller  37 . 
     According to exemplary embodiments of the present invention, the radio communication unit  31  is responsible for the radio communication function of the femto eNB  10 . The radio communication unit  31  transmits signals to the UE  10  and receives signals transmitted to the UE  10 . The radio communication unit  31  may receive a request for handover to another femto eNB from the UE according to an exemplary embodiment of the present invention. 
     According to exemplary embodiments of the present invention, the femto interface unit  33  is responsible for interfacing the femto eNB with another node. At this time, the femto interface unit  330  interfaces with security gateway  150  for communication with the femto manager  160  or the femto switch  170 . For example, the femto interface unit  33  may receive the clustering information from the femto manager  160  according to an exemplary embodiment of the present invention. 
     According to exemplary embodiments of the present invention, the femto memory  35  stores the programs for executing hand over to another femto eNB  30  according to an exemplary embodiment of the present invention. The femto memory  35  stores the identity information and the location information on the femto eNB  30  according to an exemplary embodiment of the present invention. The femto memory  35  also stores the clustering information according to an exemplary embodiment of the present invention. As an example, the clustering information includes the identity information and pseudo noise code of another femto eNB belonging to the same cluster as the current femto eNB  30 . 
     According to exemplary embodiments of the present invention, the femto controller  37  is responsible for controlling overall operations of the femto eNB  30 . The femto controller  37  controls to store the clustering information received from the femto manager  160 . The femto controller  37  performs handover to another eNB  30  in response to a request from the UE  10  according to an exemplary embodiment of the present invention. In order to perform such a handover, the femto controller  37  includes a determination unit  39 . The determination unit  39  checks other femto eNBs  30  based on the pseudo noise code received from the UE. At this time, the determination unit  39  determines, based on the clustering information, whether there is any other femto eNB corresponding to the pseudo noise code received from the UE  10  within the same cluster as the current femto eNB  30 . If another femto eNB exists, the femto controller  37  requests the femto switch  170  for handover using the identity information of the other femto eNB according to an exemplary embodiment of the present invention. That is, the femto controller  37  acquires the identity information on the other femto eNB from the clustering information and sends the acquired identity information to the femto switch  170 . 
       FIG. 12  is a flowchart illustrating a handover procedure of a femto eNB according to an exemplary embodiment of the present invention.  FIG. 13  is a diagram illustrating a format of a handover request message for use in a handover according to an exemplary embodiment of the present invention.  FIG. 14  is a diagram illustrating a format of a handover request message for use in a handover according to another exemplary embodiment of the present invention. 
     Referring to  FIG. 12 , the handover procedure of the femto eNB  30  according to an exemplary embodiment of the present invention starts with the power-on of the femto eNB at step  511 . If the power-on of the femto eNB  30  is detected at step  511 , the femto controller  37  sends the femto manager  160  the identity information and the location information of the femto eNB  30  at step  513 . For example, the location information includes the latitude and longitude information of the installation location of the femto eNB  30 . The femto controller  37  monitors receipt of the clustering information at step  515 . If the clustering information is received, the femto controller  37  saves the clustering information at step  517 . That is, the femto eNB  30  checks the identity information and pseudo noise codes of other femto eNBs of the same cluster to which the current femto eNB  30  belongs from the clustering information and stores the identity information and pseudo noise codes. If clustering information is not received at step  515 , then the femto controller  37  continues monitoring for receipt of the clustering information. 
     Next, the femto controller  37  monitors receipt of handover request from the currently connected UE  10  at step  519 . If the handover request is received, the femto controller  37  determines whether the target eNB of the handover is another femto eNB at step  521 . For example, this can be determined based on the pseudo noise code received from the UE  10 . 
     If it is determined that the target eNB of the handover is not another femto eNB at step  521 , the femto controller  37  controls the handover to a macro eNB  20  at step  522 . That is, if it is determined that the UE  10  has requested for the handover to the macro eNB  20 , the femto controller  37  requests the femto switch  170  for the handover to the macro eNB  20 . At this time, the femto controller  37  may generate a handover request message formatted as shown in  FIG. 13  to the femto switch  170 . The femto controller  37  configures the handover request message with 24-bit Mobile Switching Center Identifier (MSCID), 12-bit cell information, and 4-bit sector information. The handover request message may also include cell identifier list information, length information, and cell identification discriminator information. If a handover response message is received from the femto switch  170  afterward, the femto controller  27  forwards the handover response message to the UE  10 . 
     If it is determined that the target eNB of the handover is another femto eNB at step  521 , the femto controller  37  determines whether the target femto eNB is included in the same cluster as the current femto eNB  30  at step  523 . At this time, the femto controller  37  may determines, based on the clustering information, whether another femto eNB corresponding to the pseudo noise code received from the UE  10  exists in the cluster to which the current femto eNB  30  belongs. If it is determined that the target femto eNB is included in the same cluster as the current eNB  30 , the femto controller  37  requests the femto switch  170  for the handover to the target femto eNB at step  525 . That is, if the handover target eNB exists in the same cluster, the femto controller  37  acquires the identity information on the target femto eNB from the clustering information and sends this information to the femto switch  170 . At this time, the femto controller  27  may send the femto switch  170  the handover request message formatted as shown in  FIG. 14 . Here, the femto controller  27  configures the handover request message has a size equal to that of the macro eNB  20  but a structure different from that of the macro eNB  20 . The handover request message includes 24-bit MSCID, 12-bit cell information, and 4-bit sector information. The femto controller  37  configures such that the two bits at the beginning of each of MSCID, cell information, and sector information are fixed as 0x while leaving the rest bits being variable. The handover request message may also include cell identifier list information, length information, and cell identification discriminator information. 
     The femto controller  37  monitors receipt of a handover response from the femto switch  170  at step  527 . If the handover response is received at step  527 , the femto control  37  forwards the handover response to the UE  10  at step  529 . That is, if the handover response message is received from the femto switch  170  in response to the handover request message, the femto controller  37  responds to the handover request of the UE  10 . At this time, the femto controller  37  generates the handover response by reflecting whether to accept or reject the handover request to the target femto eNB. If the target femto eNB has accepted the handover request, the femto controller  37  commands the UE  10  to execute the handover. In this procedure, the UE  10  can perform handover from one femto eNB to another femto eNB. If a handover response is not received at step  527 , then the process ends. 
     If no handover request is received at step  519 , the femto controller  27  determines whether there is a handover request from the femto switch  170  at step  531 . If the handover request is received from the femto switch  170 , the femto controller  37  responds to the handover request sent by the femto switch  170  at step  533 . At this time, if the handover is requested by the femto switch  170  to another femto eNB, the femto controller  37  determines whether to accept the handover of the UE  10  to the target femto eNB. Finally, the femto controller  37  responds to the handover request sent by the femto switch  170  by reflecting the determination result. If no handover request is received from the femto switch  170 , then the process ends. 
       FIG. 15  is a block diagram illustrating a configuration of a femto switch according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 15 , the femto switch  170  according to an exemplary embodiment of the present invention includes a switching interface  171 , a switching memory  173 , and a switching controller  175 . 
     According to exemplary embodiments of the present invention, the switching interface  171  is responsible for interfacing the femto switch  170  with another node. The switching interface  171  is responsible for interfacing with the security gateway  150  for communication with the femto eNB  30 . 
     According to exemplary embodiments of the present invention, the switching memory  173  stores the program for supporting handover between femto eNBs  30  according to an exemplary embodiment of the present invention. 
     According to exemplary embodiments of the present invention, the switching controller  175  is responsible for controlling overall operations of the femto switch  170 . The switching controller  175  supports handover between femto eNBs  30  and between femto eNB  30  and macro eNB  20 . In order to accomplish this, the switching controller  175  is provided with a determination unit  177 . The determination unit  177  determines whether to accept the handover between the femto eNBs  30  or between the femto eNB  30  and macro eNB  20 . 
       FIG. 16  is a flowchart illustrating a femto switch procedure for handover between femto eNBs according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 16 , the handover procedure of the femto switch  170  according to an exemplary embodiment of the present invention, the switching controller  175  starts with receipt of the handover request from the femto eNB  30  at step  611 . Upon receipt of the handover request, the switching controller  175  determines whether the target eNB indicated by handover request is another femto eNB at step  613 . As an example, at this time, the switching controller  175  may check the MSCID, cell information, and sector information carried in the handover request message to determine whether the target eNB is a femto eNB. 
     If it is determined that the target eNB is a macro eNB at step  613 , the switching controller  175  controls the handover to the macro eNB  20  at step  614 . If the handover request message is received in the format of  FIG. 13 , the switching controller  175  can control the handover from the femto eNB  30  to the macro eNB  20 . That is, the switching controller  175  requests for the handover to the macro eNB  20 . If a handover response is received from the macro eNB  20 , the switching controller  175  forwards the handover response to the femto eNB  30  in response to the handover request. 
     Otherwise, if it is determined that the target eNB is another femto eNB at step  613 , the switching controller  175  requests the target femto eNB  30  for the handover at step  615 . That is, the femto controller  175  checks the target femto eNB based on the identity information received from the current femto eNB  30  and requests the target femto eNB to prepare the handover. If the handover request message is received in the format of  FIG. 14 , the switching controller  175  can request for the handover from the current femto eNB  30  another femto eNB. The switching controller  175  monitors receipt of the handover response at step  617 . If the handover response is received, the switching controller  175  forwards the handover response to the current femto eNB  30  at step  619 . That is, the switching controller  175  responds to the handover request sent by the current femto eNB  30  by reflecting whether the target femto eNB has accepted the handover. At this time, the switching controller  175  may generate the handover response message to the current femto eNB  30  in response to the handover request message. 
     The system and method of the present invention is capable of supporting handover between femto eNBs as well as between macro eNBs, and between macro and femto eNBs, thereby securing reliable connectivity of the UE on the move while receiving the communication service. As a result, the system and method of the present invention is capable of improving communication service quality. 
     As described above, the communication system, femto eNB thereof, and clustering and handover method of the same according to the present invention are capable of supporting handover between femto eNBs as well as handover between macro eNBs and between macro and femto eNBs. Also, the present invention is capable of securing reliable connectivity of the UE on the move while receiving the communication service provided by the communication system. Furthermore, the present invention is capable of improving the communication service quality. 
     While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.