Abstract:
The present disclosure relates to a pre-5 th -Generation (5G) or 5G communication system to be provided for supporting higher data rates Beyond 4 th -Generation (4G) communication system such as Long Term Evolution (LTE). A method for communicating by a user equipment with a macro cell base station and a small cell base station in a communication system is provided. The method comprises applying a first base station security key to a first communication link with the macro cell base station; generating a second base station security key to be used for a second communication link with the small cell base station based on the first base station security key; applying the second base station security key to the second communication link with the small cell base station; and communicating through at least one of the first communication link and the second communication link.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application is a U.S. National Stage application under 35 U.S.C. §371 of an International application filed on Aug. 7, 2014 and assigned application number PCT/KR2014/007312, which claimed the benefit of a Korean patent application filed on Aug. 9, 2013 in the Korean Intellectual Property Office and assigned Serial number 10-2013-0094952, the entire disclosure of which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to wireless communications. More particularly, the present disclosure relates to methods and apparatuses for generating and maintaining security keys for a plurality of base stations connected to one terminal in a wireless communication system supporting a plurality of wireless link connections to one terminal. 
       BACKGROUND 
       [0003]    To meet the demand for wireless data traffic having increased since deployment of 4G (4 th -Generation) communication systems, efforts have been made to develop an improved 5G (5 th -Generation) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘beyond 4G network’ or a ‘post LTE system’. 
         [0004]    The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), full dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems. 
         [0005]    In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud radio access networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, coordinated multi-points (CoMP), reception-end interference cancellation and the like. 
         [0006]    In the 5G system, hybrid FSK and QAM modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed. 
         [0007]    Installing multiple small cells is being researched to increase the wireless network capability of a macro cell to respond to soring mobile data traffic. 
         [0008]    Small cells with small cell coverage may recycle limited frequency resources and enables a high data rate of data transmission and transmit power savings since small cell base stations are positioned relatively close to users. The nature of the small cell base station having small cell coverage may cause frequent handover and radio link failure. A scheme attracting attention to address such issue is the dual connectivity that may allow a terminal to maintain a connection with the base station of the macro cell while simultaneously receiving data through a small cell link at a high data rate. 
         [0009]    Presumably, small cell base stations may suffer from weak security as compared with macro cell base stations. Such assumption comes from the fact that, while macro cell base stations are directly managed by the communication network provider, small cell base stations scattered indoor (or in a home) are confronted with difficulty in physical management for security maintenance. 
         [0010]    The dual connectivity may allow a macro cell base station to play a role as an anchor for controlling multiple small cell base stations. Thus, if the security information regarding the macro cell base station is exposed through the small cell base stations, personal information leaks, illegal billing, or other security issues are more likely to happen. 
         [0011]    Further, assuming a few tens or a few hundreds of small cells to be installed to increase network cell capacity, control overhead and latency issues may arise due to procedures such as security key request and response ensuing when receiving the respective security keys of the small cells from a higher network (e.g., a mobility management entity (MME)). 
         [0012]    Therefore, there is a need for a scheme and procedure for effectively generating and managing an independent security key by a layered network having a macro cell and multiple small cells. 
         [0013]    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 disclosure. 
       SUMMARY 
       [0014]    Aspects of the present disclosure 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 disclosure is to provide a method and apparatus for generating and maintaining a security key for a plurality of base stations connected to a single terminal in a wireless communication system. In particular, the instant disclosure provides a method and apparatus for generating and maintaining a security key in a wireless communication system under a dual connectivity environment, i.e., under the circumstance where a terminal is simultaneously linked to a macro cell base station and a small cell base station. 
         [0015]    Another aspect of the present disclosure describes a scheme for applying and operating separate security keys for the respective packet data convergence protocol (PDCP) layers of a macro cell and a small cell, if the macro cell and the small cell have the PDCP layers, in a network where a terminal is simultaneously linked to the macro cell base station and the small cell base station that coexists in the coverage of the macro cell. 
         [0016]    Another aspect of the present disclosure is to provide a method and apparatus for generating and managing an independent security key between a macro cell and a small cell layered network in a wireless communication system simultaneously supporting multiple transmission links. 
         [0017]    Another aspect of the present disclosure provides an apparatus and method for generating, removing, or exchanging security keys when a base station is connected, released, or exchanged considering the network layer of a source base station and a target base station due to an issue that arises owing to a difference in security capacity between per-layer base stations in the layered network. 
         [0018]    Another aspect of the present disclosure provides a process for generating a security key and transmitting security key-related information when a small cell is added to a macro cell base station and data radio bearer (DRB) starts to be transmitted, when a small cell is changed (e.g., another small cell is connected), or when a small cell is released so that the macro cell resumes serving a corresponding DRB. 
         [0019]    Another aspect of the present disclosure provides an apparatus and method for independently maintaining the security keys of the macro cell and the small cells while selectively maintaining the independence of security keys between the small cells in order to address the control overhead and latency issues when independently generating security keys from a higher network (a mobility management entity (MME)). 
         [0020]    Another aspect of the present disclosure provides an apparatus and method for addressing the control overhead and latency issues in such a way as to simultaneously generate multiple security keys when independently generating security keys from a higher network (a MME). 
         [0021]    Another aspect of the present disclosure provides an apparatus and method for continuously maintaining a count value that is information to identify user data forwarded between base stations upon handover through a radio resource control (RRC) reconfiguration process and using the same PDCP configuration to prevent data loss when changing a connected base station (adding, releasing, or exchanging). 
         [0022]    In accordance with another aspect of the present disclosure, a method for communicating by a user equipment with a macro cell base station and a small cell base station in a communication system is provided. The method includes applying a first base station security key to a first communication link with the macro cell base station; generating a second base station security key to be used for a second communication link with the small cell base station based on the first base station security key; applying the second base station security key to the second communication link with the small cell base station; and communicating through at least one of the first communication link and the second communication link. 
         [0023]    In accordance with another aspect of the present disclosure, a method for communicating by a macro cell base station with a user equipment and a small cell base station in a communication system is provided. The method includes determining a first base station security key for a first communication link with the user equipment; generating a second base station security key to be used for a second communication link between the small cell base station and the user equipment based on the first base station security key; and transmitting the generated second base station security key to the small cell base station. 
         [0024]    In accordance with another aspect of the present disclosure, a user equipment communicating with a macro cell base station and a small cell base station in a communication system is provided. The user equipment includes a controller configured to apply a first base station security key to a first communication link with the macro cell base station, generate a second base station security key to be used for a second communication link with the small cell base station based on the first base station security key, and apply the second base station security key to the second communication link with the small cell base station; and a transceiver configured to communicate through at least one of the first communication link and the second communication link. 
         [0025]    In accordance with another aspect of the present disclosure, a macro cell base station communicating with a user equipment and a small cell base station in a communication system is provided. The macro cell base station includes a controller configured to determine a first base station security key for a first communication link with the user equipment, and generate a second base station security key to be used for a second communication link between the small cell base station and the user equipment based on the first base station security key; and a transceiver configured to transmit the generated second base station security key to the small cell base station. 
         [0026]    In accordance with another aspect of the present disclosure, a method of performing communication by a user terminal forming a communication link for data transmission with a macro cell base station and a small cell base station located in a communication system is provided. The method includes applying a first base station security key to a communication link with the macro cell base station, generating a second base station security key to be used for a communication link with the small cell base station, applying the second base station security key to the communication link with the small cell base station, and communicating user data through the communication links to which the security keys are applied. 
         [0027]    In accordance with another aspect of the present disclosure, a method for performing communication by a macro cell base station forming a communication link with a user terminal in a communication system is provided. The method includes a small cell base station and the user terminal, comprising: determining to add a small cell base station to form a communication link with the user terminal, sending a request for a next hop (NH) and a next hop chaining counter (NCC) to a MME, receiving a response including the NCC, generating a first base station security key to be used for a communication link between the added small cell base station and the user terminal using the NH included in the received response, and transmitting the generated first base station security key to the added base station. 
         [0028]    In accordance with yet another aspect of the present disclosure, a user terminal forming a communication link for data transmission with a macro cell base station and a small cell base station located in a communication system is provided. The user terminal includes a controller configured to apply a first base station security key to a communication link with the macro cell base station, to generate a second base station security key to be used for a communication link with the small cell base station, and to apply the second base station security key to the communication link with the small cell base station, and a transceiver configured to communicate user data through the communication links to which the security keys are applied. 
         [0029]    In accordance with still another aspect of the present disclosure, proposes a macro cell base station forming a communication link with a user terminal in a communication system including a small cell base station and the user terminal is provided. The macro cell base station includes a controller configured to determine to add a small cell base station to form a communication link with the user terminal, to send a request for a NH and a NCC to a MME, to receive a response including the NCC, and to generate a first base station security key to be used for a communication link between the added small cell base station and the user terminal using the received NH, and a transceiver configured to transmit the generated first base station security key to the added base station. 
         [0030]    Another aspect of the present disclosure, the PDCP layer exists in each of the macro cell and the small cells and separate security keys apply and operate for them in a network where the small cells coexist within the coverage of the macro cell so that two or more links are simultaneously connected to the terminal. Thus, the security of the macro cell base station may be maintained even when using the small cell base stations with relatively weak security as compared with the macro cell base station. 
         [0031]    Further, according to another aspect of the present disclosure, there is provided a method for generating security keys or transmitting information relating to the same under the circumstance where a small cell is added to the macro cell base station so that a DRB starts to be transmitted or the small cell is released so that the macro cell resumes serving the corresponding DRB. Thus, security keys for multiple small cells may be generated and control-related overhead issues may be addressed while maintaining the security of the macro cell base station for the small cell base stations. That is, an aspect of the present disclosure is to minimize security key control-related overhead while maintaining the security for an interface (X2) between the macro cell base station and a small cell base station with a relatively weak security as compared with the macro cell base station. 
         [0032]    Other aspects, advantages, and salient features of the disclosure will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses various embodiments of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    The above and other aspects, features, and advantages of certain embodiments of the present disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
           [0034]      FIG. 1  is a view illustrating a case in which a terminal establishes dual connectivity for a macro cell and small cells that coexist in a wireless communication system according to an embodiment of the present disclosure; 
           [0035]      FIG. 2  is a view illustrating a network control plane and user plane under a dual connectivity situation where a macro cell and a small cell are simultaneously connected to a terminal in a wireless communication system according to an embodiment of the present disclosure; 
           [0036]      FIG. 3  is a view illustrating an example of a protocol stack structure to support dual connectivity according to an embodiment of the present disclosure; 
           [0037]      FIG. 4  is a view illustrating another example of a protocol stack structure to support dual connectivity according to an embodiment of the present disclosure; 
           [0038]      FIG. 5  is a view illustrating another example of a protocol stack structure to support dual connectivity according to an embodiment of the present disclosure; 
           [0039]      FIG. 6  is a view illustrating an example of communication between a macro cell base station, a small cell base station, and a user terminal in a protocol stack structure where a macro cell and a small cell have an independent packet data convergence protocol. (PDCP) layer according to an embodiment of the present disclosure; 
           [0040]      FIG. 7  is a view illustrating a structure of a security key used in a 3rd Generation Partnership Project (3GPP) long term evolution (LTE) system according to an embodiment of the present disclosure; 
           [0041]      FIG. 8A  is a view illustrating an example of generating a security key and transmitting related information when handover occurs between base stations in a communication system according to an embodiment of the present disclosure; 
           [0042]      FIG. 8B  is a flowchart illustrating an example of generating a security key and transmitting related information when handover occurs between base stations in a communication system according to an embodiment of the present disclosure; 
           [0043]      FIG. 9A  is a view illustrating an example of generating a security key and transmitting related information for maintaining a separate security key when a small cell is added according to an embodiment of the present disclosure; 
           [0044]      FIG. 9B  is a flowchart illustrating an example of generating a security key and transmitting related information for maintaining a separate security key when a small cell is added according to an embodiment of the present disclosure; 
           [0045]      FIG. 10  is a view illustrating a process of generating and transmitting a security key and an initial setup process of a small cell link when a macro cell determines to add a small cell on the control plane according to an embodiment of the present disclosure; 
           [0046]      FIG. 11  is a view illustrating an process of discarding a security key and an process in which a terminal resumes communication with a macro cell base station when the macro cell determines to release a small cell on the control plane according to an embodiment of the present disclosure; 
           [0047]      FIG. 12  is a view illustrating an procedure in which a base station obtains information for generating a security key from an MME according to an embodiment of the present disclosure; 
           [0048]      FIG. 13  is a view illustrating another procedure in which a base station obtains information for generating a security key from an MME according to an embodiment of the present disclosure; 
           [0049]      FIG. 14  is a view illustrating an process of generating security keys for a macro cell base station, a small cell base station, and a user terminal and applying the generated security keys when the macro cell base station determines to add a small cell according to an embodiment of the present disclosure; 
           [0050]      FIG. 15  is a view illustrating an process of generating security keys for a macro cell base station, a small cell base station, and a user terminal and applying the generated security keys when the macro cell base station determines to release or replace a small cell according to an embodiment of the present disclosure; 
           [0051]      FIGS. 16A and 16B  are views illustrating an example of generating and transferring a security key when adding a small cell base station, when changing a small cell base station, and when releasing all of the linked small cells in a dual connectivity environment where the macro cell and the small cells use separated security keys according to an embodiment of the present disclosure; 
           [0052]      FIGS. 17A and 17B  are views illustrating an example of establishing a data radio bearer (DRB) path and generating and transmitting a security key on the control plane and user plane when adding a small cell base station, when changing a small cell base station, and when releasing all of the linked small cells in a dual connectivity environment where the macro cell and the small cells use separated security keys according to an embodiment of the present disclosure; 
           [0053]      FIG. 18  is a view illustrating a configuration of a user terminal according to an embodiment of the present disclosure; and 
           [0054]      FIG. 19  is a view illustrating a configuration of a base station according to an embodiment of the present disclosure. 
       
    
    
       [0055]    Throughout the drawings, like reference numerals will be understood to refer to like parts, components, and structures. 
       DETAILED DESCRIPTION 
       [0056]    The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the present disclosure 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 various embodiments described herein can be made without departing from the scope and spirit of the present disclosure. In addition descriptions of well-known functions and constructions may be omitted for clarity and conciseness. 
         [0057]    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 present disclosure. Accordingly, it should be apparent to those skilled in the art that the following description of various embodiments of the present disclosure is provided for illustration purpose only and not for the purpose of limiting the present disclosure as defined by the appended claims and their equivalents. 
         [0058]    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. 
         [0059]    Such denotations as “first,” “second,” “A,” “B,” “(a),” and “(b),” may be used in describing the components of the present disclosure. These denotations are provided merely to distinguish a component from another, and the essence of the components is not limited by the denotations in light of order or sequence. When a component is described as “connected,” “coupled,” or “linked” to another component, the component may be directly connected or linked to the other component, but it should also be appreciated that other components may be “connected,” “coupled,” or “linked” between the components. 
         [0060]    Before detailing the present disclosure, examples of meanings or denotations applicable to some terms used in this disclosure are proposed. However, it should be noted that the present disclosure is not limited thereto. 
         [0061]    The present disclosure targets wireless communication networks. Tasks performed over a wireless communication network may be done while a system (e.g., a base station) in charge of the wireless communication network controls the network and transmits data or may be done by a terminal coupled with the wireless network. 
         [0062]    The wireless communication system includes at least one base station (BS). Each base station provides communication services within a particular geographical area (generally referred to as a cell). A cell may be divided into multiple areas (referred to as sectors). 
         [0063]    A base station is an entity communicating with a terminal and may be denoted as, e.g., a BS, a base transceiver system (BTS), a NodeB (NB), an eNodeB (eNB), or an access point (AP). 
         [0064]    A cell should be comprehensively interpreted to denote some area covered by a base station and collectively means a mega cell, a macro cell, a small cell, a micro cell, a pico cell, a femto cell, or other various coverage areas. It should be noted that according to the context of the present disclosure the term “macro cell” may mean a base station of the macro cell, and the term “small cell” may mean a base station of the small cell. 
         [0065]    The macro cell base station may also be referred to as a macro cell eNB, macro eNB, or MeNB. 
         [0066]    The small cell is a cell with a smaller cell area than the macro cell and may include a pico cell, a femto cell, or a micro cell. The small cell base station may also be denoted as a small cell eNB, small eNB, or SeNB. 
         [0067]    A user equipment is a mobile or stationary entity communicating with a base station and may be denoted as UE, mobile station (MS), mobile equipment (ME), device, wireless device, handheld device, terminal, mobile terminal (MT), user terminal (UT), or subscriber station (SS). 
         [0068]    Downlink means communication from a base station to a terminal, and uplink means communication between a terminal to a base station. For downlink, a transmitter may be part of a base station, and a receiver may be part of a terminal. For uplink, a transmitter may be part of a terminal, and a receiver may be part of a base station. 
         [0069]      FIG. 1  is a view illustrating a case in which a terminal establishes dual connectivity for a macro cell  102  and small cells that coexist in a wireless communication system. 
         [0070]    Discussion is underway for systems that offload soring mobile traffic data by adding small cell networks to a wireless communication system (e.g., a macro cell network). As an example, the coverage of a macro cell  102  that is served by a macro cell base station  100  is denoted in solid lines, and coverages  112  and  122  of small cells that are served by small cell base stations  110  and  120  are circled in dotted lines as shown in  FIG. 1 . 
         [0071]    The small cells  112  and  122  including at least one pico cell, femto cell, or micro cell have smaller coverage but may have multiple small cell base stations installed therein, and thus, the small cells may play a role to offload soring mobile data. The small cells have a smaller transmission distance and good channel environment and they may thus provide services to users at a higher data rate and may easily recycle limited frequency resources (frequency bands) while saving power consumed by the terminal. 
         [0072]    The small cells may be confronted with frequent handoff when supporting the mobility of terminals  114  and  124  due to their smaller coverage. To support such frequent handoff, the terminal needs to simultaneously connect to the macro cell base station. Further, one terminal may be served by multiple small cell base stations. 
         [0073]    Hereinafter, dual connectivity refers to a network structure in which a terminal is served from two or more base stations connected thereto. The terminal may be served by a base station through a control channel or data channel and may be provided expanded services from multiple base stations without limited to those from two cells (the macro cell and the small cell). 
         [0074]      FIG. 2  is a view illustrating a network control plane and user plane under a dual connectivity situation where a macro cell and a small cell are simultaneously connected to a terminal in a wireless communication system. 
         [0075]    Referring to  FIG. 2 , under the dual connectivity situation where a terminal is simultaneously connected to a macro cell and a small cell, the user terminal  200  is linked to the macro cell base station (macro cell eNB (MeNB))  202  and the small cell base station (small cell eNB (SeNB))  204  through the connection of a network control lane or user plane. The macro cell base station  202  and the small cell base station  204  may be connected through, e.g., an X2 interface  206 . 
         [0076]    Under the dual connectivity situation where the small cell is added to the existing macro cell (e.g., the cellular network), the terminal is not controlled by the macro cell alone. That is, under the dual connectivity situation, the terminal may also be controlled (e.g., resource allocation) by one or more small cell base stations. 
         [0077]    A protocol stack structure in which the small cell base stations, as well as the macro cell base station, also has an independent PDCP to support dual connectivity according to an embodiment of the present disclosure is described with reference to  FIGS. 3 to 5 . 
         [0078]      FIG. 3  is a view illustrating an example of a protocol stack structure for supporting dual connectivity according to an embodiment of the present disclosure. 
         [0079]    The macro cell base station  300  and the small cell base station  310 , respectively and independently, include packet data convergence protocol (PDCP) layers  302  and  312 , radio link control (RLC) layers  304  and  314 , medium access control (MAC) layers  306  and  316 , and physical (PHY) layers  308  and  318 . 
         [0080]    Referring to,  FIG. 3  exemplifies the structure in which the user plane of the small cell base station  310  is directly connected to a core network (CN) via a separate S1 interface  320  distinguished from the user plane S1 interface  330  of the macro cell base station  300 . 
         [0081]    The small cell base station  310  is directly connected to the core network (CN) via the S1 interface  320 , and the small cell base station  310  may transmit user plane data via a separate path, not through the macro cell base station or inter-base station connection (e.g., the X2 interface). 
         [0082]      FIG. 4  is a view illustrating another example of a protocol stack structure for supporting dual connectivity according to an embodiment of the present disclosure. 
         [0083]    Referring to  FIG. 4 , the macro cell base station  300  and the small cell base station  310 , respectively and independently, include PDCP layers  302  and  312 , RLC layers  304  and  314 , MAC layers  306  and  316 , and PHY layers  308  and  318 . 
         [0084]    In particular,  FIG. 4  exemplifies the structure in which the user plane of the small cell base station  310  is connected to the core network (CN) via the macro cell base station  300  through the Xn interface (e.g., the X2 interface)  420 . 
         [0085]    In such case, since the data of all the user planes is transmitted via the macro cell base station through the inter-base station connection (X2), the transmission capacity may be limited by the latency and restricted capacity of the backhaul. That is, the connection structure shown in  FIG. 4  is a structure in which one DRB for small cells is served through one base station (any one of the macro cell base station or small cell base stations) without occurrence of DRB split for small cells. 
         [0086]      FIG. 5  is a view illustrating another example of a protocol stack structure for supporting dual connectivity according to an embodiment of the present disclosure. 
         [0087]    Referring to  FIG. 5 , the macro cell base station  300  and the small cell base station  310  respectively and independently include PDCP layers  502  and  302  and a PDCP layer  312 , RLC layers  504  and  304  and an RLC layer  314 , MAC layers  506  and  306  and an MAC layer  316 , and PHY layers  508  and  308  and a PHY layer  318 , and the user plane of the small cell base station  310  is connected to the small cell base station via the macro cell base station from the CN. 
         [0088]    In such case, since the data of all the user planes is transmitted via the macro cell base station through the inter-base station connection (the Xn interface  520 ), the transmission capacity may be limited by the latency and restricted capacity of the backhaul. The connection structure shown in  FIG. 5  is a structure in which a DRB split for small cells occurs (a split from the S1 interface  500  to the Xn interface  520 ), and one DRB is served through multiple base stations (the macro cell base station and small cell base stations). 
         [0089]    Now described is a scheme for generating, managing, and operating a security key when a macro cell and a small cell respectively include independent PDCP layers as described above in connection with  FIGS. 3 to 5 . 
         [0090]      FIG. 6  is a view illustrating an example of communication between a macro cell base station, a small cell base station, and a user terminal in a protocol layer structure where a macro cell and a small cell have an independent PDCP layer according to an embodiment of the present disclosure. 
         [0091]    Data of the control plane is transferred through the macro cell base station  300  (a link  600 ) in the embodiment of  FIG. 6 . Further, data of the user plane is served to the user terminal  630  through two DRBs, i.e., one through the macro cell base station  300  (particularly between EPS bearer  1  and the PDCP  612  of the link  602 ) and the other through the small cell base station  310  (particularly between EPS bearer  2  and the PDCP  614  of the link  604 ). 
         [0092]    The user terminal  630  forms links  600  and  602  with the macro cell base station  300  and a link  604  with the small cell base station  310  and communicate data via separate protocol layers (PHY, MAC, RLC, and PDCP layers). 
         [0093]    The PDCP layers  610 ,  612 , and  614  independently present in the macro cell and the small cell are in charge of security. In this case, required is a design as to whether the same or separate and independent security keys are used for the link  602  through the macro cell in the PDCP layer  612  of the macro cell and the link  604  through the small cell in the PDCP layer  614  of the small cell. 
         [0094]      FIG. 7  is a view illustrating a structure of a security key used in a 3GPP LTE system according to an embodiment of the present disclosure. 
         [0095]    A lower level key may be generated from a higher level key. 
         [0096]    The top level key K ( 700 ) used to authenticate the user terminal is present in the universal subscriber identity module (USIM) or authentication center (AuC). 
         [0097]    The lower level keys CK, IK ( 702 ) that may be generated from the higher level key K ( 700 ) may be known only to the UE or the home subscriber server (HSS). That is, the lower level keys CK, IK ( 702 ) is present in the UE or the HSS. 
         [0098]    The MME may generate K ASME  ( 704 ), which is a key of the access security management entity (ASME), based on the lower level key CK, IK ( 702 ). The ASME is an entity receiving the top level key of the access network from the HSS, and for the evolved universal mobile telecommunications system terrestrial radio access network (E-UTRAN), the MME may correspond to the ASME. 
         [0099]    The UE or the base station may generate an integrity key K NASint  ( 708 ) and an encryption key K NASenc  ( 706 ) for the non access stratum (NAS) based on K ASME  ( 704 ). Further, the UE or base station may generate K eNB    710  which is a base key for the security key of each base station. 
         [0100]    The UE or the base station may generate K RRCint  ( 716 ), K RRCenc  ( 714 ), and K UPenc  which are security keys for the access stratum (AS) based on K eNB  ( 710 ). 
         [0101]    Subsequently, generation and transfer of a security key are described, focusing on the authentication process. 
         [0102]    UE-MME LTE mutual authentication is carried out via an evolved packet system authentication and key agreement (EPS AKA) procedure performed between the UE, the MME, and the HSS as mutual authentication between the UE and the network. In the EPS AKA procedure, the HSS transmits an authentication vector (AV) to the MME, and the MME and the UE may perform mutual authentication using the authentication vector. As a result of the authentication, the UE and the MME share K ASME  ( 704 ) and obtain, from K ASME  ( 704 ), K NASint  ( 708 ) and K NASenc  ( 706 ), which are security keys of the NAS, and K eNB  ( 710 ), which is the base key of the base station. 
         [0103]    It is K ASME  ( 704 ) that the MME receives from the HSS. Since K ASME  ( 704 ) cannot be transferred to the UE via the E-UTRAN, K ASME  ( 704 ) may be identified by KSI ASME  that corresponds to K ASME  ( 704 ) in a one-to-one manner and replaces K ASME  ( 704 ). 
         [0104]    An integrity check and encryption may be performed on the NAS signaling message that is the control plane protocol between the UE and the MME. The integrity check is a mandatory function, and encryption is an optional function. The base key for NAS security is K ASME  ( 704 ) that is positioned in the UE and the MIME and is obtained through authentication between the subscriber and the network. NAS security keys are obtained from K ASME  ( 704 ) in the UE and the MME and come in such types as integrity keys K NASint  ( 708 ) and encryption keys K NASenc  ( 706 ). 
         [0105]    The control plane between the UE and the eNB performs an integrity check (mandatory) and encryption (optional) on RRC signaling, and the user plane performs encryption (optional) on IP packets. For access stratum (AS) security, the base key is K eNB  ( 710 ), and K eNB  ( 710 ) is positioned in the UE and the eNB and is obtained from K ASME  ( 704 ). The eNB does not include K ASME  ( 704 ). Thus, the MME generates K eNB  ( 710 ) from K ASME  ( 704 ) and transfers to the eNB. 
         [0106]    AS security keys are obtained from K eNB  ( 710 ) in the UE and the eNB and come in such types as K RRCint  ( 716 ), K RRCenc  ( 714 ), and K UPenc  ( 712 ). K RRCint  ( 716 ) and K RRCenc  ( 714 ), respectively, are used for integrity check and encryption on RRC signaling, and K UPenc  ( 712 ) is used for encryption on user plane data (IP packets). 
         [0107]    The base station (macro cell base station or small cell base station) may receive a next hop (NH)  718  and a next hope chaining counter (NCC)  720  from the MME. When the NCC  720  is transferred to the terminal, the terminal may determine the NH  718  using the NCC  720 . The terminal or the base station may generate KeNB* ( 722 ) that is a security key of the base station using the NH  718  and may apply the generated security key as the security key of the base station. KeNB* ( 722 ) may be generated based on the NH  718  or may be generated based on a previous security key  710  of the base station. 
         [0108]      FIG. 8A  is a view illustrating an example of generating a security key and transmitting related information when handover occurs between base stations in a communication system according to an embodiment of the present disclosure. 
         [0109]      FIG. 8B  is a flowchart illustrating an example of generating a security key and transmitting related information when handover occurs between base stations in a communication system according to an embodiment of the present disclosure. 
         [0110]    If the user terminal  800  reports a result of channel measurement (measurement report) to the serving base station  810 , the serving base station  810  or the MIME  830  determines to hand the user terminal  800  over to the target base station  820  based on the channel measurement result at operation  801 . 
         [0111]    The serving base station  810  generates a base station security key, KeNB*, to be used in the new base station (target base station)  820  at operation  802 . That is, the procedure of generating (or regenerating) the base station security key KeNB* to be used in the target base station  820  may be initiated by the serving base station  810  or the MME  830 . 
         [0112]    The serving base station  810  forwards KeNB* and the NCC to the target base station  820  at operation  803 . 
         [0113]    The target base station  820  informs the user terminal  800  of the NCC ( 804 ), and the user terminal  800  generates KeNB* based on the NCC at operation  805 . 
         [0114]    The user terminal  800  and the target base station  820  perform data transmission by applying the new base station security key KeNB* at operation  806 . 
         [0115]    Selectively, the target base station  820  may also receive a new NH-NCC pair (i.e., {NH, NCC}) from the MIME  830  in preparation for next handover at operation  807 . 
         [0116]    Meanwhile, the communication network including a small cell may be installed so that the coverage of the small cell overlaps the coverage of the macro cell within the macro cell coverage, and the user terminal may establish a link with each of the macro cell base station and the small cell base station at the same time (i.e., two or more links). Although handover occurs between equivalent base stations in the 3GPP LTE system, the small cell network according to an embodiment of the present disclosure overlaps the macro cell, and thus, the small cell link may be added to the link that is served only by the macro cell, the small cell link may be released, or the small cell link may be replaced with a link with a new small cell base station (by handover between the small cells). 
         [0117]    If the same security key as the one used in the macro cell is used for small cells subjected to RRC control by the macro cell, the complexity of generation of security keys may be reduced (because there is no computation for generating security keys for small cells), but if the security keys of the small cells that are relatively security vulnerable leak, the security key of the macro cell would be highly likely to leak as well. By contrast, if the macro cell and the small cell use separate independent security keys, they may have robust security but may cause complicated control and overhead in generating and managing security keys. Receiving security keys from a higher network (e.g., the MME) every handover or when installing a few tens or a few hundreds of small cells to increase network cell capacity would cause control overhead and latency issues. 
         [0118]    Accordingly, the present disclosure proposes a scheme for independently generating and managing each security key or sharing and managing a single security key according to a predetermined standard or condition in a network where a macro cell and small cells coexist. 
         [0119]    Further, the present disclosure proposes a scheme for generating, transmitting, and managing a security key for a transmission link based on a security key corresponding to another transmission link and a scheme in which each base station directly generates and manages a security key. 
         [0120]    In other words, there are proposed a scheme for generating and managing a security key considering layers between a macro cell and a small cell in generating and managing a security key per network layer, a scheme for generating, sharing, and using the same security key between the network layers of the macro cell link and the small cell link, a scheme for generating and managing separate security keys between the network layers of the macro cell link and the small cell link, a scheme for generating a security key for the small cell link based on a security key of the macro cell link, transmitting the security key for the small cell link to the small cell base station, and managing the same, and a scheme for generating a security key for the small cell link independently from a security key of the macro cell link, transmitting the security key to the small cell base station, and managing the same. 
         [0121]      FIG. 9A  is a view illustrating an example of generating a security key and transmitting related information for maintaining a separate security key when a small cell is added according to an embodiment of the present disclosure. 
         [0122]      FIG. 9B  is a flowchart illustrating an example of generating a security key and transmitting related information for maintaining a separate security key when a small cell is added according to an embodiment of the present disclosure. 
         [0123]    If the user terminal  900  reports a result of channel measurement (measurement report) to the macro cell base station  910 , the macro cell base station  910  determines whether to add the small cell base station  920  to the user terminal  900  based on the channel measurement result at operation  901 . 
         [0124]    The macro cell base station  910  or the small cell base station  920  may generate a security key, KeNB*, of the small cell base station  920  ( 902 ). That is, the procedure of generating (or regenerating) the base station security key KeNB* to be used in the small cell base station  920  may be initiated by the macro cell base station  910  or the small cell base station  920 . Selectively, the procedure of generating (or regenerating) the base station security key KeNB* to be used in the small cell base station  920  may also be initiated by the procedure of varying (or regenerating) the base station security key used in the macro cell base station  910  (the variation or regeneration of the macro cell base station security key may be initiated in the MIME or the macro cell). 
         [0125]    The security key of the small cell base station  920  may be used by the user terminal  900  for a new link for connection with the small cell base station  920 . Selectively, the macro cell base station  910  may inquire and obtain information to generate a base station security key to be used for the new link from the MME  930 . 
         [0126]    The macro cell base station  910  forwards the generated KeNB* and NCC to the added small cell base station  920  at operation  903 . 
         [0127]    The macro cell base station  910  informs the user terminal  900  of security key-related information (e.g., the NCC) at operation  904 , and the user terminal  900  generates a base station security key KeNB* to be used for the link with the small cell base station based on the security key-related information. 
         [0128]    Thereafter, the user terminal  900  and the added small cell base station  920  may perform any DRB transmission by applying the security key KeNB* that they generated on their own or received at operation  905 . 
         [0129]    Embodiments of sharing security key information considering the relationship between base stations forming links with a user terminal are now described. 
         [0130]    According to an embodiment of the present disclosure, the security key of the macro cell base station may be shared by the small cell base station. 
         [0131]    In such case, the small cell base station may generate and use the security key of the small cell base station using (based on) the shared (transferred) security key of the macro cell base station. 
         [0132]    Further, the small cell base station may transmit the security key of the macro cell base station or the generated security key of the small cell base station to another small cell base station so that the other small cell base station by itself may generate and use its security key. 
         [0133]    Meanwhile, the security key of the other small cell base station may also be generated by the macro cell base station. That is, the security key of the small cell base station may be transferred (shared) to the macro cell base station, and the macro cell base station receiving the security key of the small cell base station may generate the security key of the other small cell base station using (based on) the security key of the small cell base station and transfer to the other base station so that the other small cell base station may use the same. 
         [0134]    According to another embodiment of the present disclosure, the security key of the macro cell base station might not be shared by the small cell base station. 
         [0135]    In this case, the small cell base station receives and uses the security key (not the security key of the macro cell base station) of the small cell base station that is generated and transferred by the macro cell base station. 
         [0136]    The security key of the small cell base station may be shared or not by the macro cell base station. 
         [0137]    In case the security key of the small cell base station is shared by the macro cell base station, the macro cell base station generates the security key of the other small cell base station using (based on) the shared security key of the small cell base station and transmits the generated security key of the other small cell base station so that the other small cell may use it. 
         [0138]    Unless the security key of the small cell base station is shared by the macro cell base station, the small cell base station generates the security key of the other small cell base station and transmits the security key to the other small cell base station, or the macro cell base station generates a security key independent from the security key of the small cell base station and transmits the independent security key to the other small cell base station so that the other small cell base station may use it. 
         [0139]      FIG. 10  is a view illustrating the process of generating and transmitting a security key and an initial setup process of a small cell link when a macro cell determines to add a small cell on the control plane according to an embodiment of the present disclosure. 
         [0140]    If the user terminal  900  reports a result of channel measurement (measurement report) to the macro cell base station  910  ( 1002 ), the macro cell base station  910  determines whether to additionally link the small cell base station to the user terminal  900  based on the channel measurement result and generates a base station security key KeNB* to be used for the new link to be connected to the small cell base station  920  ( 1004 ). 
         [0141]    The macro cell base station  910  includes KeNB* and/or NCC in a small cell add request message (SCELL ADDITION REQUEST) and forwards the same to the small cell base station  920  ( 1006 ). 
         [0142]    The added small cell base station  920  responds by transferring a small cell add acknowledgement message (SCELL ADDITION ACK) to the macro cell base station  910  ( 1008 ). Selectively, the small cell add acknowledgement message may contain an NCC value. 
         [0143]    The macro cell base station  910  transfers a message for RRC reconfiguration (rrcConnectionReconfiguration) to the user terminal  900  ( 1010 ). Selectively, the message for RRC reconfiguration may contain the NCC value transferred from the small cell base station  920 . 
         [0144]    Having received security key-related information (e.g., the NCC) through the RRC reconfiguration message  1010 , the user terminal  900  may generate a base station security key KeNB* for a new link to be connected with the small cell base station  920  ( 1012 ). 
         [0145]    The macro cell base station  910  receives a message (rrcConnectionReconfigurationComplete) responding to the RRC reconfiguration message from the user terminal  900  ( 1014 ). 
         [0146]    The macro cell base station  910  transmits the RRC reconfiguration message rather than sending out an RRC release message or RRC reset message to the user terminal  900  in order to prevent data transferred to the user terminal  900  from being lost while forwarded from the macro cell base station  910  to the small cell base station  920 . That is, use of the RRC release message or RRC reset message may cause the COUNT value forwarded to identify the user data to be initialized, thus leading to data loss. Accordingly, upon RRC configuration, an RRC reconfiguration process is performed to maintain the COUNT value. The COUNT value is a value recorded in the PDCP layer and is an index to identify data transferred to the user terminal. For example, the COUNT value may be a value ranging from  0  to  500  and may be used for retransmission of data that may be lost upon data forwarding. 
         [0147]    Selectively, the macro cell base station  910  may perform a process  1016  of transferring the COUNT value through, e.g., a sequence number status transfer (SN STATUS TRANSFER) message from the previous transmission link (i.e., the macro cell base station) to the target transmission link (i.e., the small cell base station) before forwarding the data to the small cell base station  920 . 
         [0148]    Subsequently, the macro cell base station  910  may forward user data to be transferred to the user terminal  900  to the small cell base station  920  ( 1018 ). Selectively, the small cell base station  920  may perform management to detect or prevent data loss during the process  1018  of forwarding data from the macro cell base station  910  to the small cell base station  920  by using the COUNT value included in the sequence number status transfer (SN STATUS TRANSFER) message  1016 . 
         [0149]    Then, the user terminal  900  may perform transmission of a DRB with the small cell base station  920  based on the newly generated security key KeNB*. Selectively, the process of transmitting the DRB may include one or more of random access of the user terminal  900  to the small cell base station  920  ( 1020 ), PDCP status reporting ( 1022 ), applying the newly generated security key KeNB* to the newly generated data radio bearer (denoted “DRB 2”) ( 1024 ), and transmitting a physical downlink shared channel (PDSCH) and/or physical uplink shared channel (PUSCH) for the DRB 2 ( 1026 ). 
         [0150]      FIG. 11  is a view illustrating a process of discarding a security key and a process in which a terminal resumes communication with a macro cell base station when the macro cell determines to release a small cell on the control plane according to an embodiment of the present disclosure. 
         [0151]    If the user terminal  900  reports the channel measurement result (measurement report) to the macro cell base station (macro ENB)  910  ( 1102 ), the macro cell base station  910  determines whether to release the link to the small cell base station  920  connected with the user terminal  900  (i.e., the small cell link) based on the channel measurement result and determines whether to apply the data radio bearer DRB2 used to be connected to the small cell base station  920  back to itself (i.e., the macro cell base station  910 ) ( 1104 ). 
         [0152]    The macro cell base station  910  transmits a small cell release request (SCELL RELEASE REQUEST) message to the small cell base station  920  ( 1106 ), and the small cell base station  920  responds by transmitting a small cell release acknowledgment (SCELL RELEASE ACK) message to the macro cell base station  910  ( 1108 ). 
         [0153]    Selectively, the small cell base station  920  may perform a process  1110  of transferring a COUNT value to identify user data to the macro cell base station  910  through a sequence number status transfer (SN STATUS TRANSFER) message before forwarding data to the macro cell base station  910  as the small cell link is released. 
         [0154]    Subsequently, the small cell base station  920  may forward user data to be transferred to the user terminal  900  to the macro cell base station  910  ( 1112 ). Selectively, the macro cell base station  920  may perform management to detect or prevent data loss during the process  1112  of forwarding data from the small cell base station  920  to the macro cell base station  910  by using the COUNT value included in the sequence number status transfer (SN STATUS TRANSFER) message  1110 . 
         [0155]    The macro cell base station  910  transmits, to the user terminal  900 , a message (rrcConnectionReconfiguration) for RRC reconfiguration ( 1114 ) and receives an RRC reconfiguration complete message (rrcConnectionReconfigurationComplete) from the user terminal  900  ( 1116 ). Selectively, the RRC reconfiguration message  1114  may contain an NCC value. 
         [0156]    The macro cell base station  910  transmits the RRC reconfiguration message rather than an RRC release message or RRC reset message to the user terminal  900  for RRC configuration in order to continuously maintain the COUNT value. 
         [0157]    The user terminal  900  may discard the security key KeNB* generated for the small cell link and applies the security key KeNB of the macro cell base station to DRB 2 ( 1118 ) and may perform DRB 2 PDSCH and/or PUSCH transmission ( 1120 ). 
         [0158]    The macro cell base station or the small cell base station may previously receive multiple security key-related information items from a higher network entity (e.g., the MME) upon generating independent security keys and may use the information items to generate security keys. 
         [0159]    At this time, the macro cell base station may access the MME and receive multiple security key-related information items (seed) to generate a security key and may transmit the security key to the small cell link. Further, a representative small cell base station may directly access the ME to receive multiple security key-related information items (seed) and forward them to a neighboring small cell base station, or each small cell base station may directly access the MME to receive security key-related information (e.g., seed) to generate an independent security key. 
         [0160]    A base station should receive an NH from the MME in order to generate a new next hope (NH)-based security key that is not based on the security key KeNB of the macro cell base station. A process for the same is described with reference to  FIGS. 12 and 13 . 
         [0161]      FIG. 12  is a view illustrating a procedure in which a base station obtains information for generating a security key from an MME according to an embodiment of the present disclosure. 
         [0162]    Referring to  FIG. 12 , an NH/NCC request (NH/NCC REQUEST) message is used as a new message to receive the NH/NCC from the MME. 
         [0163]    If the macro cell base station  910  determines to generate a security key which requires a new NH-NCC pair (i.e., generating a vertical key) ( 1202 ), the macro cell base station  910  transmits an NH/NCC request (NH/NCC REQUEST) message to the MME  930  ( 1204 ). The MME  930  responds by sending out an NH/NCC response (NH/NCC RESPONSE) message to the macro cell base station  910  ( 1206 ). 
         [0164]    Here, the NH/NCC response message  1206  may contain the NH-NCC pair. Selectively, the NH/NCC response message  1206  may include a plurality of NH-NCC pairs {NH, NCC}. Since the small cell base station has small cell coverage, handover or adding, release, or change of small cells may be frequent, and thus, more NH-NCC pairs {NH, NCC} are required for generating security keys for base stations. 
         [0165]    Although the macro cell base station is an entity to send out the NH/NCC request message  1204  to request {NH, NCC}, for example, the small cell base station may, in some cases, send out the message to obtain {NH, NCC} from the MME. 
         [0166]      FIG. 13  is a view illustrating another procedure in which a base station obtains information for generating a security key from an MME according to an embodiment of the present disclosure. 
         [0167]    A path switch request (PATH SWITCH REQUEST) message is a message transmitted from a base station to the MME to change or switch data transmission paths (PATH) when handover occurs between base stations. In the embodiment shown in  FIG. 13 , the path switch request (PATH SWITCH REQUEST) message is used to receive the NH/NCC from the MME. 
         [0168]    If the macro cell base station  910  determines to generate a security key which requires a new NH-NCC pair (i.e., generating a vertical key) ( 1302 ), the macro cell base station  910  transmits the path switch request (PATH SWITCH REQUEST) message to the MME  930  ( 1304 ). 
         [0169]    In case a small cell base station is added, released, or changed, the data transmission path to the small cell base station may be varied or not depending on the structure of the user plane. 
         [0170]    As an example, since the data transmission path interface  320  of the small cell becomes different from the data transmission path interface  330  of the macro cell in the case shown in  FIG. 3 , a path switch occurs. 
         [0171]    As another example, in the case shown in  FIGS. 4 and 5 , although a small cell is added, the data transmission path is not changed (the same path passing through the macro cell as the one before the small cell is added). In such case, the path switch request message may contain, e.g., the same transmission layer address as the present one and E-radio access bearer (E-RAB) information. 
         [0172]    Unless the path switch request (PATH SWITCH REQUEST) message received from the macro cell base station  910  indicates a path switch or change (e.g., the case shown in  FIGS. 4 and 5 ), the MME  930  performs only operations regarding the NH and NCC without a path switch, while in case the path switch request (PATH SWITCH REQUEST) message indicates a path switch, the MME  930  performs operations regarding the NH and NCC and a data path switching operation ( 1306 ). That is, the MME  930  transmits a path switch request acknowledgment (PATH SWITCH REQUEST ACK) message containing the NH-NCC pair to the macro cell base station  910  in response to the path switch request message ( 1308 ). 
         [0173]    Selectively, the path switch request acknowledgment (PATH SWITCH REQUEST ACK) message  1308  may include a plurality of NH-NCC pairs {NH, NCC}. Since the small cell base station has small cell coverage, handover or adding, release, or change of small cells may be frequent, and thus, more NH-NCC pairs {NH, NCC} are required for generating security keys for base stations. 
         [0174]    Although the macro cell base station is an entity to send out the path switch request (PATH SWITCH REQUEST) message  1304  to request {NH, NCC}, for example, the small cell base station may, in some cases, send out the message to obtain {NH, NCC} from the MME. 
         [0175]    Described is an embodiment of independently generating and managing a security key when a user terminal adds a link with a small cell base station. 
         [0176]    According to an embodiment of the present disclosure, there is proposed a scheme for selectively maintaining the independence of security keys between small cell base stations while managing a security key of a macro cell and security keys of small cells to be independently generated. That is, the security keys between the small cells may be determined to be generated independently from each other in some cases or to be dependent, the same or similar to each other in other cases. 
         [0177]    Specifically, a small cell first added to the macro cell coverage may be rendered to generate a security key independent from the security key of the macro cell base station. A small cell added next to the first small cell may generate a dependent security key based on the security key of the first small cell. 
         [0178]    Further, under a certain condition, it may be determined to generate a security key independent from a small cell base station added. For example, the condition to generate a security key independent from a small cell base station added may include when a cluster of small cells positioned within a predetermined physical distance is varied or when a timer to generate an independent security key expires (i.e., a predetermined time or more elapses after a previous independent security key is generated). 
         [0179]      FIG. 14  is a view illustrating a process of generating security keys for a macro cell base station, a small cell base station, and a user terminal and applying the generated security keys when the macro cell base station determines to add a small cell according to an embodiment of the present disclosure. 
         [0180]    Selectively, the small cell base station may be notified that small cell ServCell_s is added by receiving information such as SCellToAddRemove information element (IE) from the macro cell base station ( 1400 ). Here, ServCell_s means a serving cell controlled by the small cell base station. For example, the SCellToAddRemove IE may be contained in a small cell add request (SCELL ADD REQUEST) message. 
         [0181]    The macro cell base station (or small cell base station or user terminal) determines whether the small cell to be added is a small cell first added ( 1402 ). Further, the user terminal may generate a security key for the small cell base station (a security key for a DRB to be added) through operations  1410  and  1412  for determining an NCC value transmitted from the macro cell base station (or small cell base station). That is, different security key generating methods may apply depending on the NCC value transmitted from the base station. 
         [0182]    A process of generating and applying a security key is described in detail. 
         [0183]    In case as a result of the determination  1402  the small cell is first added, the user terminal sets the sum of the received NCC_s value and one, i.e., NCC_s+1, to a local NCC_s value ( 1404 ). Here, NCC_s is an NCC maintained for the security key KeNB_s of the small cell base station. The next hop chaining counter (NCC) may be represented in three bits, and at this time, may be used to distinguish the security keys for eight base stations for one K ASME . 
         [0184]    The user terminal selects the ID of the macro cell base station, physical cell ID (PCI), and operation frequency, downlink-EUTRAN absolute radio frequency channel number (DL-EARFCN), according to a predetermined rule ( 1406 ). 
         [0185]    Subsequently, the user terminal may generate a security key KeNB_s* for the small cell base station as in Equation 1 by applying the received NCC_s, the PCI, and the DL_EARFCN to a key derivation function (KDF) ( 1408 ). 
         [0000]      KeNB_ s *=KDF[NH(NCC_ s ),PCI,DL-EARFCN]   Equation 1
 
         [0186]    Here, NH(NCC_s) is a function to calculate next hop (NH) using NCC_s. 
         [0187]    As such, the method of generating a new security key using the NH value to independently maintain the security keys of the macro cell base station and the small cell base station is called vertical security key derivation. That is, when the received NCC value differs from the previous NCC (local NCC), it may be determined to generate a new security key according to the vertical security key derivation method. 
         [0188]    If it is determined in  1402  that the small cell is not first added (i.e., when there are one or more small cells already added), the user terminal determines whether the NCC_s has been signaled from the macro cell base station ( 1410 ). 
         [0189]    In case it is determined in  1410  that the NCC_s has not been signaled, the user terminal may determine to use (recycle) the existing security key of the small cell base station without generating a security key ( 1424 ). 
         [0190]    Thus, the user terminal applies the security key KeNB_s for a logical channel LCH_s served by the small cell and applies the security key KeNB_m for a logical channel LCH_m served by the macro cell ( 1422 ). 
         [0191]    Meanwhile, in case it is determined in  1410  that the NCC_s has been signaled, the user terminal determines whether the signaled (received) NCC_S is the same as the local NCC_S ( 1412 ). 
         [0192]    In case a result of the  1412  determination indicates “the same,” the user terminal may select a PCI and DL-EARFCN according to a predetermined rule ( 1416 ) and may apply the present security key KeNB_s of the small cell base station, the PCI, and the DL-EARFCN to the key derivation function to generate a security key KeNB_s* for the small cell base station as in Equation 2 ( 1418 ). 
         [0000]      KeNB_ s *=KDF[KeNB_ s ,PCI,DL-EARFCN]  Equation 2
 
         [0193]    The method of generating a new security key based on an existing base station security key as shown in Equation 2 is called a horizontal security key derivation method. That is, when the received NCC value is the same as the previous NCC (local NCC), it may be determined to generate a new security key according to the horizontal security key derivation method. 
         [0194]    In other words, even though the small cell base station (or the macro cell base station) does not receive control information (e.g., NH or NCC) to generate a new security key from the MME, the user terminal generates a new security key based on the existing small cell base station security key. This way may reduce overhead that may be caused by control signaling in the MME due to frequent security key generation tasks. 
         [0195]    Unless the result of the  1412  determination indicates “the same,” the user terminal sets the received NCC_s value to the local NCC_s value ( 1414 ) and performs the selection of PCI and DL-EARFCN ( 1406 ) and generation of a key using the NCC_s ( 1408 ). 
         [0196]    If the security key of the small cell base station is generated through the operation  1418  or  1408 , the user terminal sets the generated base station security key KeNB_s* to the new base station security key KeNB_s ( 1420 ) and may apply the security key to the logical channel for data transmission ( 1422 ). 
         [0197]    Selectively, independent security keys respectively for the macro cell base station and the small cell base station may be generated according to a determination by the macro cell or a rule under a particular condition or need. For example, the macro cell base station or the small cell base station (or the user terminal) may operate a security key generation timer to perform management so that if a predetermined time elapses, security keys may be independently and periodically generated or may perform management so that independent security keys may be generated when the number of small cell base stations recycling security keys exceeds a predetermined number. 
         [0198]      FIG. 15  is a view illustrating a process of generating security keys for a macro cell base station, a small cell base station, and a user terminal and applying the generated security keys when the macro cell base station determines to release or replace a small cell according to an embodiment of the present disclosure. 
         [0199]    Selectively, the small cell base station may be notified that small cell ServCell_s is released by receiving information such as SCellToAddRemove IE from the macro cell base station ( 1500 ). For example, the SCellToAddRemove IE may be contained in a small cell release request (SCELL RELEASE REQUEST) message. 
         [0200]    The macro cell base station (or small cell base station or user terminal) determines whether the small cell to be released is a small cell released last ( 1502 ). Further, the user terminal may determine a base station security key to be used through the operation of determining an NCC value transmitted from the macro cell base station (or the small cell base station). That is, different base station security key applying methods may apply depending on the NCC value transmitted from the base station. 
         [0201]    A process of generating and/or applying a security key is described in detail. 
         [0202]    In case it is determined in  1502  that the small cell is released last, the user terminal releases the existing security key KeNB_s of the small cell base station and resets (i.e., initializes) the local NCC_s value ( 1504 ). Accordingly, the security key of the macro cell base station, KeNB_m, may apply to both the logical channel LCH_m served by the macro cell and the logical channel LCH_s served by the small cell (i.e., the logical channel to replace the released small cell) ( 1506 ). Here, although the logical channel when the small cell is released is denoted as LCH_s for convenience, the logical channel LCH_s with the small cell base station released (accordingly not present any longer) should be interpreted to mean a logical channel with the macro cell base station (however it is denoted). 
         [0203]    If it is determined in  1502  that the small cell is not released last (i.e., when there are one or more small cells left after the release), the user terminal determines whether the NCC_s has been signaled from the macro cell base station ( 1508 ). 
         [0204]    In case it is determined in  1508  that the NCC_s has not been signaled, the user terminal does not generate a security key. The user terminal determines to use (i.e., recycle) the existing security key of the small cell base station as a base station security key to replace the released small cell ( 1514 ). 
         [0205]    In case it is determined in  1508  that the NCC_s has been signaled, the user terminal generates a security key of the small cell base station to replace the released small cell using the vertical security key derivation method or horizontal security key derivation method according to the signaled (or received) NCC_s value similar to when a small cell is added ( 1510 ). 
         [0206]    If the security key of the small cell base station is determined (generated) according to operation  1514  or  1510 , the security key KeNB_m of the macro cell base station applies to the logical channel LCH_m served by the macro cell, and the determined (generated) security key KeNB_s of the small cell base station applies to the logical channel LCH_s served by the small cell (i.e., the logical channel with the base station to replace the released small cell) ( 1512 ). 
         [0207]      FIGS. 16A and 16B  are views illustrating an example of generating and transferring a security key when adding a small cell base station, when changing a small cell base station, and when releasing all of the linked small cells in a dual connectivity environment where the macro cell and the small cells use separated security keys according to an embodiment of the present disclosure. 
         [0208]    The embodiment shown in  FIGS. 16A and 16B  is regarding a scenario in which the user terminal initially receiving two DRBs through the macro cell base station using the security key KeNB of the macro cell base station receives one of the DRBs (referred to as DRB 2 and indicating the LCH_s as shown in  FIGS. 16A and 16B ) through a small cell base station 1 added and then hands over to a small cell base station 2 for the DRB 2 and finally releases the link with the small cell base station 2 to receive both the DRBs from the macro cell base station. 
         [0209]    The security key KeNB_m of the macro cell base station may be obtained, as in Equation 3, using a key derivation function whose input values include the macro cell base station ID, which is the physical cell ID (PCI), the operation frequency, which is the downlink-EUTRAN absolute radio frequency channel number (DL_EARFCN), and the next hop (NH) ( 1610 ). 
         [0000]      KeNB_ m =KDF[NH( n ),PCI,DL-EARFCN]=K1  Equation 3
 
         [0210]    Initially, the user terminal  1600  is served two DRBs (DRB 1 indicating LCH_m and DRB 2 indicating LCH_s) by the macro cell base station  1602 , and the security key of the macro cell base station is commonly used for the two DRBs ( 1612  and  1614 ). 
         [0211]    In case a variation in channel status is detected based on a channel measurement report of the user terminal, the macro cell base station  1602  adds a new small cell and determines to connect one (here, DRB 2) of the user DRBs to the small cell base station  1604  ( 1616 ). The macro cell base station  1602  generates an NH value for use in vertical security key generation ( 1618 ). 
         [0212]    At this time, the security key for the new small cell base station  1604  is obtained as in Equation 4 using the KDF whose input values include the PCI, which is the ID of the small cell base station, the DL-EARFCN, which is the operation frequency, and the new NH value ( 1620 ). That is, the security key for the new small cell base station  1604  may be generated by the vertical security key derivation method. 
         [0000]      KeNB_ s *=KDF[NH( n +1),PCI,DL-EARFCN]=K2   Equation 4
 
         [0213]    Here, n is the value of NCC_m which is an NCC maintained by the security key of the macro cell base station. 
         [0214]    If the macro cell base station  1602  transmits the security key KeNB_s* generated for the small cell base station to the small cell base station ( 1656 ), the small cell base station applies the received KeNB_s* as a security key of the new base station. 
         [0215]    The macro cell base station transmits the NCC_s information to the user terminal  1600  as well ( 1658 ). The user terminal  1600  may restore (generate) and use the security key KeNB_s* for the DRB 2 connected to the small cell base station 1  1604  based on the received NCC_s. 
         [0216]    Specifically, the user terminal  1600  makes comparison as to the received NCC_s ( 1622 ). The NCC_s is initialized to the local NCC when handover occurs (or the initial small cell is added), and in case the new NCC_s value is signaled, it is updated with the received NCC_s value. When the received NCC_s value differs from the local NCC_s value, the user terminal  1600  may generate an NH with the received NCC_s value ( 1624 ) and may generate a value K2 of the security key KeNB_s of the small cell base station using the PCI and the DL-EARFCN values ( 1626 ). Then, the security key KeNB_m may apply to DRB 1 ( 1628 ), and the security key KeNB_s may apply to DRB 2 ( 1630 ). 
         [0217]    Next, the channel condition changes, and the macro cell base station  1602  determines to switch (i.e. handover) the DRB 2 connection of the user terminal  1600  from the small cell base station 1  1604  to the new small cell base station 2  1606  ( 1632 ). At this time, the security key for the small cell base station 2  1606  is obtained as in Equation 5 using the key derivation frequency (KDF) based on the PCI, which is the ID of the small cell base station 2  1606 , the operation frequency DL-EARFCN, and the security key KeNB_s of the previous small cell base station. That is, the security key for the replacing small cell base station  1606  may be generated by the horizontal security key derivation method ( 1634 ). 
         [0000]      KeNB_ s *=KDF[KeNB_ s ,PCI,DL-EARFCN]=K3  Equation 5
 
         [0218]    The macro cell base station  1602  may transmit the security key KeNB_s* (K3) generated for the small cell base station 2  1606  to the small cell base station 2  1606  ( 1660 ). Further, the macro cell base station  1602  transmits the NCC_s information to the user terminal  1600  as well ( 1662 ). The user terminal  1600  may make comparison as to the received NCC_s (, SeNB NCC), and restore (generate) and use the security key KeNB_s* for the DRB 2 connected to the small cell base station 2  1606  based on the NCC_s ( 1636  and  1638 ). As a result, the user terminal may apply the base station security keys for DRB 1 and DRB 2 ( 1640  and  1642 ). 
         [0219]    Next, the channel condition changes, and if the macro cell base station  1602  determines to release the DRB 2 which is the connection of the user terminal  1600  to the small cell base station 2  1606  and to connect the same to the macro cell base station  1602  (hand over) ( 1644 ), the macro cell base station  1602  transmits a small cell release (SCELL RELEASE) message to the user terminal  1600  ( 1646 ). 
         [0220]    When releasing the last small cell base station ( 1648 ) (i.e., when there is no more serving cell to be linked to the LCH_s), the macro cell base station  1602  discards the security key KeNB_s of the small cell base station 2 that has been used and resets the NCC_s ( 1650 ). It performs transmission with the security key KeNB_m (k1) of the macro cell base station  1602  applied to DRB 2 as well as DRB 1 ( 1652  and  1654 ). 
         [0221]      FIGS. 17A and 17B  are views illustrating an example of establishing a DRB path and generating and transmitting a security key on the control plane and user plane when adding a small cell base station, when changing a small cell base station, and when releasing all of the linked small cells in a dual connectivity environment where the macro cell and the small cells use separated security keys according to an embodiment of the present disclosure. 
         [0222]    The embodiment shown in  FIGS. 17A and 17B  is also regarding a scenario in which the user terminal initially receiving two DRBs through the macro cell base station using the security key KeNB of the macro cell base station receives one of the DRBs (referred to as DRB 2 and indicating the LCH_s as shown in  FIGS. 17A and 17B ) through a small cell base station 1 added and then hands over to a small cell base station 2 for the DRB 2. 
         [0223]    In case the user terminal  1700  which has initially been connected with the macro cell base station  1702  alone determines to add a small cell base station to receive a service ( 1710 ), the macro cell base station  1702  transmits a small cell add request to the small cell base station  1704  while generating and forwarding a security key KeNB_s* ( 1712 ). The small cell base station 1  1704  responds to the request  1712  ( 1714 ). The macro cell base station  1702  transmits a sequence number status transfer (SN STATUS TRANSFER) message including a COUNT value to the small cell base station  1704  ( 1720 ), and forwards user data to be transmitted to the user terminal to the small cell base station  1704  ( 1722 ). 
         [0224]    The macro cell base station  1702  stops the logical channel LCH_s through which the user terminal  1700  connects to the macro cell base station  1702  ( 1716 ) and transmits an RRC reconfiguration message for connection with the small cell base station 1  1704  and the user terminal  1700  to the user terminal  1700  while informing the user terminal  1700  of the NCC_s information ( 1718 ). 
         [0225]    The user terminal  1700  stops the logical channel LCH_s connecting with the macro cell base station ( 1724 ) and generates the KeNB_s based on the NCC_s information and applies the same to the DRB 2 ( 1726 ). Here, although the logical channel through which the user terminal communicates with the macro cell base station is denoted LCH_s for convenience, the LCH_s when the communication with the macro cell base station is stopped for communicating with a new small cell base station should be interpreted to mean a logical channel with the macro cell base station (regardless of however it is denoted). 
         [0226]    Subsequently, the user terminal  1700  sends an RRC reconfiguration complete message to the macro cell base station  1702  ( 1728 ). The user terminal  1700  starts to transmit DRB 2 with the small cell base station  1704  ( 1732 ). In this case, the security key KeNB_s applies for a logical channel LCH_s served by the small cell, and the security key KeNB_m applies for a logical channel LCH_m served by the macro cell ( 1734  and  1736 ). 
         [0227]    Selectively, PDCP status reporting  1738  may occur between the user terminal  1700  and the small cell base station  1704 , and radio environment measurement reporting  1740  may occur from the user terminal  1700  to the macro cell base station  1702 . 
         [0228]    Next, in case the macro cell base station  1702  determines to change the small cell base station to be used for DRB 2 from the small cell base station 1  1704  to the small cell base station 2  1706  ( 1742 ), the macro cell base station  1702  sends a request message to release the small cell base station 1  1704  to the small cell base station 1  1704  ( 1744 ). At this time, the small cell base station 1  1704  stops transmitting the DRB 2 ( 1746 ) and sends a response to the request message  1744  to the macro cell base station  1702  ( 1748 ). At this time, the macro cell base station  1702  may generate and transfer the security key KeNB_s* while sending a request to add a small cell to the small cell base station 2  1706  ( 1750 ). The small cell base station 2  1706  responds to the add request  1750  ( 1752 ). 
         [0229]    The macro cell base station  1702  performs RRC reconfiguration for connection with the new small cell base station 2  1706  and the user terminal  1700  ( 1754  and  1760 ). The macro cell base station  1702  may pass the NCC_s over while sending the RRC connection reconfiguration message  1754  to the user terminal  1700 . The user terminal  1700  stops the logical channel LCH_s for DRB 2 ( 1756 ), generates a new small cell base station security key KeNB_s* (i.e., a horizontal base station security key) based on the received NCC_s, and applies the same to the DRB 2 ( 1758 ). 
         [0230]      FIG. 18  is a view illustrating a configuration of a user terminal according to an embodiment of the present disclosure. 
         [0231]    The UE  1800  includes a transceiver  1810  to communicate signals and a controller  1805  to control the overall operation of the UE  1800 . The transceiver  1810  may play a role to communicate various control signals and data with a macro cell base station or a small cell base station. The operations of the user terminal as exemplified herein may be appreciated as performed under the control the controller  1805 . 
         [0232]    Although the transceiver  1810  and the controller  1805  are shown as if they are separate components, they may also be implemented in a single component. 
         [0233]      FIG. 19  is a view illustrating a configuration of a base station according to an embodiment of the present disclosure. 
         [0234]    The base station  1900  is an example of a macro cell base station or small cell base station as described herein. 
         [0235]    The base station  1900  includes a transceiver  1910  to communicate signals and a controller  1905  to control the overall operation of the base station  1900 . The transceiver  1910  may play a role to communicate various control signals and data with a user terminal or a small cell base station. The operations of the macro cell base station or small cell base station as exemplified herein may be appreciated as performed under the control the controller  1905 . 
         [0236]    Although the transceiver  1910  and the controller  1905  are shown as if they are separate components, they may also be implemented in a single component. 
         [0237]    It should be noted that examples of inter-system signal transfer, generation of security keys, and configuration of the apparatus as illustrated in  FIGS. 3 to 19  are not intended to limit the scope of the present disclosure. In other words, all the entities, operations or components illustrated in  FIGS. 3 to 19  should not be construed as essential components to practice the present disclosure, and the present disclosure may be rather implemented with only some of the components without departing from the gist of the present disclosure. 
         [0238]    The above-described operations may be realized by equipping a memory device retaining their corresponding codes in the entity, base station, or user terminal of the communication system. That is, the controller in the entity, the base station, or user terminal may execute the above-described operations by reading and executing the program codes stored in the memory device by a processor or central processing unit (CPU). 
         [0239]    As described herein, various components or modules in the entity, the base station, or the user terminal may be operated using a hardware circuit, e.g., a complementary metal oxide semiconductor-based logic circuit, firmware, software, and/or using a hardware circuit such as a combination of hardware, firmware, and/or software embedded in a machine-readable medium. As an example, various electric structures and methods may be executed using electric circuits such as transistors, logic gates, or ASICs. 
         [0240]    While the present disclosure has been shown and described with reference to various 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 disclosure as defined by the appended claims and their equivalents.