Abstract:
A method and radio network controller (RNC) in a radio access network (RAN) for controlling access to a cellular telecommunication system. Upon receiving an access request from a given user equipment (UE) through a given radio base station (RBS), the RNC retrieves authorization information from an access control database within the RAN. The authorization information indicates whether the given UE is authorized to access the system through the given RBS. The RNC alternatively grants access or denies access to the UE based on the retrieved authorization information. The RBS is particularly useful for controlling access through small cells with limited capacity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. Provisional Application No. 60/747,693 filed May 19, 2006, the disclosure of which is incorporated herein by reference. 
     
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT:  
       [0002]    NOT APPLICABLE 
       REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX:  
       [0003]    NOT APPLICABLE 
       BACKGROUND OF THE INVENTION 
       [0004]    This invention relates to mobile communication systems. More particularly, and not by way of limitation, the invention is directed to an apparatus and method for fine-grained access control in a mobile communication network. 
         [0005]    In cellular communication networks such as GSM, small cells known as picocells may serve small areas such as parts of a building, a street corner, or an airplane cabin. A picocell is generally smaller than a microcell, which in turn, is smaller than a macrocell. Picocells are usually used to extend coverage to indoor areas where outdoor signals do not reach well, or to add network capacity in areas with very dense phone usage, such as train stations. The picocells are traditionally provided as coverage or capacity extensions and do not include an access control mechanism. This means that all users allowed to access the macrocells of a public land mobile network (PLMN) will also be allowed to access the microcells and picocells of that same PLMN. 
         [0006]    In addition, the term “femtocell” is being introduced (to indicate that the coverage area is relatively small and in some cases even smaller than the coverage of picocells). These femtocells are normally meant for a limited subset of the subscribers allowed access to the macrocells of a PLMN. For example, a femtocell may cover only the geographical area of a house and the allowed users may be the members of the family living in that house. So clearly a new access control mechanism is needed to control access to the femtocells. If the existing core network (CN)-level access control is used for the access control mechanism for the femtocells, it would add a significant signaling load to the core network because the core network would be involved in all access control procedures. 
         [0007]    In Wideband Code Division Multiple Access (WCDMA) cellular networks, there is currently no equivalent to femtocells with access control support. Current access control procedures are not fine-grained enough to support a large number of very small cells. 
         [0008]    It would be advantageous to have an access control apparatus and method, which reduces the signaling load on the core network while providing access control sufficiently fine-grained to support a large number of very small cells. The present invention provides such an apparatus and method. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    In one embodiment, the present invention provides access control in the RAN without interaction with the CN. Thus, the signaling load on the CN is reduced, and the risk of information getting out of synchronization is minimized (i.e., when the CN believes that a subscriber is registered on a particular area of the network, but this is not the case because the RAN has rejected access to that area of the network). The invention does not require functional changes in the UE or in the CN. 
         [0010]    In one aspect, the present invention is directed to a method of controlling access in a cellular telecommunication system having a radio access network (RAN) in communication with a core network (CN). The method includes implementing access logic within the RAN for determining whether a given mobile user equipment (UE) is authorized to access the system through a given radio base station (RBS); and alternatively granting access or denying access to the UE based on a determination by the access logic in the RAN. 
         [0011]    In another aspect, the present invention is directed to an apparatus for controlling access in a cellular telecommunication system having a RAN in communication with a CN. The apparatus includes means within the RAN for determining whether a given UE is authorized to access the system through a given RBS; and means within the RAN for alternatively granting access or denying access to the UE based on a determination by the access logic in the RAN. 
         [0012]    In another aspect, the present invention is directed to a radio network controller (RNC) in a RAN in a cellular telecommunication system. The RNC controls access to a CN in communication with the RAN. The RNC includes first communication means for receiving an access request from a given UE through a given RBS; and means responsive to receiving the access request, for retrieving authorization information from an access control database within the RAN. The authorization information indicates whether the given UE is authorized to access the CN through the given RBS. The RNC also includes means for alternatively granting access or denying access to the UE based on the retrieved authorization information. 
         [0013]    In another aspect, the present invention is directed to a radio network controller (RNC) in a RAN in a cellular telecommunication system. The RNC controls access to a CN in communication with the RAN. The cellular telecommunication system includes macrocells, microcells, picocells and smaller femtocells. The RNC includes means for controlling access through the macrocells, microcells and picocells, wherein access requests are forwarded to the CN for access decisions; and means for controlling access through the femtocells, wherein access decisions are made in the RNC and then also in the CN (i.e. both the RAN-level and the CN-level access control procedures are performed). 
     
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0014]    In the following, the essential features of the invention will be described in detail by showing preferred embodiments, with reference to the attached figures in which: 
           [0015]      FIG. 1  is a simplified block diagram of a first exemplary embodiment of the system of the present invention; 
           [0016]      FIG. 2  is a simplified block diagram of a second exemplary embodiment of the system of the present invention; and 
           [0017]      FIG. 3  is a signaling diagram illustrating the flow of messages in an exemplary embodiment of the method of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Although described herein in terms of an embodiment in a WCDMA network, the invention may also be applied to other types of radio technologies such as GSM and Long Term Evolution (LTE) networks. 
         [0019]      FIG. 1  is a simplified block diagram of a first exemplary embodiment of the system of the present invention. In a radio access network (RAN) a number of small WCDMA Radio Base Stations (RBSs) referred to herein as Femto-RBSs  11   a - 11   n  serve small coverage areas referred to herein as femtocells  12   a - 12   n.  The prefix “femto” is utilized to indicate that the coverage area is relatively small, while distinguishing the cells from microcells or picocells, which are generally associated with other types of cellular networks. A Macro-RBS  13  likewise serves a large macrocell area  14 . 
         [0020]    Each Femto-RBS provides WCDMA coverage for user equipments such as UE  15  and are connected to a Femto-RNC (Radio Network Controller)  16 . The interface  20  between each Femto-RBS and the Femto-RNC is known as the “extended lub” interface (the “lub+” interface). The lub+ interface is preferably formed by an internet protocol (IP) connection over an IP network. In some implementations, the lub+ interface resembles the lub interface, but is modified for conveying additional information. In the content of this description, the “+” indicates that the Femto-RBS signals the preconfigured Femto-RBS Identity (Femto-RBS-ID) to the Femto-RNC e.g. during the initial power-on procedure. In one embodiment, the IP-based transmission utilizes Fixed Broadband access (such as, for example, xDSL, Cable, and the like) to connect the Femto-RBSs to the Femto-RNC. In another embodiment, a variant of Wireless Broadband access (for example, WiMAX) may be utilized. The Femto-RNC may then connect to a core network (CN)  17  utilizing an lu interface. The Macro-RBS  13  may connect to a Macro-RNC  18  utilizing an lub interface, and the Macro-RNC connects to the CN utilizing an lu interface. 
         [0021]    An Access Control Database (ACDB)  19  is introduced in the RAN. The ACDB stores information regarding the Femto-RBSs  11   a - 11   n  and which UEs  15  are authorized to access each Femto-RBS. Each UE is identified by its International Mobile Subscriber Identity (IMSI) or by some other permanent mobile identity, and each Femto RBS is identified using, for example, a preconfigured Femto-RBS Identity (Femto-RBS-ID). The ACDB is shown in  FIG. 1  as a standalone unit. However, it is a functional entity that alternatively may be incorporated in the Femto-RNC. 
         [0022]    Access control is crucial to gain end-user acceptance for the Femto-RBS concept. When a Femto-RBS is used to enhance local coverage, for example in a home or Small Office or Home Office (SOHO) environment, only terminals belonging to the home or enterprise should be allowed to access the Femto-RBS. The end user or SOHO may have purchased the Femto-RBS, or may be paying for the transmission towards the Femto-RNC and CN. Femto-RBSs may serve only a limited number of end users, so it is important that authorized users do not experience bad quality or are denied access because unauthorized users have used the available bandwidth or exceeded the maximum number of allowed users. 
         [0023]    The current Access Class concept in the RAN cannot be used for fine-grained Access Control required for Femto-RBSs. Only ten different Access Classes are available for normal end-users and this is not sufficient to build any logic for the use of Femto-RBSs. The same problem also applies to the mechanisms between the UE  15  and the CN  17 . The CN can also reject a location updating request, but the CN is not aware of the current Femto-RBS being used for the location updating. In an alternative embodiment, this may be solved by defining a unique Service Area Identity (SAI) for each Femto-RBS. Although effective, this solution may not scale well, and it also creates additional load in the signaling towards the CN. It also creates dependency on needed CN functionality. Thus a femtocell network solution cannot be deployed without the CN functionality (i.e., it is preferred to deploy the femtocell network as consisting only of RAN-level components). 
         [0024]    The present invention provides fine-grained RAN-based access control in a WCDMA-based system. During an access attempt, the Femto-RNC  16  queries the ACDB  19  for access identification information. Thus, the identification procedure, which is normally implemented in the CN  17 , is implemented in the RAN/RNC. When a RRC connection is established between a UE and the Femto-RNC in order to perform a location updating procedure, the location updating request triggers the Femto-RNC to retrieve the IMSI from the UE. When the Femto-RNC receives the IMSI, the Femto-RNC queries the ACDB to determine whether the specific UE  15  is authorized to access this particular Femto-RBS. If so, access is granted, and the location updating request is forwarded to the CN. If the UE is not authorized to access this particular Femto-RBS, access is denied, and the Femto-RNC rejects the location updating request. This process is described in more detail in  FIG. 3  below. 
         [0025]      FIG. 2  is a simplified block diagram of a second exemplary embodiment of the system of the present invention. This embodiment is similar to  FIG. 1  except that the Femto-RNC  16  and Macro-RNC  18  are implemented as components of a combined RNC  21 . The Femto-RBSs  11   a - 11   n  communicate with a Femto-RNC component  22  utilizing the lub+ interface  20 . As noted above, the lub+ interface is preferably formed by an IP connection over an IP network. The Macro-RBS  13  communicates with a Macro-RNC component  23  utilizing an lub interface. The combined RNC has lur interfaces to other RNCs (Femto-RNCs, Macro-RNCs and combined RNCs) in the RAN. When a RRC connection is established between a UE and the Femto-RNC component in order to perform a location updating procedure, the location updating request triggers the Femto-RNC component to retrieve the IMSI from the UE. When the Femto-RNC component receives the UE&#39;s IMSI, the Femto-RNC component queries the ACDB  19  to determine whether the specific UE  15  is authorized to access this particular Femto-RBS. If so, access is granted, and the location updating request is forwarded to the CN  17 . If the UE is not authorized to access this particular Femto-RBS, access is denied, and the Femto-RNC component rejects the location updating request. 
         [0026]    A number of Location Area Identities (LAI) are reserved for the Femto-cell layer. These LAIs are defined in the CN  17  as normal. If the CN knows the LAI where a specific UE is located, and if the CN needs to page this UE, the CN knows which Femto-RNC(s) is/are controlling a specific Location Area and triggers the paging request towards the relevant Femto-RNC(s). When the Femto-RBS is powered on, it performs predefined activities and contacts a Femto-RNC for the initial and automatic configuration of the needed cell configuration and other information. 
         [0027]    An LAI is also allocated for each femtocell during the automatic configuration procedure. If the number of femtocells is higher than the number of the reserved LAIs, the same LAI is shared by multiple femtocells. The LAI allocation mechanism may be a random allocation mechanism, round-robin algorithm, or other suitable allocation algorithm. 
         [0028]      FIG. 3  is a signaling diagram illustrating the flow of messages in an exemplary embodiment of the method of the present invention. The UE  15  is initially camped on a macrocell such as macrocell  14  when it detects a femtocell and reads relevant femtocell system information  31  from the Femto-RBS  11 . At  32 , the UE attempts to camp on the femtocell. At  33 , the system information is passed to the Non-Access Stratum (NAS) layers (i.e., Mobility Management, MM) which detects that the Location Area Identity (LAI) of the femtocell is different than the previous LAI on which NAS registration was performed. Therefore, the NAS triggers a Location Updating procedure towards the network via lower layers, e.g., the Radio Resource Control (RRC) layer in the WCDMA RAN case. 
         [0029]    The RRC layer in the UE triggers RRC connection establishment  34  by sending an RRC CONNECTION REQUEST message to the Femto-RNC  16  on the Random Access Channel (RACH). The RRC CONNECTION REQUEST message contains an Establishment Cause Information Element (IE) with e.g., the value “Registration” (indicating Location Updating procedure). The RRC connection may be established using existing principles as defined in 3GPP TS 25.331. Once the RRC connection is successfully established between the UE and the Femto-RNC, the UE sends a LOCATION UPDATING REQUEST message  35  to the Femto-RNC. This message contains mobile identity and other information (e.g., Location Updating type, Classmark, and so on). 
         [0030]    Normally the Temporary Mobile Subscriber Identity (TMSI) is used as the mobile identity as shown in  FIG. 3 , but in some cases the IMSI (or some other permanent mobile identity) may be used as the mobile identity. The Location updating type indicates normally e.g., “Normal location updating” when the UE  15  moves between Location Areas. The Femto-RNC  16  knows also the Femto-RBS-ID used for the RRC connection establishment  34  and stores this information. The Femto-RBS-ID was provided to the Femto-RNC as part of the lub+interface during the initial Femto-RBS startup procedure. The Femto-RNC also stores the received LOCATION UPDATING REQUEST message  35 . 
         [0031]    If the TMSI was used as the mobile identity in the LOCATION UPDATING REQUEST message  35 , the Femto-RNC  16  triggers an Identification procedure towards the UE  15  by sending an IDENTITY REQUEST message  36  to the UE. The identity type requested may indicate “IMSI” or some other permanent mobile identity. The messages and information elements in the Identification procedure are as defined in 3GPP TS 24.008 with the main exception that the procedure is performed from the RAN (i.e., from the Femto-RNC) towards the UE rather than from the CN  17 . The UE responds to the IDENTITY REQUEST message by sending an IDENTITY RESPONSE message  37  to the Femto-RNC. This message contains the IMSI of the UE. The Femto-RNC now holds both the IMSI of the UE  15  and the Femto-RBS-ID of the Femto-RBS  11 . The Femto-RNC then performs an Access Control procedure  38 . Note that if the IMSI was used as the mobile identity in the LOCATION UPDATING REQUEST message  35 , the Femto-RNC skips the Identification procedure  36 - 37  and triggers the Access Control procedure  38  upon receiving the LOCATION UPDATING REQUEST message  35 . 
         [0032]    In the Access Control procedure, the Femto-RNC queries the ACDB  19  with the Femto-RBS-ID and the IMSI to determine whether this particular UE  15  is authorized to access this particular Femto-RBS  11 . The ACDB uses the information configured in the database and returns an indication of whether access is to be denied or allowed. 
         [0033]    If access to this particular Femto-RBS is denied, the Femto-RNC  16  rejects the Location updating procedure by sending a (MM) LOCATION UPDATING REJECT message  41  to the UE  15 . The Femto-RNC also indicates a reject cause (for example, “Location Area not allowed” or “No Suitable Cells In Location Area” or other valid reject cause) so that the UE does not reattempt the Location updating procedure from this LA and femtocell. The UE may behave in one of several ways, depending on the type of reject cause received. For example, if the reject cause indicates “Location Area not allowed”, the UE may store the LAI in the list of “forbidden location areas for regional provision of service” and perform a cell selection when back to the MM IDLE state. If the reject cause indicates “Roaming not allowed in this location area”, the UE may store the LAI in the list of “forbidden location areas for roaming” and perform a PLMN selection instead of a cell selection when back to the MM IDLE state. If the reject cause indicates “No Suitable Cells In Location Area”, the UE may store the LAI in the list of “forbidden location areas for roaming” and search for a suitable cell in another location area in the same PLMN. 
         [0034]    The Femto-RNC  16  then initiates an RRC connection release procedure  42  by sending an RRC CONNECTION RELEASE message to the UE  15 . The Femto-RNC may also include in the message, the Redirection Info IE, which is used to redirect the UE to another WCDMA macro layer frequency or to GSM. 
         [0035]    If access to this particular Femto-RBS is allowed, the Femto-RNC  16  triggers the establishment of an lu signaling connection  44 . The Femto-RNC sends a message INITIAL UE MESSAGE  45  to the CN  17 . This message contains the stored LOCATION UPDATING REQUEST message  35  and other information. The lu signaling connection may be established using existing principles as defined in 3GPP TS 25.413 and in other relevant 3GPP specifications. In response, the CN may trigger optional MM procedures  46  such as Identification and/or Authentication towards the UE. If these procedures are successful, the CN accepts the Location Updating procedure and sends a LOCATION UPDATING ACCEPT message  47  to the UE. At  48 , the CN releases the lu signaling connection. The Femto-RNC releases the RRC connection by sending an RRC CONNECTION RELEASE message  49  without including the Redirection Info IE. 
         [0036]    It should be noted that the message type octet of each NAS/L3 message contains a send sequence number, N(SD), field in bits  7  and  8  (see 3GPP TS 24.007). The main purpose of this field is to identify messages that have been duplicated (i.e., messages that the UE needs to resend in some scenarios). If the CN  17  is indicated in the system information as being a version earlier than Rel-99, then bit  8  shall be set to 0 and bit  7  is reserved for the N(SD) in messages sent from the UE  15 . In messages sent from the CN, bits  7  and  8  are coded with a “0”. If the CN  17  is indicated in the system information as being Rel-99 or later, bits  7  and  8  are reserved for the N(SD) in messages sent from the UE. In messages sent from the CN, bits  7  and  8  are coded with a “0”. The two-bit N(SD) is always used in UMTS/WCDMA networks. In other words, core networks supporting UTRAN/3G access are always Rel-99 or later. Both variants are possible in GSM networks when GERAN/2G access is used. 
         [0037]    Since the RAN in the present invention is injecting NAS messages in the communication between the UE  15  and the CN  17 , there is the possibility that the send sequence number, N(SD), may get out of synchronization between the UE  15  and the CN  17 . This may be a problem if the CN is performing any checks on the received N(SD). 
         [0038]    Consider the following example:
   a) The UE  15  sends the LOCATION UPDATING REQUEST message  35  with N(SD)=0.   b) The Femto-RNC  16  sends the IDENTITY REQUEST message  36  to the UE, and the UE responds with the IDENTITY RESPONSE message  37  and N(SD)=1.   c) If access is allowed, the Femto-RNC forwards the LOCATION UPDATING REQUEST message with N(SD)=0 to the CN  17 .   
 
         [0042]    Therefore, the next NAS procedure triggered by the CN may create the problem because the next expected N(SD) in the CN is 1, but the UE will use N(SD)=2 or N(SD)=0 depending on the indicated core network release. 
         [0043]    In an embodiment of the present invention, a procedure is performed to synchronize the N(SD). If the CN  17  is performing checks on the N(SD), the Femto-RNC  16  can be configured to perform N(SD)-mapping during the Location updating procedure when the Identification procedure was triggered from the Femto-RNC. The N(SD)-mapping procedure may include the steps:
   a) The initial LOCATION UPDATING REQUEST message  35  from the UE with N(SD)=0 is forwarded to the CN.   b) The N(SD) for all uplink NAS-messages is decreased by one, also taking into account the maximum value of N(SD) depending on the indicated core network release.   
 
         [0046]    As described above, the present invention implements access control in the RAN without interaction with the CN. Thus, the signaling load on the CN is reduced, and the risk of information getting out of synchronization is minimized. It should further be noted that no functional changes are needed in the UE or in the CN. Although described in terms of an embodiment in a WCDMA network, the invention may also be applied to other types of radio technologies such as GSM and Long Term Evolution (LTE) networks. 
         [0047]    Although preferred embodiments of the present invention have been illustrated in the accompanying drawings and described in the foregoing Detailed Description, it is understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the scope of the invention. The specification contemplates all modifications that fall within the scope of the invention defined by the following claims.