Patent Publication Number: US-9426704-B2

Title: Apparatus and method for managing an access mode of a node B in a wireless communication system

Description:
PRIORITY 
     The present application is a Continuation of U.S. Ser. No. 12/797,271, which was filed in the U.S. Patent and Trademark Office on Jun. 9, 2010, and claims priority under 35 U.S.C. §119(a) to Korean Patent Application Serial No. 10-2009-0051131, which was filed in the Korean Intellectual Property Office on Jun. 9, 2009, the entire disclosure of each of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an apparatus and a method for managing an access mode of a Node B (NB) in a wireless communication system. More particularly, the present invention relates to an apparatus and a method for determining and changing an access mode of an NB, depending on different factors in a wireless communication system. 
     2. Description of the Related Art 
     In a conventional cellular communication system, an NB, which is connected to a mobile communication core network, provides communication services to User Equipments (UEs) that located within a cell area of the NB. 
     When an amount of communication in a cell area of an NB is expected to increase temporarily, for example, during a large gathering such as a party or concert, a load of the cell is also expected to increase temporarily, which may affect the quality of the communication services provided by the NB. When this occurs, the load of the cell may be distributed by temporarily adding a mobile NB, e.g., an NB mounted on a vehicle, to the area where the amount of communication is expected to be large. 
     Additionally, in a conventional cellular communication system, a channel state often deteriorates due to a geographical condition inside a cell, an increased distance between a UE and an NB, or movement of a UE. For example, a shadowing area may be formed inside a building, such as an office or a house, which is located inside a coverage area of an NB. If a UE is located in the shadowing area, an NB may not be able perform sufficient communication because a channel state associated with the UE is poor. 
     In order to improve communication in areas with poor channel conditions, a femto-cell service may be utilized. A femto cell is a small cell area formed by a compact NB that is commonly installed inside an office or a house and that accesses a mobile communication core network via a broadband network. The compact NB is a low power NB, which is often installed by a user, not the service provider and the compact NB may also be referred to as a micro NB, a self configurable NB, an indoor NB, a home NB, or a femto NB. In the following description, the compact NB is referred to as a femto NB. 
     There are three access modes of a femto NB that are currently under discussion in the current 3 rd  Generation Partnership Project Long Term Evolution (3GPP LTE), which are defined as follows. 
     1. Open Access Mode: a mode that grants access to all users that are not registered in an NB. 
     2. Closed Access Mode: a mode that grants access only to users that are registered in an NB. 
     3. Hybrid Access Mode: a mode that that grants access to a user that is registered in an NB, and also grants an access to users that are not registered in the NB. 
     The above-defined various access modes of the femto NB may also be applied to a mobile NB and a general macro NB. 
     Currently, when an NB is installed, e.g., a femto NB, a mobile NB, or a general macro NB, an NB access mode is permanently set. Accordingly, in an environment where neighbor NBs use the same frequency, when a UE that receives a service from an NB that utilizes an open access mode enters a coverage of an NB that utilizes a closed access mode, a serious interference factor is generated between the neighbor NBs that use the different access modes, which deteriorates communication quality. 
     SUMMARY OF THE INVENTION 
     The present invention is designed to address at least the above-mentioned problems and/or disadvantages occurring in the prior art and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide an apparatus and a method for managing an access mode of an NB in a wireless communication system. 
     Another aspect of the present invention is to provide an apparatus and a method for solving an interference problem that occurs between neighbor NBs using different access modes, but the same frequency, and providing a load balancing effect in a specific area or when an event occurs by determining and changing an access mode of an NB depending on different factors in a wireless communication system. 
     In accordance with an aspect of the present invention, a method is provided for changing an access mode of a second node B (NB), by a core network (CN), in a wireless communication system. The method includes receiving, from a first NB, a request to change the access mode of the second NB from a first mode into a second mode; determining, by the CN, to change the access mode of the second NB into the second mode; transmitting, to the second NB, a request message requesting to change the access mode of the second NB; and receiving, from the second NB, a response message informing of an access mode control result. If the first mode allows access only to users or terminals having a registration in the second NB, and the second mode allows access to users or terminals, regardless of the registration in the second NB, the second NB further grants access to at least one user or terminal not registered in the second NB, after changing the access mode, and if the first mode allows access to the users or the terminals, regardless of the registration in the second NB, and the second mode allows access only to the users or the terminals having the registration in the second NB, the second NB controls at least one user or terminal not registered in the second NB to handover to a neighbor NB. 
     In accordance with another aspect of the present invention, a method is provided for changing an access mode by a node B (NB) of a wireless communication system. The method includes receiving, from a core network or another NB, a request message requesting to change the access mode of the NB from a first mode into a second mode; determining to change the access mode; transmitting, to the core network or the another NB, a response message informing of an access mode control result; if the first mode allows access only to users or terminals having a registration in the NB, and the second mode allows access to users or terminals, regardless of the registration in the NB, further granting access to at least one user or terminal not registered in the NB, after changing the access mode; and if the first mode allows access to the users or the terminals, regardless of the registration in the NB, and the second mode allows access only to the users or the terminals having the registration in the NB, controlling at least one user or terminal not registered in the NB to handover to a neighbor NB. 
     In accordance with another aspect of the present invention, a method is provided for changing an access mode by a node B (NB) in a wireless communication system. The method includes determining to change the access mode of the NB from a first mode into a second mode; transmitting, to a core network (CN), a request message requesting to change the access mode of the NB; receiving, from the CN, a response message informing of an access mode control result to change the access mode; if the first mode allows access only to users or terminals having a registration in the NB, and the second mode allows access to users or terminals, regardless of the registration in the NB, further granting access to at least one user or terminal not registered in the NB, after changing the access mode; and if the first mode allows access to the users or the terminals, regardless of the registration in the NB, and the second mode allows access only to the users or the terminals having the registration in the NB, controlling at least one user or terminal not registered in the NB to handover to a neighbor NB. 
     In accordance with another aspect of the present invention, a first node B (NB) of a wireless communication system is provided, which changes an access mode of a second NB. The first NB includes a controller for determining to change the access mode of the second NB from a first mode into a second mode; a transmitter for transmitting, to the second NB, a request message requesting to change the access mode of the second NB; and a receiver for receiving, from the second NB, a response message informing of an access mode control result. If the first mode allows access only to users or terminals having a registration in the second NB, and the second mode allows access to users or terminals, regardless of the registration in the second NB, the second NB further grants access to at least one user or terminal not registered in the second NB, after changing the access mode, and if the first mode allows access to the users or the terminals, regardless of the registration in the second NB, and the second mode allows access only to the users or the terminals having the registration in the second NB, the second NB controls at least one user or terminal not registered in the second NB to handover to a neighbor NB. 
     In accordance with another aspect of the present invention, a node B (NB) of a wireless communication system is provided, which changes an access mode of the NB. The NB includes a controller for determining to change the access mode of the NB from a first mode into a second mode; a transmitter for transmitting, to a core network (CN), a request message requesting to change the access mode of the NB; and a receiver for receiving, from the CN, a response message informing of an access mode control result to change the access mode. If the first mode allows access only to users or terminals having a registration in the NB, and the second mode allows access to users or terminals, regardless of the registration in the NB, further grants access to at least one user or terminal not registered in the NB, after changing the access mode, and the controller, if the first mode allows access to the users or the terminals, regardless of the registration in the NB, and the second mode allows access only to the users or the terminals having the registration in the NB, controls at least one user or terminal not registered in the NB to handover to a neighbor NB. 
     In accordance with another aspect of the present invention, a node B (NB) of a wireless communication system is provided, which changes an access mode of the NB. The NB includes a receiver for receiving, from a core network (CN) or another NB, a request message requesting to change the access mode of the NB from a first mode into a second mode; a controller for determining whether to change the access mode; and a transmitter for transmitting, to the CN or the another NB, a response message informing of an access mode control result to change the access mode. The controller, if the first mode allows access only to users or terminals having a registration in the NB, and the second mode allows access to users or terminals, regardless of the registration in the NB, further grants access to at least one user or terminal not registered in the NB, after changing the access mode, and the controller, if the first mode allows access to the users or the terminals, regardless of the registration in the NB, and the second mode allows access only to the users or the terminals having the registration in the NB, controls at least one user or terminal not registered in the NB to handover to a neighbor NB. 
     In accordance with another aspect of the present invention, a method is provided for changing an access mode of a second node B (NB), by a first NB, of a wireless communication system. The method includes determining to change the access mode of the second NB from a first mode into a second mode; transmitting, to a core network (CN), a request message requesting to change the access mode of the second NB; and receiving, from the CN, a response message informing of an access mode control result to change the access mode. If the first mode allows access only to users or terminals having a registration in the second NB, and the second mode allows access to users or terminals, regardless of the registration in the second NB, the second NB further grants access to at least one user or terminal not registered in the second NB, after changing the access mode, and if the first mode allows access to the users or the terminals, regardless of the registration in the second NB, and the second mode allows access only to the users or the terminals having the registration in the second NB, the second NB controls at least one user or terminal not registered in the second NB to handover to a neighbor NB. 
     In accordance with another aspect of the present invention, a method is provided for changing an access mode of a second node B (NB), by a core network (CN), of a wireless communication system. The method includes determining to change the access mode of the second NB from a first mode into a second mode; transmitting, to the second NB, a request message requesting to change the access mode of the second NB; and receiving, from the second NB, a response message informing of an access mode control result to change the access mode. If the first mode allows access only to users or terminals having a registration in the second NB, and the second mode allows access to users or terminals, regardless of the registration in the second NB, the second NB further grants access to at least one user or terminal not registered in the second NB, after changing the access mode, and if the first mode allows access to the users or the terminals, regardless of the registration in the second NB, and the second mode allows access only to the users or the terminals having the registration in the second NB, the second NB controls at least one user or terminal not registered in the second NB to handover to a neighbor NB. 
     In accordance with another aspect of the present invention, a first node B (NB) of a wireless communication system is provided. The first NB includes a controller for determining to change an access mode of a second NB from a first mode into a second mode; a transmitter for transmitting, to core network (CN), a request message requesting to change the access mode of the second NB; and a receiver for receiving, from the CN, a response message informing of an access mode control result to change the access mode. If the first mode allows access only to users or terminals having a registration in the second NB, and the second mode allows access to users or terminals, regardless of the registration in the second NB, the second NB further grants access to at least one user or terminal not registered in the second NB, after changing the access mode, and if the first mode allows access to the users or the terminals, regardless of the registration in the second NB, and the second mode allows access only to the users or the terminals having the registration in the second NB, the second NB controls at least one user or terminal not registered in the second NB to handover to a neighbor NB. 
     In accordance with another aspect of the present invention, a method is provided for changing an access mode of a second node B (NB), by a first node NB, of a wireless communication system. The method includes determining to change the access mode of the second NB from a first mode into a second mode; transmitting, to the second NB, a request message requesting to change the access mode of the second NB; and receiving, from the second NB, a response message informing of an access mode control result to change the access mode. If the first mode allows access only to users or terminals having a registration in the second NB, and the second mode allows access to users or terminals, regardless of the registration in the second NB, the second NB further grants access to at least one user or terminal not registered in the second NB, after changing the access mode, and if the first mode allows access to the users or the terminals, regardless of the registration in the second NB, and the second mode allows access only to the users or the terminals having the registration in the second NB, the second NB controls at least one user or terminal not registered in the second NB to handover to a neighbor NB. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain embodiments of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates an example of an interference factor of an access mode decision and change factors of an NB in a wireless communication system according to an embodiment of the present invention; 
         FIG. 2  is a signal flow illustrating a method for changing an NB access mode by an interference factor at an NB of a wireless communication system according to an embodiment of the present invention; 
         FIG. 3  is a signal flow illustrating a method for changing an NB access mode by an interference factor at an NB of a wireless communication system according to an embodiment of the present invention; 
         FIG. 4  is a signal flow illustrating a method for changing an NB access mode by a cell load distribution necessity factor at an NB of a wireless communication system according to an embodiment of the present invention; 
         FIG. 5  is a signal flow illustrating a method for changing an NB access mode by a cell load distribution necessity factor at an NB of a wireless communication system according to an embodiment of the present invention; 
         FIG. 6  is a signal flow illustrating a method for changing an NB access mode by a cell load distribution necessity factor at an NB of a wireless communication system according to an embodiment of the present invention; 
         FIG. 7  is a signal flow illustrating a management method for determining, at an NB, an access mode, and reporting the access mode to a Core Network (CN) in a wireless communication system according to an embodiment of the present invention; 
         FIG. 8  is a signal flow illustrating a management method for determining, at a CN, an access mode of an NB and reporting the access mode to the NB in a wireless communication system according to an embodiment of the present invention; 
         FIG. 9  is a signal flow illustrating a management method for requesting, by a neighbor NB, an NB to change an access mode for the purpose of reducing an inter-cell interference in a wireless communication system according to an embodiment of the present invention; 
         FIG. 10  is a signal flow illustrating a management method for requesting, by a neighbor NB, an NB to change an access mode for the purpose of reducing an inter-cell interference in a wireless communication system according to an embodiment of the present invention; 
         FIG. 11  is a signal flow illustrating an operating method for cases  1  and  2  for changing an access mode of an NB from a hybrid access mode or an open access mode to a closed access mode in a wireless communication system according to an embodiment of the present invention; 
         FIG. 12  is a signal flow illustrating an operating method for case  3  for changing an access mode of an NB from a closed access mode to a hybrid access mode in a wireless communication system according to an embodiment of the present invention; 
         FIG. 13  is a signal flow illustrating an operating method for case  3  for changing an access mode of an NB from a closed access mode to a hybrid access mode in a wireless communication system according to an embodiment of the present invention; 
         FIG. 14  is a signal flow illustrating an operating method for case  3  for changing an access mode of an NB from a closed access mode to a hybrid access mode in a wireless communication system according to an embodiment of the present invention; 
         FIG. 15  is a signal flow illustrating an operating method for case  4  for changing an access mode of an NB from an open access mode to a hybrid access mode in a wireless communication system according to an embodiment of the present invention; 
         FIG. 16  is a flowchart illustrating an operating method of an NB, for changing an access mode of a neighbor NB according to a request from the NB in a wireless communication system according to an embodiment of the present invention; 
         FIG. 17  is a flowchart illustrating an operating method of a neighbor NB, for changing an access mode of the neighbor NB according to a request from an NB in a wireless communication system according to an embodiment of the present invention; 
         FIG. 18  is a flowchart illustrating an operating method of an NB, for changing an access mode of the NB according to a request from the NB in a wireless communication system according to an embodiment of the present invention; and 
         FIG. 19  is a block diagram illustrating an NB according to an embodiment of the present invention. 
     
    
    
     Throughout the drawings, like reference numerals will be understood to refer to like parts, components and structures. 
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Various embodiments of the present invention are described herein below. The matters defined in the description such as detailed constructions and elements are provided to assist in a comprehensive understanding of the embodiments of the invention. 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. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
     In the following description, a wireless communication system denotes, for example, a communication system that uses an Orthogonal Frequency Division Multiplexing (OFDM) scheme or an Orthogonal Frequency Division Multiple Access (OFDMA) scheme. Although certain embodiments of the present invention are described using a broadband wireless access communication system as an example, the present invention is also applicable to different wireless communication systems. 
     In the following description, an NB may be a macro NB or a femto NB. Further, an NB that operates in an open access mode is referred to as an open NB, an NB that operates in a closed access mode is referred to as a closed NB, and an NB that operates in a hybrid access mode is referred to as a hybrid NB. In addition, a UE that is registered in an NB that operates in the closed access mode and that is granted access is referred to as a closed user or a Closed Subscriber Group (CSG) UE, and the other UEs are referred to as open users or non-CSG UEs. Additionally, a coverage area of an NB that operates in an open access mode is referred to as an open cell, a coverage area of an NB that operates in a closed access mode is referred to as a closed cell, and a coverage area of an NB that operates in a hybrid access mode is referred to as a hybrid cell. Here, the closed cell may also be referred to as a CSG cell, and the open cell and the hybrid cell may also be referred to as a non-CSG cell. 
     In the following description, a Core Network (CN) denotes a Mobile Management Entity (MME), a Serving General packet radio service (GPRS) Support Node (SGSN), a Mobile Switching Center (MSC), an Access Control Router (ACR), Operations &amp; Maintenance (OAM), a Gate-Way (GW), etc. 
     The determination to change a current access mode and selection of a new access mode of the NB may be performed right after a new NB is installed or power is turned on, and an access mode change during an operation may be performed due to the following other factors. Also, an access mode change may be performed right after a new NB is installed or power is turned on due to the following other factors. 
     1. Interference factor: When neighbor NBs use the same frequency and a non-CSG UE receives a service from an NB that utilizes an open access mode, i.e., an open NB, or an NB that utilizes a hybrid access mode, i.e., a hybrid NB, enters a coverage of an NB hat utilizes a closed access mode, i.e., a closed NB, a serious interference factor is generated between the neighbor NBs that use the different access modes, seriously deteriorating communication quality. In this case, to reduce the interference generated between the neighbor NBs, the closed NB may provide a service by temporarily changing its access mode and granting access to a non-CSG UE. 
     2. Load distribution necessity factor: When an amount of communication in an area will greatly increase, e.g., during a concert or sporting event, and a load of a cell of an NB will increase, load distribution may be required. In this case, with respect to a closed NB, when the access mode is temporarily changed to an open access mode with permission of a person who has installed the closed NB, the load of the cell may be distributed without a separate procedure. 
     When a CSG UE registered in a femto NB is located inside a coverage of the femto NB in operation in the open access mode, the femto NB may provide a service by temporarily changing its access mode and allowing access by the CSG UE. 
     Due to the change of an NB access mode by the above-described factors, system information such as a Physical Cell Identifier (PCI) for identifying cells, a CSG indicator representing whether a cell is a CSG cell, a CSG IDentifier (ID) for identifying CSG cells, etc., is changed, and an NB notifies a UE that receives a service from the NB that the system information has changed and informs the UE of the changed system information. 
       FIG. 1  illustrates an example of an interference factor of an NB in a wireless communication system according to an embodiment of the present invention. 
     Referring to  FIG. 1 , when neighbor NBs (open or hybrid NB  110  and closed NB  120 ) use the same frequency band and a non-CSG UE  100  receives a service from an open NB or a hybrid NB  110  enters a coverage of a closed NB  120 , interference is generated between the neighbor NBs that use the different access modes. 
     The open NB or hybrid NB  110 , which is a serving NB of the non-CSG UE  100  may recognize that a CSG cell exists near the non-CSG UE  100  through a measurement report message of the non-CSG UE  100 . That is, the serving NB  110  may recognize that a communication environment of the non-CSG UE  100  is degrading and that the CSG cell is transmitting a strong signal exists through the measurement report message of the non-CSG UE  100 . At this point, the communication between the non-CSG UE  100  and the serving NB  110  becomes an interference factor acting on the CSG cell, and a strong signal that the non-CSG UE  100  receives from the CSG cell deteriorates communication quality of the non-CSG UE  100 . 
     When the non-CSG UE  100  receives a strong signal in the CSG cell, and the non-CSG UE  100  causes interference while moving with low speed and remaining in the area, the serving NB  110  requests the closed NB  120  to change an access mode and allows the closed NB  120  to accept the non-CSG UE  100 , thereby solving the interference problem. 
       FIG. 2  is a signal flow illustrating a method for changing an NB access mode by an interference factor at an NB of a wireless communication system according to an embodiment of the present invention. 
     Referring to  FIG. 2 , it is assumed that a CSG UE  210  is connected to a closed NB  230  and receives a service in step  201 , and a non-CSG UE  200  is connected to a open or hybrid NB  220  and receives a service in step  203 . Under these assumptions, the non-CSG UE  200  enters a coverage of the closed NB  230 , the CSG UE  210  and the non-CSG UE  200  receive closed access system info.-message transmitted by the closed NB  230  in step  205  and step  207 , respectively. The non-CSG UE  200  may detect a CSG cell that transmits a strong signal, and recognize a PCI of the detected CSG cell through the system info.-message received from the detected CSG cell. Therefore, the non-CSG UE  200  informs its serving NB, i.e., the open or hybrid NB  220 , of the PCI of the CSG cell that transmits a strong signal by transmitting a measurement report message to the serving NB  220  in step  209 . 
     The serving NB  220  that has received the measurement report message from the non-CSG UE  200  determines whether the non-CSG UE  200  receives interference from the neighbor CSG cell, which is greater than a reference level, based on the PCI included in the measurement report message. If the non-CSG UE  200  receives interference that is greater than the reference level from the neighbor CSG cell, the non-CSG UE  200  may reduce the interference of the UE and improve communication quality by handing over the UE to the neighbor CSG cell, such that it receives a service from the neighbor CSG cell, i.e., the closed NB  230 , rather than receiving a service from the serving NB  220 . Therefore, when determining that the non-CSG UE  200  receives interference greater than the reference level from the neighbor CSG cell, in step  211 , the serving NB  220  determines to perform a handover of the non-CSG UE  200 , and determines an access mode change request to the closed NB  230 , i.e., target NB, so that the target NB  230  may allow access of the non-CSG UE  200 . Here, to prevent the handover decision and access mode change request decision from frequently occurring, the handover decision and the access mode change request decision may be performed when the non-CSG UE  200  receives interference greater than the reference level from the neighbor CSG cell and the velocity of the non-CSG UE  200  is less than a reference level. 
     To handover the non-CSG UE  200  and request an access mode change by the closed NB  230 , the serving NB  220  requests additional CSG cell information, such as an Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (E-UTRAN) Cell Global Identifier (ECGI), for more accurate cell identification than just the PCI, by transmitting an additional measurement control message to the non-CSG UE  200 , in step  213 . The non-CSG UE  200  collects the CSG cell information and transmits the collected information to the serving NB  220 . 
     In step  215 , the serving NB  220  transmits a Handover (HO) &amp; access mode change request message to a CN  240  using the CSG cell information transmitted by the non-CSG UE  200 . Here, the HO &amp; access mode change request message requests the non-CSG UE  200  to perform a handover to the target NB  230 , and requests the target NB  230  to change its access mode so that the non-CSG UE  200  may access the target NB  230 . Though the handover request and the access mode change request are performed together in  FIG. 2 , they also may be performed separately, e.g., using two separate request messages. 
     The CN  240  controls an access mode of the target NB  230  in step  217 . Specifically, when the CN  240  has an allowance authority regarding the handover and the access mode change, the CN  240  may determine whether to allow the handover and the access mode change through a separate handover &amp; access mode change algorithm. However, when the CN  240  does not have the allowance authority regarding the handover and the access mode change, the CN  240  may determine allowance with respect to the handover and the access mode change unconditionally. When the CN  240  determines allowance regarding the handover of the non-CSG UE  200  and the access mode change of the target NB  230 , the CN  240  transfers the HO &amp; access mode change request message to the target NB  230  in step  219 . 
     The target NB  230  controls its access mode in step  221 . That is, the target NB  230  determines whether to accept the handover of the non-CSG UE  200 , and determines whether to allow an access mode change so that the non-CSG UE  200  may access the target NB  230 . When the target NB  230  is a femto NB, the target NB  230  may additionally receive an allowance from an owner of the femto NB to change the access mode. 
     The target NB  230  transmits an HO &amp; access mode change response message including an access mode control result to the CN  240  in step  225 , and the CN  240  transfers the HO &amp; access mode change response message including a final access mode control result to the serving NB  220  in step  227 . When allowance is determined with respect to the handover of the non-CSG UE  200  and the access mode change of the target NB  230  as the final access mode control result, the serving NB  220  orders the non-CSG UE  200  to perform the handover to the target NB  230  by transmitting a handover command message to the non-CSG UE  200  in step  229 . 
     When the target NB  230  determines an allowance with respect to the handover of the non-CSG UE  200  and the access mode change of itself through the access mode control, the target NB  230  informs the CSG UE  210  that the access mode has changed, and accordingly the system information has changed, by transmitting a system information modification message to the CSG UE  210  in step  223 . For example, the target NB  230  may change its access mode from a closed access mode to a hybrid access mode so that not only the CSG UE  210 , but also the non-CSG UE  200 , may access the target NB  230 . In this case, the target NB  230  may transmit a hybrid access system info.-message including system information of the changed access mode to the CSG UE  210  and the non-CSG UE  200  in steps  231  and step  233 . 
     In step  235 , the non-CSG UE  200  accesses the target NB  230  and receives a service from the target NB  230  by performing a general handover procedure. 
     Though both the CN  240  and the target NB  230  perform the access mode control in  FIG. 2 , alternatively, only one of them may perform the access mode control. 
     In  FIG. 2 , the transfer of the HO &amp; access mode change request message has been described using an example in which the serving NB  220  transfers the HO &amp; access mode change request message to the target NB  230  via the CN  240 . However, when direct communication, such as an X2 interface between NBs is possible, the serving NB  220  may directly transfer the HO &amp; access mode change request message to the target NB  230 , which will be described in more detail with reference to  FIG. 3 . 
       FIG. 3  is a signal flow illustrating a method for changing an NB access mode by an interference factor at an NB of a wireless communication system according to an embodiment of the present invention. 
       FIG. 3  is basically the same as  FIG. 2 , i.e., steps  201  to  213  of  FIG. 2  are the same as steps  301  to  313  of  FIG. 3 , and thus, a repetitive description will be avoided. 
     Referring to  FIG. 3 , in step  315 , a serving NB  320  transmits an HO &amp; access mode change request message to a target NB  330  using CSG cell information transmitted by a non-CSG UE  300 . Here, the HO &amp; access mode change request message requests the non-CSG UE  300  to perform a handover to the target NB  330 , and requests the target NB  330  to change its access mode so that the non-CSG UE  300  may access the target NB  330 . Though the handover request and the access mode change request are performed together in  FIG. 3 , alternatively, they may be performed separately. 
     In step  317 , the target NB  330  controls its access mode. That is, the target NB  330  determines whether to accept a call of the non-CSG UE  300 , and determines whether to allow an access mode change so that the non-CSG UE  300  may access the target NB  330 . If the target NB  330  is a femto NB, the target NB may additionally receiving an allowance from an owner of the femto NB to change the access mode. 
     When the target NB  330  determines an allowance with respect to the handover of the non-CSG UE  300  and the access mode change of the target NB  330 , the target NB  330  transfers the HO &amp; access mode change request message to a CN  340  in step  319 . 
     In step  321 , the CN  340  controls the access mode of the target NB  330 . Specifically, when the CN  340  has an allowance authority regarding the handover and the access mode change, the CN  340  determines whether to allow the handover and the access mode change through a separate handover &amp; access mode change algorithm. However, when the CN  340  does not have the allowance authority regarding the handover and the access mode change, the CN  340  may determine allowance with respect to the handover and the access mode change unconditionally. 
     The CN  340  transmits an HO &amp; access mode change response message including an access mode control result to the target NB  330  in step  323 , and the target NB  330  transfers an HO &amp; access mode change response message including a final access mode control result to the serving NB  320  in step  327 . When allowance is determined with respect to the handover of the non-CSG UE  300  and the access mode change of the target NB  330  as the final access mode control result, the serving NB  320  may order the non-CSG UE  300  to perform the handover to the target NB  330  by transmitting a handover command message to the non-CSG UE  300  in step  329 . 
     When the target NB  330  determines an allowance with respect to the handover of the non-CSG UE  300  and its access mode change as the final access mode control result, the target NB  330  informs the CSG UE  310  that the access mode has changed, and accordingly that the system information has changed, by transmitting a system information modification message to the CSG UE  310  in step  325 . For example, the target NB  330  may change its access mode to a hybrid access mode so that not only the CSG UE  310 , but also the non-CSG UE  300 , may access the target NB  330 . In this case, the target NB  330  may transmit a hybrid access system info.-message including system information of the changed access mode to the CSG UE  310  and the non-CSG UE  300  in step  331  and step  333 , respectively. 
     The non-CSG UE  300  accesses the target NB  330  and receives a service from the target NB  330  by performing a general handover procedure in step  335 . 
     Though both the CN  340  and the target NB  330  perform the access mode control in  FIG. 3 , alternatively, only one of them may perform the access mode control. That is, the target NB  330  that has received an HO &amp; access mode change request message from the serving NB  320  may directly transmit an HO &amp; access mode change response message, including only an access mode control performance result of the target NB, to the serving NB  320 . 
       FIG. 4  is a signal flow illustrating a method for changing an NB access mode by a cell load distribution necessity factor at an NB of a wireless communication system according to an embodiment of the present invention. 
     Referring to  FIG. 4 , it is assumed that a CSG UE  410  is connected to a closed NB  430  as a closed user to receive a service in step  401 , and a non-CSG UE  400  is connected to an open or hybrid NB  420  as an open user to receive a service in step  405 . In addition, it is assumed that the closed NB  430 , which is a neighbor NB to the non-CSG UE  400 , and open or hybrid NB, i.e., the serving NB,  420  of the non-CSG UE  400  transmit load info.-messages including cell load information to a CN  440  in step  403  and step  407 , respectively. Based on these assumptions, the non-CSG UE  400  enters a coverage of the neighbor NB  430 , the CSG UE  410  and the non-CSG UE  400  receive closed access system info.-messages transmitted by the neighbor NB  430  of the closed access mode in steps  409  and  411 , respectively. 
     The CN  440  determines whether an NB whose cell load is greater than a reference level and an NB whose cell load is less than the reference level exist among NBs whose coverage areas overlap, based on the load information for each cell and an inter-NB coverage mapping table. If an NB whose cell load is greater than the reference level and an NB whose cell load is less than the reference level exist among the NBs with overlapping coverage areas, the CN  440  may distribute a cell load by changing an access mode of the NB whose cell load is less than the reference level so that a UE that receives a service from an NB whose cell load is greater than the reference level may perform a handover to the NB whose cell load is less than the reference level. 
     Referring to  FIG. 4 , when an NB whose cell load is greater than the reference level and an NB whose cell load is less than the reference level exist among the NBs with overlapping coverage areas, the CN  440  determines whether to request the NB whose cell load is less than the reference level to change an access mode in step  413 . For example, when the coverage areas of the serving NB  420  and the neighbor NB  430  of the non-CSG UE  400  overlap each other, and the cell load of the serving NB  420  is greater than the reference level and the cell load of the neighbor NB  430  is less than the reference level, the CN  440  determines to request the neighbor NB  430  to change an access mode so that the non-CSG UE  400  may perform a handover to the neighbor NB  430  whose cell load is less than the reference level and receive service from the neighbor NB  430 . 
     In step  415 , the CN  440  transmits an access mode change request message to the neighbor NB  430 . Here, the access mode change request message requests the neighbor NB  430  to change its access mode so that the non-CSG UE  400  may access the neighbor NB  430 . 
     In step  417 , the neighbor NB  430  controls its access mode. Specifically, the neighbor NB  430  determines whether to allow its access mode to change so that the non-CSG UE  400  may access the neighbor NB  430 . When the neighbor NB  430  is a femto NB, the neighbor NB may additionally receive an allowance from an owner of the femto NB to change its access mode. That is, the neighbor NB may transmit an access mode change request message to the owner  410  of the femto NB and receive an access mode change response message from the owner  410  of the femto NB in step  419 . 
     In step  421 , the neighbor NB  430  transmits an access mode change response message including an access mode control result to the CN  440 . When determining an allowance with respect to its access mode, the neighbor NB  430  informs the CSG UE  410  that the access mode has changed, and accordingly, that the system information has changed, by transmitting a system information modification message to the CSG UE  410  in step  423 . For example, the neighbor NB  430  may change its access mode to a hybrid access mode so that not only the CSG UE  410 , but also the non-CSG UE  400 , may access the neighbor NB  430 . In this case, the neighbor NB  430  may transmit a hybrid access system info.-message including system information of the changed access mode to the CSG UE  410  and the non-CSG UE  400  in steps  425  and  427 , respectively. 
     In step  429 , the non-CSG UE  400  generates a measurement report message based on the system info.-message received from the neighbor NB  430 , and transmits the same to the serving NB  420 . The serving NB  420  determines a handover of the non-CSG UE  400  based on the measurement report message in step  431 . When the serving NB transmits a handover command message to the non-CSG UE  400  according to the handover decision of the non-CSG UE  400 , the non-CSG UE  400  is connected to the neighbor NB  430  and receives a service from the neighbor NB  430  by performing a general handover procedure in step  433 . 
     In  FIG. 4 , the CN  440  requests an NB whose cell load is less than a reference level to change an access mode to allow a UE inside a coverage area of an NB whose cell load is greater than the reference level to access the NB whose cell load is less than the reference level and receive service from the NB whose cell load is less than the reference level. Specifically,  FIG. 4  illustrates the changing an access mode of an NB based on a request by a CN. However, it is also possible for an access mode of an NB to be changed based on a request of a neighbor NB. That is, an NB whose cell load is greater than a reference level may request an NB whose cell load is less than the reference level to change an access mode, which will be described in more detail with reference to  FIG. 5 . 
     As another example, an NB may spontaneously request to change its access mode. That is, an NB whose cell load is less than a reference level may request to change its own access mode, which will be described in more detail with reference to  FIG. 6 . 
       FIG. 5  is a signal flow illustrating a method for changing an NB access mode by a cell load distribution necessity factor at an NB of a wireless communication system according to an embodiment of the present invention. 
     Referring to  FIG. 5 , it is assumed that a CSG UE  510  is connected to a closed NB  530  as a closed user to receive a service in step  501 , and a non-CSG UE  500  is connected to a open or hybrid NB  520  as an open user to receive a service in step  503 . Under these assumptions, the non-CSG UE  500  enters a coverage area of the closed NB  530 , which is a neighbor NB, and the CSG UE  510  and the non-CSG UE  500  receive closed access system info.-messages transmitted by the neighbor NB  530  in steps  505  and  507 , respectively. 
     In addition, NBs  520  and  530  exchange a load info.-message, including cell load information, with each other in steps  509  and  511 . The NBs  520  and  530  determine whether an NB whose cell load is greater than a reference level and an NB whose cell load is less than the reference level exist among NBs including the NBs  520  and  530 , whose coverage areas overlaps those of the NBs  520  and  530 , based on the load information for each cell collected through the load info.-message exchange and an inter-NB coverage mapping table. 
     When an NB whose cell load is greater than the reference level and an NB whose cell load is less than the reference level exist among the NBs whose coverage areas overlap, the NBs  520  and  530  may distribute the cell load by changing an access mode of the NB whose cell load is less than the reference level so that a UE that receives a service from the NB whose cell load is greater than the reference level may perform a handover to the NB whose cell load is less than the reference level. Therefore, when an NB whose cell load is greater than the reference level and an NB whose cell load is less than the reference level exist among the NBs whose coverage areas overlap those of the NBs  520  and  530 , the NB  520  or  530  determines whether the NB whose cell load is greater than the reference level is itself, and when the NB is itself, the NB  520  or  530  determines to request the NB whose cell load is less than the reference level to change an access mode in step  513 . 
     For example, when the coverage of the serving NB  520  of the non-CSG UE  500  overlaps the coverage of the neighbor NB  530 , the cell load of the serving NB  520  is greater than the reference level, and the cell load of the neighbor NB  530  is less than the reference level, the serving NB  520  determines whether to request the neighbor NB  530  to change an access mode so that the non-CSG UE  500  may perform a handover to the neighbor NB  530  whose cell load is less than the reference level to receive a service from the neighbor NB  530 . 
     More specifically, the serving NB  520  transmits an access mode change request message to the neighbor NB  530  in step  515 . Here, the access mode change request message requests the neighbor NB  530  to change its access mode so that the non-CSG UE  500  may access the neighbor NB  530 . 
     In step  517 , the neighbor NB  530  controls its access mode. Specifically, the neighbor NB  530  determines whether to allow its access mode to change so that the non-CSG UE  500  may access the neighbor NB  530 . 
     When the neighbor NB  530  is a femto NB, the neighbor NB  530  may additionally receive an allowance from an owner of the femto NB to change the access mode. That is, the neighbor NB  530  may additionally transmit an access mode change request message to the owner  510  of the femto NB and receive an access mode change response message from the owner  510  of the femto NB in step  519 . 
     When the neighbor NB  530  determines to allow its access mode to change, the neighbor NB  530  transfers the access mode change request message to a CN  540  in step  521 . 
     In step  523 , the CN  540  controls an access mode of the neighbor NB  530 . Specifically, when the CN  540  has an allowance authority regarding the access mode change, the CN  540  may determine whether to allow the access mode change through a separate access mode change algorithm. However, when the CN  540  does not have the allowance authority regarding the access mode change, the CN  540  may unconditionally determine allowance with respect to the access mode change. 
     In step  525 , the CN  540  transmits an access mode change response message including an access mode control result to the neighbor NB  530 , and in step  527 , the neighbor NB  530  transfers an access mode change response message including a final access mode control result to the serving NB  520 . 
     When determining an allowance with respect to its access mode change as the final access mode control result, the neighbor NB  530  informs the CSG UE  510  that the access mode has changed, and accordingly, that the system information has changed, by transmitting a system information modification message to the CSG UE  510  in step  529 . For example, the neighbor NB  530  may change its access mode to a hybrid access mode so that the CSG UE  510  and also the non-CSG UE  500  may access the neighbor NB  530 . In this case, the neighbor NB  530  may transmit a hybrid access system info.-message including system information of the changed access mode to the CSG UE  510  and the non-CSG UE  500  in steps  531  and  533 , respectively. 
     In step  535 , the non-CSG UE  500  generates a measurement report message based on the system info.-message received from the neighbor NB  530 , and transmits the same to the serving NB  520 . The serving NB  520  determines a handover of the non-CSG UE  500  based on the measurement report message in step  537 . When the serving NB  520  transmits a handover command message to the non-CSG UE  500  according to a handover decision of the non-CSG UE  500 , the non-CSG UE  500  receives a service from the neighbor NB  530  by performing a general handover procedure and accessing the neighbor NB  530  in step  539 . 
       FIG. 6  is a signal flow illustrating a method for changing an NB access mode by a cell load distribution necessity factor at an NB of a wireless communication system according to an embodiment of the present invention. 
     Referring to  FIG. 6 , it is assumed that a CSG UE  610  is connected to a closed NB  630  as a closed user to receive a service in step  601 , and a non-CSG UE  600  is connected to an open or hybrid NB  620  as an open user to receive a service in step  603 . Under these assumptions, the non-CSG UE  600  enters a coverage of the closed NB  630 , which is a neighbor NB, and the CSG UE  610  and the non-CSG UE  600  receive closed access system info.-message transmitted by the neighbor NB  630  in steps  605  and  607 , respectively. 
     In addition, the NBs  620  and  630  exchange load info.-message including cell load information with each other. The NBs  620  and  630  determine whether an NB whose cell load is greater than a reference level and an NB whose cell load is less than the reference level exist among NBs including the NBs  620  and  630 , whose coverage areas overlap those of the NBs  620  and  630 , based on the load information for each cell collected through the load info.-message exchange and an inter-NB coverage mapping table. 
     When an NB whose cell load is greater than the reference level and an NB whose cell load is less than the reference level exist among the NBs whose coverage areas overlap, the NBs  620  and  630  may distribute the cell load by changing an access mode of the NB whose cell load is less than the reference level so that a UE that receives a service from the NB whose cell load is greater than the reference level may perform a handover to the NB whose cell load is less than the reference level. 
     Therefore, when an NB whose cell load is greater than the reference level and an NB whose cell load is less than the reference level exist among the NBs whose coverage areas overlap those of the NBs  620  and  630 , the NB  620  or  630  determines whether the NB whose cell load is less than the reference level is itself, and when the NB is itself, the NB  620  or  630  determines whether to change its own access mode in step  609 . 
     For example, when the coverage area of the serving NB  620  of the non-CSG UE  600  overlap the coverage area of the neighbor NB  630 , the cell load of the serving NB  620  is greater than the reference level, and the cell load of the neighbor NB  630  is less than the reference level, the neighbor NB  630  determines to change its access mode so that the non-CSG UE  600  may perform a handover to the neighbor NB  630  to receive service from the neighbor NB  630 . 
     Additionally, when the neighbor NB  630  is a femto NB, the neighbor NB may additionally receive an allowance from an owner of the femto NB to change the access mode. That is, the neighbor NB may additionally transmit an access mode change request message to the owner  610  of the femto NB and receive an access mode change response message from the owner  610  of the femto NB in step  611 . 
     The neighbor NB  630  transmits an access mode change request message to the CN  640  in step  613 . Here, the access mode change request message requests the neighbor NB  630  to change its access mode so that the non-CSG UE  600  may access the neighbor NB  630 . 
     In step  615 , the CN  640  controls the access mode of the neighbor NB  630 . Specifically, when the CN  640  has an allowance authority regarding the access mode change, the CN  640  may determine whether to allow the access mode change through a separate access mode change algorithm. However, when the CN  640  does not have the allowance authority regarding the access mode change, the CN  640  may unconditionally determine an allowance with respect to the access mode change. 
     The CN  640  transmits an access mode change response message including an access mode control result to the neighbor NB  630  in step  617 . When determining an allowance with respect to the access mode change of the neighbor NB  630 , the neighbor NB  630  informs the CSG UE  610  that the access mode has changed, and accordingly, that the system information has changed, by transmitting a system information modification message to the CSG UE  610  in step  619 . 
     For example, the neighbor NB  630  may change its own access mode to a hybrid access mode so that the CSG UE  610  and the non-CSG UE  600  may access the neighbor NB  630 . In this case, the neighbor NB  630  may transmit hybrid access system info.-messages including system information of the changed access mode to the CSG UE  610  and the non-CSG UE  600  in steps  621  and  623 , respectively. 
     In step  625 , the non-CSG UE  600  generates a measurement report message based on the system info.-message received from the neighbor NB  630 , and transmits the same to the serving NB  620  of the non-CSG UE  600 . The serving NB  620  determines a handover of the non-CSG UE  600  based on the measurement report message in step  627 . When the serving NB  620  transmits a handover command message to the non-CSG UE  600 , the non-CSG UE  600  is connected to the neighbor NB  630  in order to receive service from the neighbor NB  630  by performing a general handover procedure in step  629 . 
       FIG. 7  is a signal flow illustrating a management method for determining, by a NB, an access mode of the NB, and reporting the access mode to a CN in a wireless communication system according to an embodiment of the present invention. 
     Referring to  FIG. 7 , a NB  710  determines its own access mode in step  710 , and reports the determined access mode by transmitting an access mode change request message to a CN  720  in step  703 . 
     In step  705 , the CN  720  controls the access mode of the NB  710 . Specifically, when the CN  720  has an allowance authority regarding the access mode decision, the CN  720  may determine whether to allow the access mode decision or determine a different access mode and allocate a suitable PCI through a separate access mode decision algorithm. However, when the CN  720  does not have the allowance authority regarding the access mode decision, the CN  720  may unconditionally determine an allowance with respect to the access mode decision, and allocate a suitable PCI. 
     In step  707 , the CN  720  transmits an access mode change response message including an access mode control result to the NB  710 . When the CN  720  determines an allowance with respect to the access mode decision of the NB  710  through the access mode control, the NB  710  transmits a system info.-message including system information of the determined access mode to a UE  700  in step  709 . 
     The UE  700  generates a measurement report message based on the system info.-message received from the NB  710 , and transmits the measurement report message to the serving NB to try a handover to the NB  710  or perform an initial access procedure to the NB  710  in step  711 . 
       FIG. 8  is a signal flow illustrating a management method for determining, by a CN, an access mode of an NB and reporting the access mode to the NB in a wireless communication system, according to an embodiment of the present invention. 
     Referring to  FIG. 8 , when an NB  810  is installed or is powered on in step  801 , the NB  810  transmits a power-on-report message to a CN  820  in step  803 . 
     The CN  820  controls an access mode of the NB  810  in step  805 . Specifically, the CN  820  determines the access mode of the NB  810  through a separate access mode decision algorithm, and allocates a suitable PCI. The CN  820  transmits an access mode change response message including an access mode control result to the NB  810  in step  807 . 
     In step  809 , the NB  810  transmits a system info.-message including system information of the determined access mode to a UE  800 . 
     The UE  800  generates a measurement report message based on the system info.-message received from the NB  800 , and transmits the measurement report message to the serving NB to try a handover to the NB  810  or to perform an initial access procedure to the NB  810  in step  811 . 
       FIG. 9  is a signal flow illustrating a management method for requesting, by a neighbor NB, an NB to change an access mode for the purpose of reducing an inter-cell interference in a wireless communication system according to an embodiment of the present invention. 
     Referring to  FIG. 9 , a UE  900  is connected to a serving NB  920  to receive a service from the serving NB  920  in step  901 . 
     A neighbor NB  910  of the UE  900  determines to request a peripheral NB to change an access mode for the purpose of reducing an inter-cell interference in step  903 . For example, the neighbor NB  910  may requests the serving NB  920  to change its access mode. The following description is described using the above example. 
     In step  905 , the neighbor NB  910  transmits an access mode change request message to a CN  930 . Here, the access mode change request message requests the serving NB  920  to change the access mode. 
     The CN  930  controls the access mode of the serving NB  920  in step  907 . Specifically, when the CN  930  has an allowance authority regarding the access mode change, the CN  930  may determine whether to allow the access mode change through a separate access mode change algorithm. However, when the CN  930  does not have the allowance authority regarding the access mode change, the CN  930  may unconditionally determine allowance with respect to the access mode change. 
     When determining to allow the access mode change of the serving NB  920 , the CN  930  transfers the access mode change request message to the serving NB  920  in step  909 . 
     The serving NB  920  that has received the access mode change request message controls its own access mode in step  911 . That is, the serving NB  920  determines whether to allow its access mode to change. 
     When the serving NB  920  is a femto NB, the serving NB  920  may additionally receive an allowance from an owner of the femto NB to change the access mode. 
     In step  913 , the serving NB  920  transmits an access mode change response message including an access mode control result to the CN  930 , and in step  917 , the CN  930  transfers the access mode change response message including the final access mode control result to the neighbor NB  910 . 
     When determining to allow the access mode change of the serving NB  920 , the serving NB  920  in forms the UE  900  that the access mode has changed, and accordingly, that the system information has changed, by transmitting a system information modification message to the UE  900  in step  915 . In addition, the serving NB  920  may transmit a changed system info.-message including system information of the changed access mode to the UE  900  in step  919 . 
     In conjunction with  FIG. 9 , the transfer of the access mode change request message has been described using an example in which the neighbor NB  910  transfers the access mode change request message to the serving NB  920  via the CN  930 . However, when direct communication, such as an X2 interface between NBs is possible, the neighbor NB  910  may directly transfer the access mode change request message to the serving NB  920 , which will be described in more detail with reference to  FIG. 10 . 
       FIG. 10  is a signal flow illustrating a management method for requesting, by a neighbor NB, an NB to change an access mode for the purpose of reducing an inter-cell interference in a wireless communication system according to an embodiment of the present invention. 
     Referring to  FIG. 10 , a UE  1000  is connected to a serving NB  1020  to receive a service from the serving NB  1020  in step  1001 . 
     A neighbor NB  1010  of the UE  1000  may determine to request a peripheral NB to change an access mode for the purpose of reducing an inter-cell interference in step  1003 . For example, the neighbor NB may request the serving NB  1020  to change its access mode. The following description is made using the above example. 
     The neighbor NB  1010  transmits an access mode change request message to the serving NB  1020  in step  1005 . Here, the access mode change request message requests the serving NB  1020  to change the access mode. 
     In step  1007 , the serving NB  1020  controls its own access mode. That is, the serving NB  1020  determines whether to allow its access mode to change. 
     When the serving NB  1020  is a femto NB, the serving NB may additionally receive an allowance from an owner of the femto NB to change the access mode. 
     When determining to allow its access mode change, the serving NB  1020  transfers the access mode change request message to a CN  1030  in step  1009 . 
     The CN  1030  that has received the access mode change request message controls the access mode of the serving NB  1020  in step  1011 . Specifically, when the CN  1030  has an allowance authority regarding the access mode change, the CN  1030  may determine whether to allow the access mode change through a separate access mode change algorithm. However, when the CN  1030  does not have the allowance authority regarding the access mode change, the CN  1030  may unconditionally determine allowance with respect to the access mode change. 
     The CN  1030  transmits an access mode change response message including an access mode control result to the serving NB  1020  in step  1013 . The serving NB  1020  transfers the access mode change response message including the final access mode control result to the neighbor NB  1010  in step  1015 . 
     Alternatively, instead of the two messages in steps  1013  and  1015 , the CN  1030  may directly transfer the access mode change response message including the access mode control result to the neighbor NB  1010 . 
     When determining to allow its access mode change, the serving NB  1020  informs the UE  1000  that the access mode has changed, and accordingly, that system information has changed, by transmitting a system information modification message to the UE  1000  in step  1017 . In addition, the serving NB  1020  may transmit a system info.-message including system information of the changed access mode to the UE  1000  in step  1019 . 
     Table 1 summarizes an operating method of an NB according to an access mode change of the NB. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 changing 
               
            
           
           
               
               
               
               
            
               
                 existing 
                 Closed 
                 Hybrid 
                 Open 
               
               
                   
               
               
                 Closed 
                 — 
                 treats existing 
                 it is possible in the 
               
               
                   
                   
                 CSG UE 
                 case where there 
               
               
                   
                   
                 considerately 
                 is not CSG UE 
               
               
                   
                   
                 (case 3) 
                 (case 5) 
               
               
                 Hybrid 
                 commands open 
                 — 
                 it is possible in the 
               
               
                   
                 user to handover 
                   
                 case where there 
               
               
                   
                 (case 1) 
                   
                 is not CSG UE 
               
               
                   
                   
                   
                 (case 6) 
               
               
                 Open 
                 commands open 
                 treats existing 
                 — 
               
               
                   
                 user to handover 
                 CSG UE 
               
               
                   
                 (case 2) 
                 considerately, and 
               
               
                   
                   
                 commands open 
               
               
                   
                   
                 user to handover 
               
               
                   
                   
                 depending on a 
               
               
                   
                   
                 resource shortage 
               
               
                   
                   
                 (case 4) 
               
               
                   
               
            
           
         
       
     
     Referring to Table 1, when an access mode change occurs, an NB notifies a system information change to a UE communicating with the NB in a previous access mode. Of course, in cases of cases  1  and  2  above, which allow all UEs that operate in a previous access mode to perform a handover, such a system information change notice is not required. 
     In cases  3  and  5 , when an NB that is requested to change an access mode is a femto NB, receiving an allowance from an owner of the femto NB to change the access mode may be additionally performed. 
       FIG. 1  is a signal flow illustrating an operating method for cases  1  and  2  of Table 1, for changing an access mode of an NB from a hybrid access mode or an open access mode to a closed access mode in a wireless communication system according to an embodiment of the present invention. 
     Referring to  FIG. 11 , a non-CSG UE  1100  is connected to an open or hybrid NB  1120  as an open user to receive a service in step  1101 . In step  1103 , the non-CSG UE  1100  receives hybrid/open access system info.-message transmitted by a serving NB  1120  of the non-CSG UE  1100 . 
     In step  1105 , the NB  1120  may determine to change its access mode to a closed access mode. When determining to change its access mode, the NB  1120  reports the access mode change to a CN  1130  by transmitting an access mode change request message to the CN  1130  in step  1107 . 
     In step  1109 , the CN  1130  controls the access mode of the NB  1120 . Specifically, when the CN  1130  has an allowance authority regarding the access mode decision, the CN  1130  may determine whether to allow the access mode decision or determine a different access mode and allocate a suitable PCI through a separate access mode decision algorithm. However, when the CN  1130  does not have the allowance authority regarding the access mode decision, the CN  1130  may unconditionally determine an allowance with respect to the access mode decision, and allocate a suitable PCI. 
     The CN  1130  transmits an access mode change response message including an access mode control result to the NB  1120  in step  1111 . When the CN  1130  determines to allow the access mode decision of the NB  1120 , the NB  1120  orders the non-CSG UE  1100  to perform a handover in step  1113 , and transmits a closed access system info.-message including system information of the determined changed access mode to the CSG UE  1110  in step  1115 . 
     The CSG UE  1110  generates a measurement report message based on the system info.-message received from the NB  1120 , and transmits the measurement report message to the serving NB to try a handover to the NB  1120  or perform an initial access procedure to the NB  1120  in step  1117 . In step  1119 , the CSG UE  1110  is connected to the NB  1120 , now in the closed access mode, to receive service. 
       FIG. 12  is a signal flow illustrating an operating method for case  3  of Table 1, for changing an access mode of an NB from a closed access mode to a hybrid access mode in a wireless communication system according to an embodiment of the present invention. 
     Referring to  FIG. 12 , a CSG UE  1210  is connected to a closed NB (serving NB)  1220  as a closed user to receive service in step  1201 . In step  1203 , the CSG UE  1210  receives a closed access system info.-message transmitted by the serving NB  1220 . 
     The NB  1220  may determine to change its access mode from a closed access mode to a hybrid access mode in step  1205 . When determining to change its access mode, the NB  1220  transmits an access mode change request message to the CN  1230  in step  1207 . 
     In step  1209 , the CN  1230  controls the access mode of the NB  1220 . Specifically, when the CN  1230  has an allowance authority regarding the access mode decision, the CN  1230  may determine whether to allow the access mode decision or determine a different access mode and allocate a suitable PCI through a separate access mode decision algorithm. However, when the CN  1230  does not have the allowance authority regarding the access mode decision, the CN  1230  may unconditionally determine an allowance with respect to the access mode decision, and allocate a suitable PCI. 
     In step  1211 , the CN  1230  transmits an access mode change response message including an access mode control result to the NB  1220 . When the CN  1230  determines to allow the access mode decision of the NB  1220 , the NB  1220  informs the CSG UE  1210  that the access mode has changed, and accordingly, that system information has changed, by transmitting a system information modification message to the CSG UE  1210  in step  1213 . In addition, the NB  1220  transmits a hybrid access system info.-message including system information of the changed access mode to the CSG UE  1210  and a non-CSG UE  1200  in steps  1215  and  1217 , respectively. 
     In step  1219 , the non-CSG UE  1200  generates a measurement report message based on the system information message received from the NB  1220 , and transmits the measurement report message to the serving NB to try a handover to the NB  1220  or perform an initial access procedure to the NB  1220 . In step  1221 , the non-CSG UE  1200  is connected to the NB  1220  of the hybrid access mode as an open user to receive service. 
     In  FIG. 12 , when the NB  1220  is a femto NB, the NB may additionally receive an allowance from an owner of the femto NB to change the access mode. That is, as illustrated in  FIG. 13 , when determining to change its access mode from a closed access mode to a hybrid access mode in step  1305 , an NB  1320  may additionally transmit an access mode change request message to the owner  1310  of the femto NB in step  1307 , and receives an access mode change response message from the owner  1310  of the femto NB in step  1309 . Because the remaining steps in  FIG. 13 , i.e., Steps  1301 ,  1303 , and  1311 - 1325 , are the same as those of  FIG. 12 , a repetitive detailed description thereof is omitted. 
     Though description has been made using an example where an NB requests a CN to change its access mode with reference to  FIGS. 12 and 13 , the CN may request the NB to change its access mode, which will be described in more detail with reference to  FIG. 14 . 
       FIG. 14  is a signal flow illustrating an operating method for case  3  in Table, for changing an access mode of an NB from a closed access mode to a hybrid access mode in a wireless communication system according to an embodiment of the present invention. 
     Referring to  FIG. 14 , a CSG UE  1410  is connected to a closed NB (serving NB)  1420  as a closed user to receive service in step  1401 . In step  1403 , the CSG UE  1410  receives a closed access system info.-message transmitted by the serving NB  1420  of the CSG UE  1410 . 
     A CN  1430  may determine to change the access mode of the NB  1420  from the closed access mode to a hybrid access mode in step  1405 . When determining to change the access mode of the NB  1420 , the CN  1430  requests the NB  1420  to change the access mode by transmitting an access mode change request message to the NB  1420  in step  1407 . 
     When the NB  1420  is a femto NB, the NB  1420  may additionally receive an allowance from an owner of the femto NB to change the access mode. That is, the NB  1420  may additionally transmit an access mode change request message to the owner  1410  of the femto NB in step  1409 , and receive an access mode change response message from the owner  1410  of the femto NB in step  1411 . 
     The NB  1420  transmits an access mode change response message including an access mode change response to the CN  1430  in step  1413 . When the owner  1410  of the femto NB allows the access mode change, the NB  1420  informs the CSG UE  1410  that the access mode has changed, and accordingly, that system information has changed, by transmitting a system information modification message to the CSG UE  1410  in step  1415 . In addition, the NB  1420  transmits a hybrid access system info.-message including system information of the changed access mode to the CSG UE  1410  and a non-CSG UE  1400  in steps  1417  and  1419 , respectively. 
     In step  1421 , the non-CSG UE  1400  generates a measurement report message based on the system information message received from the NB  1420 , and transmits the measurement report message to the serving NB to try a handover to the NB  1420  or perform an initial access procedure to the NB  1420 . In step  1423 , the non-CSG UE  1400  is connected to the NB  1420 , now operating in the hybrid access mode, as an open user to receive service. 
       FIG. 15  is a signal flow illustrating an operating method for case  4  of Table 1, for changing an access mode of an NB from an open access mode to a hybrid access mode in a wireless communication system according to an embodiment of the present invention. 
     Referring to  FIG. 15 , a non-CSG UE  1500  is connected to an open NB (serving NB)  1520  as an open user to receive service in step  1501 . In step  1503 , the non-CSG UE  1500  receives an open access system info.-message transmitted by the serving NB  1520 . 
     The NB  1520  may determine to change its access mode to a hybrid access mode in step  1505 . When determining to change its access mode, the NB  1520  transmits an access mode change request message to the CN  1530  in step  1507 . 
     In step  1509 , the CN  1530  controls an access mode of the NB  1520 . Specifically, when the CN  1530  has an allowance authority regarding the access mode decision, the CN  1530  may determine whether to allow the access mode decision or determine a different access mode and allocate a suitable PCI through a separate access mode decision algorithm. However, when the CN  1530  does not have the allowance authority regarding the access mode decision, the CN  1530  may unconditionally determine an allowance with respect to the access mode decision, and allocate a suitable PCI. 
     In step  1511 , the CN  1530  transmits an access mode change response message including an access mode control result to the NB  1520 . When the CN  1530  determines an allowance with respect to the access mode decision of the NB  1520 , the NB  1520  may determine a handover of the non-CSG UE  1500  depending on a resource shortage in step  1513 . In addition, the NB  1520  informs the non-CSG UE  1500  that the access mode has changed, and accordingly, that system information has changed, by transmitting a system information modification message to the non-CSG UE  1500  in step  1515 . In addition, the NB  1520  transmits a hybrid access system info.-message including system information of the changed access mode to the non-CSG UE  1500  and a CSG UE  1510  in steps  1517  and  1519 , respectively. 
     In step  1521 , the CSG UE  1510  generates a measurement report message based on the system information message received from the NB  1520 , and transmits the measurement report message to the serving NB to try a handover to the NB  1520  or perform an initial access procedure to the NB  1520 . In step  1523 , the CSG UE  1510  is connected to the NB  1520 , now operating in the hybrid access mode, as a closed user to receive a service. 
     Because cases  5  and  6  are the same as the foregoing, repetitive detailed descriptions thereof are omitted. 
       FIG. 16  is a flowchart illustrating an operating method of an NB, for changing an access mode of a neighbor NB according to a request of the NB in a wireless communication system according to an embodiment of the present invention. 
     Referring to  FIG. 16 , the NB determines to change an access mode of a neighbor NB for the purpose of reducing interference or distributing a cell load in step  1601 . In step  1603 , the NB transmits, to a CN or directly to the neighbor NB, an access mode change request message requesting the neighbor NB to change the access mode, and in step  1605 , receives an access mode change response message including an access mode control result from the CN or the neighbor NB. 
       FIG. 17  is a flowchart illustrating an operating method of a neighbor NB, for changing its access mode according to a request of an NB in a wireless communication system, according to an embodiment of the present invention. 
     Referring to  FIG. 17 , in step  1701 , a neighbor NB determines whether an access mode change request message requesting the neighbor NB to change an access mode is received from a CN or directly from an NB. 
     When the access mode change request message requesting the neighbor NB to change the access mode is received from the CN or directly from the NB, the neighbor NB controls its access mode in step  1703 . That is, the neighbor NB determines whether to allow its access mode to change. 
     When the neighbor NB is a femto NB, the neighbor NB may additionally receive an allowance from an owner of the femto NB to change the access mode. In addition, the neighbor NB may transfer the access mode change request message to the CN, receive an access mode change response message including an access mode control result from the CN, and finally determine whether to allow the access mode change of itself based on an access mode control result performed by the CN and an access mode control result performed by the neighbor NB. 
     The neighbor NB transmits an access mode change response message including a final access mode control result to the CN or the NB in step  1705 . 
     The neighbor NB determines whether an allowance has been determined with respect to the access mode change of the neighbor NB as a final access mode control result in step  1707 . 
     When the access mode change of the neighbor NB is not allowed in step  1707 , the neighbor NB ends the procedure. 
     However, when the access mode change of the neighbor NB is allowed in step  1707 , the neighbor NB informs a UE that the access mode has changed, and accordingly, that system information has changed, by transmitting a system information modification message to the UE in step  1709 . In step  1711 , the neighbor NB transmits a system information message including system information of the changed access mode to the UE. 
       FIG. 18  is a flowchart illustrating an operating method of an NB, for changing an access mode of the NB according to a request of the NB in a wireless communication system, according to an embodiment of the present invention. 
     Referring to  FIG. 18 , the NB determines to change its own access mode in step  1801 . When the NB is a femto NB, the NB may additionally receive an allowance from an owner of the femto NB to change the access mode. 
     In step  1803 , the NB transmits an access mode change request message to a CN, requesting to change its access mode, and in step  1805 , receives an access mode change response message including an access mode control result from the CN. 
     The NB determines whether an allowance has been determined with respect to the access mode change of the NB as an access mode control result in step  1807 . 
     When the access mode change of the NB has not been allowed in step  1807 , the NB ends the procedure. 
     However, when the access mode change of the NB has been allowed in step  1807 , the NB informs a UE that the access mode has changed, and accordingly, that system information has changed, by transmitting a system information modification message to the UE in step  1809 . In step  1811 , the NB transmits a system information message including system information of the changed access mode to the UE. 
       FIG. 19  is a block diagram illustrating an NB according to an embodiment of the present invention. 
     Referring to  FIG. 19 , the NB includes a duplexer  1901 , a reception modem  1903 , a message processor  1905 , a controller  1907 , an access mode change unit  1909 , a message generator  1911 , and a transmission modem  1913 . 
     The duplexer  1901  transmits a transmission signal provided from the transmission modem  1913  via an antenna, and provides a reception signal from the antenna to the reception modem  1903  according to a duplexing scheme. 
     The reception modem  1903  recovers data from a signal provided from the duplexer  1901 , and transfers the data to the message processor  1905 . For example, the reception modem  1903  includes a Radio Frequency (RF) reception block, a demodulation block, and a channel-decoding block. The RF reception block includes a filter and an RF pre-processor. When a wireless communication system utilizes OFDM scheme, the demodulation block includes a Fast Fourier Transform (FFT) operator for extracting data contained in each subcarrier. The channel-decoding block includes a demodulator, a deinterleaver, and a channel decoder. 
     The message processor  1905  extracts control information from a signal provided from the reception modem  1903  and provides the extracted control information to the controller  1907 . For example, the message processor  1905  extracts a message related to an access mode change provided from a CN or a neighbor NB, and provides the extracted message to the controller  1907 . 
     The controller  1907  controls an entire transmission/reception operation of the NB. Specifically, the controller  1907  determines to change an access mode of a neighbor NB or the NB for the purpose of reducing an interference or distributing a cell load, controls the message generator  1911  to generate an access mode change request message requesting the access mode change according to the decision, and controls the transmission modem  1913  to transmit the generated message to the CN or the neighbor NB. In addition, when an access mode change request message requesting the NB to change the access mode of the NB is received from the CN or the neighbor NB via the message processor  1905 , the controller  1907  controls the access mode of the NB to determine whether to allow the access mode change of the NB, controls the message generator  1911  to generate an access mode change response message including an access mode control result according to the decision, and controls the transmission modem  1913  to transmit the generated message to the CN or the neighbor NB. 
     When the NB is a femto NB, the controller  1907  may additionally receive an allowance from an owner of the femto NB to change the access mode. In addition, when determining to change the access mode of the NB or performing an access mode control in response to a request of the CN or the neighbor NB to allow the access mode change of the NB, the controller  1907  controls the message generator  1911  to generate a system information modification message informing that the access mode has changed, and accordingly, that system information has changed, and generate a system information message including system information of the changed access mode, and controls the transmission modem  1913  to transmit the generated messages to the UE. 
     The access mode change unit  1909  changes the access mode of the NB according to the decision of the controller  1907 . 
     The message generator  1911  generates a message to be transmitted to the CN, the neighbor NB, or the UE under control of the controller  1907 , and provides the generated message to the transmission modem  1913 . For example, the message generator  1911  generates a message related to the access mode change and provides the same to the transmission modem  1913 . 
     The transmission modem  1913  converts a message or transmission data provided from the message generator  1911  into a form for transmission via a radio resource and provides the same to the duplexer  1901 . For example, the transmission modem  1913  includes a channel encoding block, a modulation block, and an RF transmission block. The channel encoding block includes a modulator, an interleaver, and a channel encoder. When a wireless communication system utilizes the OFDM scheme, the modulation block includes an Inverse Fast Fourier Transform (IFFT) operator for mapping data to each subcarrier. The RF transmission block includes a filter and an RF pre-processor. 
     In the above description of  FIG. 19 , the controller  1907  controls the message processor  1905 , the access mode change unit  1909 , and the message generator  1911 . Alternatively, the controller  1907  may perform the functions of the message processor  1905 , the access mode change unit  1909 , and the message generator  1911 . The separate configuration and illustration of the controller  1907  controls the message processor  1905 , the access mode change unit  1909 , and the message generator  1911  in  FIG. 19  is provided for separately describing each function. Therefore, in actual realization, all or some of the functions of them may be processed by the controller  1907 . 
     As described above, the various embodiments of the present invention solve an interference problem generated between neighbor NBs that use different access modes, while using the same frequency, and provide a load balancing effect in a specific area or when an event occurs by changing an access mode of an NB depending on different factors in a wireless communication system. 
     While the present invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims and their equivalents.