Patent Publication Number: US-2005118992-A1

Title: Method of transmitting and receiving service availability information about a multimedia broadcast/multicast service

Description:
PRIORITY  
      This application claims priority under 35 U.S.C. § 119 to an application entitled “Method of Transmitting/Receiving Service Availability Information about Multimedia Broadcast/Multicast Service” filed in the Korean Intellectual Property Office on Oct. 2, 2003 and assigned Serial No. 2003-68949, the contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to a method of providing an MBMS (Multimedia Broadcast/Multicast Service) service in a mobile communication system, and in particular, to a method of transmitting and receiving service availability information about an MBMS service in order to support UE (User Equipment) mobility.  
      2. Description of the Related Art  
      In addition to a basic voice service, recent communication technology and growing user demands have driven the development of CDMA (Code Division Multiple Access) mobile communication systems for transmitting a large volume of data through circuit communication and packet communication in a multimedia multicast service. To support multimedia multicast communications, the 3 rd  generation standardization groups have presented a variety of criteria for providing an MBMS in which the same data is transmitted to multiple UEs.  
       FIG. 1  illustrates a conventional MBMS mobile communication system. More specifically,  FIG. 1  illustrates a GSM (Global System for Mobile Communications)/GPRS (General Packet Radio Services)-based system that provides an MBMS service under the 3 rd  generation asynchronous mobile communication standards of the 3GPP (3 rd  Generation Project Partnership).  
      Referring to  FIG. 1 , UEs  101  to  108  are devices capable of receiving MBMS data. Cells  111 ,  112 , and  113  are the coverage areas of a Node B  121 , which provides service to the UEs  101  to  108 . More specifically, the Node B  121  exchanges radio signals with wired signals according to the 3GPP standards, and provides MBMS to the UEs  101  to  108  within the cells  111 ,  112 , and  113 .  
      An RNC (Radio Network Controller)  131  manages a plurality of Node Bs. More specifically, the RNC  131  controls radio channels established between UEs and Node Bs supporting MBMS, including the Node B  121 , in order to provide MBMS data to the Node Bs. The RNC  131  and the Node B  121  collectively form a UTRAN (UMTS Terrestrial Radio Access Network). The UTRAN.connects the UEs  101  to  108  to a CN (Core Network).  
      An SGSN (Serving GPRS Support Node)  141  controls an MBMS service for each MBMS subscriber. For example, the SGSN  141  manages billing data for each subscriber and selectively provides multimedia data to a particular RNC. An HLR (Home Location Register)  151 , which is connected to the SGSN  141 , authenticates subscribers. A GGSN (Gateway GPRS Support Node)  161  receives MBMS data destined for the UEs  101  to  108  from a content provider  191  and multicast/broadcast sources  192  and  193  via a BM-SC (Broadcast/Multicast Service Center)  181  and a BG (Border Gateway)  171 , provides the MBMS data to the SGSN  141 , manages the mobility of the UEs  101  to  108 , and manages the QoS (Quality of Service) of the MBMS service for the UEs  101  to  108 . The contents provider  191  and the multicast/broadcast sources  192  and  193  are the sources of the MBMS data transmission.  
      The BG  171  provides the GGSN  161  with the MBMS data received from the multicast[broadcast source  192  in a network that is not controlled by the present service provider. The BM-SC  181  provides the GGSN  161  with MBMS data received from the contents provider  191 , authenticates the contents provider  191 , determines the MBMS QoS, compensates for MBMS data loss, charges for the contents from the contents provider  191 , and notifies the UEs  101  to  108  of on-going MBMS services.  
      MBMS data streams are delivered to the GGSN  161  directly from the multicast/broadcast source  193  within the network, from the contents provider  191  via the BM-SC  181 , or from the multicast/broadcast source  192  via the BG  171 . The UEs  101  to  108  receive the MBMS data streams via the Node B  121  under the control of the SGSN  141  and the RNC  131 .  
      Although not shown, a plurality of SGSNs may be connected to one MBMS service, each SGSN having a plurality of RNCs. Each SGSN selectively transmits data to RNCs and each RNC selectively transmits data to Node Bs. Accordingly, the SGSN has a list of RNCs to receive data and the RNC has a list of Node Bs to receive data.  
       FIG. 2  is a diagram illustrating a signal flow for a conventional MBMS service procedure. For better understanding of the MBMS service procedure in a particular cell, only signal flows among a UE  201 , an RNC  211 , and an SGSN  221  are illustrated.  
      Referring to  FIG. 2 , the SGSN  221  provides basic information about MBMS services to the UE  201  via the RNC  211 , e.g., what the MBMS services are and whether they are available, in step  231  (Announcement). If there is a desired MBMS service among the MBMS services, the UE  201  notifies the SGSN  331  through the RNC  211  that it will join the desired MBMS service in step  241  (Joining). In the Joining step  241 , the UE  201  notifies the SGSN  221  the ID of the desired MBMS service. The SGSN  221  authenticates the UE  201  and notifies the UE  201  whether or not the MBMS service is available to the UE  201 . The SGSN  221  stores a list of UEs requesting the MBMS service and their locations through the Joining.  
      When a BM-SC announces the initiation of the MBMS service that the UE  201  has joined, the SGSN  221  transmits a Session Start message to the RNC  211 , which controls the UE  201 , in step  251 . In step  252 , the RNC  211  transmits an MBMS Paging message to the UE  201  to notify the initiation of the MBMS service (Notification). The MBMS Paging message is transmitted to a plurality of UEs that have joined the MBMS service. Accordingly, the MBMS paging in step  252  is called “group paging” in order to be distinguishable from conventional paging.  
      In response to the MBMS paging, the UE  201  transmits a Notification Response message in step  261 . The RNC  211  determines the number of UEs that are to receive the MBMS service and the type of a radio channel (i.e., service type) for the MBMS service in each cell. If the number of UEs to receive the MBMS service is at least equal to a predetermined threshold, the RNC  211  provides the MBMS service on a common channel in a point-to-multipoint (PtM) mode. However, if the number of UEs to receive the MBMS service is lower than the threshold, the RNC  211  provides the MBMS service in a point-to-point (PtP) mode by establishing an RRC (Radio Resource Control) connection and a dedicated channel with the individual UEs.  
      In step  271 , the RNC  211  transmits MBMS radio bearer information to the UE  201 . The MBMS radio bearer information includes information about the radio channel that delivers the MBMS service, for example, OVSF (Orthogonal Variable Spreading Factor) code information, transport format information, RLC (Radio Link Control) information, and PDCP (Packet Data Convergence Control) information. The UE  201  receives the MBMS service according to the MBMS radio bearer information in step  281 .  
      The MBMS radio bearer information can be transmitted to all UEs receiving the MBMS service in the cell. Therefore, it is efficient to transmit MBMS service-related control signals on a common control channel established for the cell. The 3GPP specifies that the MBMS control signals are periodically transmitted on a logical channel called an MCCH (MBMS Control Channel).  
      A plurality of cells in which a UE may be located provide a particular MBMS service either by agreement with service providers or depending on the availability of radio resources. However, the UE cannot detect if a desired MBMS service is available or there are sufficient resources to provide the MBMS service in a cell that it now belongs to. Consequently, a UE may establish an RRC connection and request an MBMS service that is not available or for which there are not sufficient resources, resulting in unnecessary signaling overhead and increased service delay.  
     SUMMARY OF THE INVENTION  
      Accordingly, the present invention has been designed to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. An object of the present invention is to provide a method of informing a UE about service availability and resource availability regarding an MBMS service for each cell in an MBMS mobile communication system.  
      Another object of the present invention is to provide a method of enabling a UE to request an MBMS service or select another cell, after recognizing that it has entered a cell not supporting the MBMS service or having insufficient resources for the MBMS service in an MBMS mobile communication system.  
      A further object of the present invention is to provide a method of informing a UE about service availability of an MBMS service for each cell by notifying the UE whether the MBMS service is NSA (Not Service Availability), SA (Service Availability), or OSA (Out of Service Area) in an MBMS mobile communication system.  
      The above and other objects are achieved by providing a method of transmitting and receiving MBMS service availability information.  
      According to one aspect of the present invention, in a method of transmitting service availability information about an MBMS service to UEs within a plurality of cells in an RNC for controlling the MBMS service for the UEs, in a mobile communication system, the RNC determines if an MBMS service is available and if resources are sufficient for the MBMS service in each of the cells. The RNC transmits a paging message including service availability information and resource availability information to UEs within cells in which the MBMS service is available. The service availability information indicates if the MBMS service is available and the resource availability information indicates if resources are available for the MBMS service. The RNC transmits to the UEs within the plurality of cells a service availability message including resource availability information about at least one MBMS service available in each of the cells.  
      According to another aspect of the present invention, in a method of receiving service availability information about an MBMS service from an RNC controlling UEs in a plurality of cells in a UE to receive the MBMS service, the UE receives from the RNC a service availability message including service availability information and resource availability information about at least one MBMS service available in a cell where the UE is located. The service availability information indicates if the MBMS service is available and the resource availability information indicates if the cell has sufficient resources for the MBMS service. The UE determines from the service availability message if a desired MBMS service is available and if there are sufficient resources for the desired MBMS service in the cell. If the desired MBMS service is available and sufficient resources exist for the desired MBMS service in the cell, the UE receives the desired MBMS service in the cell. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
       FIG. 1  illustrates a conventional MBMS mobile communication system;  
       FIG. 2  is a diagram illustrating a signal flow for a conventional MBMS service procedure;  
       FIG. 3A and 3B  illustrate transmission of MCCH control messages;  
       FIG. 4  illustrates a movement of a UE while receiving an MBMS service;  
       FIG. 5  is a diagram illustrating a signal flow for transmitting MBMS control information according to a preferred embodiment of the present invention;  
       FIG. 6  illustrates an MBMS Service Availability message on the MCCH according to a preferred embodiment of the present invention;  
       FIG. 7  is a flowchart illustrating an operation in an RNC for transmitting an MBMS Paging message according to a preferred embodiment of the present invention;  
       FIG. 8  is a flowchart illustrating an operation in an RNC for initially transmitting an MBMS Service Availability message according to a preferred embodiment of the present invention;  
       FIGS. 9A, 9B , and  10  are flowcharts illustrating an operation in a UE for determining service availability according to a preferred embodiment of the present invention; and  
       FIG. 11  is a flowchart illustrating an operation in an RNC for periodically transmitting an MBMS Service Availability message according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Preferred embodiments of the present invention will be described in detail herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail when they would obscure the invention in unnecessary detail.  
      The present invention, as will be described herein below, pertains to transmission of service availability information from an RNC to a UE by a periodic MBMS control message on an MCCH in an MBMS mobile communication system.  
      Typically, MBMS services are categorized into three types in a target cell that a UE moves to or a cell where the UE starts its communication: NSA (Not Service Availability), SA (Service Availability), or OSA (Out of Service Area). If the cell does not support a particular MBMS service, the MBMS service is NSA. Although the cell supports the MBMS service, it may not provide the MBMS service temporarily for a reason such as a lack of resources at current time. Then, the MBMS service is OSA. If the MBMS service is SA, the cell can provide the MBMS service.  
      If an MBMS service is NSA or OSA in a particular cell, a UE does not establish an RRC connection to request the MBMS service. More specifically, in the case of NSA, the cell cannot provide the MBMS service. Therefore,,there is no need for MBMS paging. However, in the case of OSA, if resources become available, the cell is capable of providing the MBMS service. In this case, the MBMS paging is needed.  
      If the MBMS service is SA, it is ongoing in a PtP or PtM mode in the cell, or while the MBMS service is not being provided due to the absence of any UE requesting the MBMS service, the cell can start the MBMS service immediately upon a request from a UE. An MCCH control message includes information indicating whether an MBMS service is SA, NSA, or OSA in a particular cell at the initiation of the MBMS service or during a current MBMS service.  
       FIGS. 3A and 3B  illustrate MCCH control messages for supporting UE mobility during an MBMS service. The MCCH control messages support the MBMS service for the UE when it is in a PMM (Packet Mobility Management) idle mode or a PMM connected mode.  
       FIG. 3A  illustrates transmission of an MBMS Service Availability message  301 . The MBMS Service Availability message  301 , which is transmitted from an RNC to a UE, informs the UE about the availability of an MBMS service during the MBMS service. The MBMS Service Availability message  301  includes the availability information of at least one MBMS service available in the cell, and MBMS service identifiers (IDs) and their service types.  
       FIG. 3B  illustrates transmission of an MBMS Radio Bearer Information message  302 . The Radio Bearer Information message  302  is used by the RNC to notify the UE of the radio bearer information of an MTCH (MBMS Traffic Channel) that delivers MBMS data. When an MBMS service is provided in a Ptp mode, a radio bearer is established for each UE. However, it is impossible to notify individual UEs of the radio bearer information on the MCCH. Accordingly, the radio bearer information of the Radio Bearer Information message  302  is valid only for an MBMS service provided in a PtM mode. The Radio Bearer Information message  302  contains an MBMS service ID and logical channel/transport channel/physical channel information.  
       FIG. 4  illustrates the movement of a UE during receiving an MBMS service. Reference numerals  401  and  402  denote cell  1  and cell  2 , respectively, and reference numeral  411  denotes a UE that moves from cell  1  to cell  2 . Cell  1  supports an MBMS service C (MBMS C), and cell  2  does not support MBMS C or does not currently have sufficient resources for MBMS C, even though it is capable of providing MBMS C. Therefore, MBMS C is SA for cell  1  and NSA or OSA for cell  2 . Because MBMS C is NSA or OSA for cell  2 , the UE  411  selects another cell capable of providing MBMS C without an unnecessary RB connection and service requesting, or stays in cell  2 . In cell  2 , if the MBMS C is NSA, the UE  411  operates according to system setting or a user request.  
       FIG. 5  is a diagram illustrating a signal flow for an operation for transmitting MBMS service availability information to a UE on the MCCH according to a preferred embodiment of the present invention. Reference numerals  501  and  502  denote UE  1  and UE M that are receiving an MBMS service, reference numeral  511  denotes an RNC that controls the radio connections of UE  1  and UE M, and reference numeral  521  denotes an SGSN that controls the RNC  511 . While both UE  1  and UE M have joined the same MBMS service, UE  1  is located in a cell for in the MBMS service is SA or OSA, and UE M is located in a cell in which the MBMS service is NSA.  
      Referring to  FIG. 5 , the RNC  511  receives a Session Start message regarding the MBMS service from the SGSN  521  in step  531 . Because the Session Start message includes the ID and QoS of the MBMS service and other parameters, the RNC  511  determines whether each of cells under its control is NSA, SA, or OSA from the parameters in step  541 . Simultaneously, the RNC  511  determines resource availability for the MBMS service in each cell. Unless an OVSF code satisfying the QoS set in the Session Start message is currently available in a cell, the RNC  511  determines that resources are insufficient for the MBMS service in the cell.  
      In step  551 , the RNC  511  transmits an MBMS Paging message regarding the MBMS service to UE  1  on an MBMS-related paging channel. The MBMS Paging message indicates the generation of an event such as the session start for the MBMS service, and includes the ID of the MBMS service, a cause value, an NSA/SA indicator, a resource availability indicator, and parameters required for counting valid UEs to receive the MBMS service. The cause value indicates that the MBMS Paging message has been generated in response to the Session Start message. The counting parameters can be further included as optional values (OVs) or conditional values (CVs) in the MBMS Paging message. The NSA/SA indicator and the resource availability indicator are set according to the resource and service states of the cell to which the MBMS Paging message is transmitted, in order to indicate whether the cell is SA, NSA, or OSA. The MBMS Paging message can be transmitted during the MBMS service in progress, and includes corresponding parameters according to the service event and the type of an MBMS control message to receive.  
      If the MBMS service is OSA in a cell, the MBMS Paging message includes the ID of the MBMS service, the cause value, the NSA/SA indicator, and the resource availability indicator. If the MBMS service is SA in the cell, the MBMS Paging message includes all the afore-described parameters.  
      In a cell for which the MBMS service is SA or OSA, a UE operates according to the MBMS service ID and the cause value set in the MBMS Paging message, and its connection state.  
      If a desired MBMS service is NSA and cell reselection is possible, the UE selects another cell capable of providing the MBMS service and moves to the another cell. However, if cell reselection is impossible and there is not any other desired MBMS service, the UE stays in the cell and ends reception of the MBMS service.  
      If the desired MBMS service is OSA and cell reselection is possible, the UE selects another cell capable of providing the MBMS service and moves to the another cell. If the desired MBMS service is OSA and cell reselection is impossible, the UE receives the MCCH in the cell and waits until MBMS resources are available. Alternatively, the UE can end the MBMS service in the cell upon user request. By receiving the MBMS Paging message or additionally receiving an MBMS Service Availability message on the MCCH through scheduling, the UE can determine if the MBMS resources are available, that is, when the MBMS service becomes SA in the cell.  
      For example, UE  1 , which is in an idle mode within a cell for which the MBMS service is SA, transmits an RRC Connection Setup Request message in response to the MBMS Paging message to the RNC  511  to receive the MBMS service, in step  552 . If it is necessary to count valid UEs, the RNC  511  counts UEs to receive the MBMS service within the cell and determines the service type of the MBMS service according to the count and radio bearer information according to the service type in step  561 . In step  562 , the RNC  511  transmits the determined radio bearer information to UE  1  by a Radio Bearer Setup message. When a radio bearer setup is completed, UE  1  can receive the MBMS service in step  563 .  
      After the initiation of the MBMS service, the RNC  511  periodically transmits the MBMS Service Availability message indicating whether the MBMS service is NSA, OSA, or SA on the MCCH in order to support the mobility of UEs moving between cells during the MBMS service. The MBMS Service Availability message includes the ID of the MBMS service, resource availability information, a current service type, and mapping information between the MBMS service ID and mRNTI (MBMS Radio Network Temporary Identifier).  
      If the ID of a desired MBMS service is not found in the MBMS Service Availability message, the MBMS service is not available in the cell (NSA).  
      The UE also determines from the resource availability information whether the MBMS service is OSA or SA. If the MBMS service is SA, the MBMS Service Availability message includes the service type of the MBMS service. The service type indicates whether the MBMS service is provided in a PtP or PtM mode, or if no UEs request the MBMS service within the cell (no UE).  
      In the case of PtM, the UE detects the radio bearer information from the Radio Bearer Information message received on the MCCH and receives the MBMS service on the MTCH according to the radio bearer information. In the case of PtP, the UE establishes an RRC connection and requests the MBMS service. In the case of no UE, the UE establishes the RRC connection and requests the MBMS service, or operates as otherwise specified. If the UE operates in the same manner in both case of PtP and no UE, the volume of transmitted information can be reduced by replacing the service type by a PtM indicator.  
      As described above, when the UE enters a particular cell, it determines if a desired MBMS service is NSA, OSA, or SA from the MBMS Service Availability message received from the cell. If the MBMS service is SA, the UE determines its service type. If the MBMS service is NSA, the UE selects another Node B where the MBMS service is available, or ends reception of the MBMS service in the current cell. If the MBMS service is OSA, the UE selects another Node B where the MBMS service is available, or receives the MCCH periodically and checks resource availability, until resources are available for the MBMS service in the cell. If the MBMS service is SA, the UE operates according to its service type.  
       FIG. 6  illustrates an MBMS Service Availability message transmitted on an MCCH according to the embodiment of the present invention. As illustrated in  FIG. 6 , the MBMS Service Availability message includes Message Type and at least one or more IEs (Information Elements) for an MBMS service being SA or OSA.  
      Referring to  FIG. 6 , the IEs include MBMS Service ID, Resource Availability, Current Service Type, and Mapping Information for mRNTI. The Current Service Type is set to no UE, PtP, or PtM, or it can be a PtM Indicator. The Current Service Type can be conditional, such that it is included only when resources are available. The Mapping Information for MRNTI is included only when the Current Service Type is PtP or PtM. Thus, it is also conditional. The Resource Availability indicates whether resources are available or not by using one bit, or is set to describe the resource availability for the MBMS service. The Resource Availability will be described in more detail herein below.  
       FIGS. 7 and 8  are flowcharts illustrating an operation in an RNC for transmitting the MBMS Paging Message and the MBMS Service Availability message according to a preferred embodiment of the present invention. Referring to  FIG. 7 , upon receiving a Session Start message for a particular MBMS service from the SGSN in step  601 , the RNC determines if the MBMS service is SA in each cell under its control in step  611 . The determination is made according to parameters included in the Session Start message, not according to resource availability for the MBMS service.  
      If the MBMS service is not SA, the RNC sets the ID of the MBMS, a cause value indicating the session start, and an NSA/SA indicator indicating NSA in the MBMS Paging message in step  612  and transmits the MBMS Paging message in step  624 . It can be further contemplated as another embodiment of the present invention that the RNC does not transmit the MBMS Paging message to cells for which the MBMS service is NSA.  
      However, if the MBMS service is SA, the RNC checks if resources are available for the MBMS service in the cell in step  621 . That is, the RNC determines if resources are sufficient to support the QoS of the MBMS service according to an MBMS QoS parameter set in the Session Start message.  
      If resources are not sufficient for the MBMS service, the RNC sets the ID of the MBMS service, the cause value indicating the session start, the NSA/SA indicator as SA, and a resource availability indicator as false (i.e., OSA) in the MBMS Paging message in step  622 .  
      However, if sufficient resources exist for the MBMS service, the RNC sets the ID of the MBMS service, the cause value indicating the session start, the NSA/SA indicator as SA, and the resource availability indicator as true in the MBMS Paging message in step  623 . When determining that it is necessary to count UEs requesting the MBMS service, the RNC includes parameters for counting the UEs in the MBMS Paging message. In step  624 , the RNC transmits the MBMS Paging message set in step  612 ,  622 , or  623 .  
       FIG. 8  is a flowchart illustrating an initial transmission of an MBMS Service Availability message following the procedure illustrated in  FIG. 7 . Referring to  FIG. 8 , the RNC determines if the MBMS service is NSA in the cell in step  631 . If the MBMS service is NSA, the RNC terminates the procedure without transmitting any message related to the MBMS service to the cell. However, if the MBMS service is not NSA, the RNC determines if the cell has resources enough to satisfy the QoS of the MBMS service in step  632 .  
      If resources are insufficient, the RNC sets MBMS Service ID to the ID of the MBMS service, Resource Availability to false, and Mapping Information for mRNTI in the MBMS Service Availability message in step  634  and proceeds to step  671 .  
      If the cell has sufficient resources in step  632 , the RNC counts the number of UEs to receive the MBMS service in a connected mode in order to determine the radio bearer type of the MBMS service in the cell in step  633 . The RNC determines if the count is  0  in step  641 . If the count is  0 , the RNC includes the ID of the MBMS service, the mRNTI mapping information, Resource Availability set to true, and Current Service Type set to no UE in the MBMS Service Availability message in step  642 .  
      If the count is not  0 , the RNC compares the count with a threshold preset to provide the MBMS service in a PtM mode in step  651 . If the count is less than the threshold, the RNC determines the radio bearer type of the MBMS service as PtP and sets radio bearer information corresponding to the PtP mode in step  652 . Accordingly, the MBMS Service Availability message includes the ID of the MBMS service, the mRNTI mapping information, Resource Availability set to true, and Current Service Type set to PtP.  
      However, if the count is equal to or greater than the threshold, the RNC sets the radio bearer type of the MBMS service as PtM and sets radio bearer information corresponding to the PtM mode in step  661 . The MBMS Service Availability message includes the ID of the MBMS service, the mRNTI mapping information, Resource Availability set to true, and Current Service Type set to PtM.  
      The MBMS Service Availability message set in step  634 ,  642 ,  652 , or  661  is transmitted on the MCCH in step  671 . Although not shown, the PtM radio bearer information determined in step  661  is transmitted on the MCCH by a Radio Bearer Information message and the PtP radio bearer information determined in step  642  or  652  is transmitted on a control channel by the Radio Bearer Information message.  
       FIGS. 9A and 9B  are flowcharts illustrating an operation in the UE for receiving an MBMS Service Availability message according to an embodiment of the present invention. The receiving operation is performed when the MS reselects a cell while roaming. That is, the operations illustrated in  FIGS. 9A and 9B  are performed at each cell reselection, irrespective of a session start.  
      Referring to  FIGS. 9A and 9B , as the UE enters a particular cell through cell reselection, and reads system information received on a BCCH (Broadcast Control Channel) in step  701 . In step  711 , the UE determines if the system information includes MCCH configuration information. If the MCCH configuration information is not included in the system information, the UE determines that the current cell is a non-MBMS service area (NMA) that does not support a desired MBMS service in step  715 . Therefore, the UE selects another cell supporting the MBMS service or ends reception of the MBMS service. In order to prevent unnecessary power consumption, the UE neither requests the desired MBMS service by establishing an RRC connection nor monitors an MBMS-related paging channel.  
      However, if the UE detects the MCCH configuration information from the system information, it connects to the MCCH based on the MCCH configuration information and receives an MBMS Radio Bearer Information message and/or an MBMS Service Availability message in step  712 . In step  713 , the UE determines if the MBMS Radio Bearer Information message includes the ID of the desired MBMS service. If the MBMS Radio Bearer Information message includes the ID of the desired MBMS service, the UE receives data of the MBMS service on the MTCH in step  714 .  
      However, if the MBMS Radio Bearer Information message does not include the ID of the MBMS service, the UE checks if the MBMS Service Availability message includes the ID of the MBMS service in step  721 . If the MBMS Service Availability message does not include the ID of the MBMS service , the UE updates its routing area (RA) when necessary in step  722 . The RA update will be described in more detail later with reference to  FIG. 10 .  
      If the MBMS Service Availability message includes the ID of the MBMS service, the UE reads Resource Availability from the MBMS Service Availability message in step  731 . If Resource Availability is false, the UE determines that the MBMS service is OSA in the cell in step  732 . Then the UE selects another cell capable of providing the MBMS service or stays in the cell until the MBMS service is available. To determine the availability of the MBMS service, the UE monitors the MBMS-related paging channel.  
      If Resource Availability is true in step  731 , the UE determines if Current Service Type is no UE in the MBMS Service Availability message in step  741 . If Current Service Type is no UE, the UE performs an operation corresponding to its current state in step  742 . More specifically, in an idle mode, the UE enters a connection mode by an RRC connection setup procedure. In the connection mode, the UE operates according to the UE state.  
      More specifically, if the UE is in a CELL_FACH state, in which it monitors an FACH (Forward Access Channel), or CELL_PCH state, in which it monitors a PCH (Paging Channel), the UE acquires PtP radio bearer information by receiving a Cell Update Confirm message or a Radio Bearer Setup message. In a URA_PCH (UTRAN Registration Area_Paging Channel) state in which a cell update occurs less frequently than in the CELL_PACH state, the UE performs the cell update and acquires PtP radio bearer information by a Cell Update Confirm message or a Radio Bearer Setup message received through the cell update procedure. Finally, in a CELL_DCH state in which the UE monitors a DCH (Dedicated Channel), the UE implements a handover procedure and receives PtP radio bearer information on a dedicated control channel during the handover procedure.  
      However, if Current Service Type is not no UE in step  741 , the UE determines if Current Service Type is PtP in step  751 . If Current Service Type is PtP in step  751 , the UE operates in correspondence with its current state in step  752 . More specifically, in the idle mode, the UE enters the connected mode through the RRC connection setup. In the connected mode, the UE operates according to its connected state. For example, in the CELL_FACH or CELL_PCH state, the UE acquires the PtP RB information by a Cell Update Confirm message or a Radio Bearer Setup message. In the URA_PCH state, the UE performs the cell update procedure and acquires the PtP RB information by a Cell Update Confirm message or a Radio Bearer Setup message received through the cell update procedure. In the CELL_DCH state, the UE receives PtP radio bearer information by a handover.  
      In step  753 , the UE receives data of the MBMS service on a DTCH (Dedicated Traffic Channel) using the PtP radio bearer information acquired in step  742  or  752 .  
      If Current Service Type is not PtP in step  751 , the UE determines if Current Service Type is PtM in step  761 . If Current Service Type is not PtM, the UE terminates the procedure, considering that an error has been generated.  
      When the Current Service Type is not PtM, the UE receives a Radio Bearer Information message including PtM radio bearer information on the MCCH in step  762  and receives the MBMS service on the MTCH activated by the PtM radio bearer information in step  763 .  
       FIG. 10  is a flowchart illustrating an RA update in a UE according to a preferred embodiment of the present invention. Referring to  FIG. 10 , if a desired MBMS service is NSA, the UE determines if an RA update is needed in step  771 . This determination is made according to whether the current cell and the previous cell are in different RAs. If the RA update is not needed, the UE selects another cell supporting the MBMS service or stays in the current cell in step  773 . The UE periodically monitors an MBMS-related paging channel to determine if the MBMS service starts in the current cell.  
      If the RA update is needed, the UE transmits an RA Update Request message to the RNC and awaits reception of an RA Update Confirm message from the RNC in step  772 .  
      In step  781 , the UE determines if the RA Update Confirm message includes MBMS service information. The MBMS service information refers to all information set in the MBMS Service Availability message and the MBMS Radio Bearer Information message. That is, the MBMS service information may include the IDs, service types and PtP radio bearer information of MBMS services being SA. If the MBMS service information is found in the RA Update Confirm message, the UE determines that a desired MBMS service is SA, NSA, or OSA according to the MBMS service information in step  782 . If the MBMS service is NSA or OSA, the UE correspondingly operates. If the MBMS service is SA, the UE sets up a PtP radio bearer based on the PtP radio bearer information included in the RA Update Confirm message, and then receives the MBMS service.  
      If the MBMS service information is not found in the RA Update Confirm message in step  781 , the UE determines that the MBMS service is SA, NSA, or OSA according to the first MBMS Service Availability message received after the RA Update Confirm message in step  783 . That is, if the MBMS service is NSA or OSA, the UE correspondingly operates. If the MBMS service is SA, the UE requests the MBMS service. When receiving the Radio Bearer Setup message including the PtP radio bearer information regarding the MBMS service before step  783 , the UE receives the MBMS service according to the PtP radio bearer information.  
      If the MBMS service Availability message received after the RA update illustrated in  FIG. 10  does not have the ID of the MBMS service, the UE considers that there is no need for performing the RA update again, which prevents repetition of unnecessary RA updating.  
       FIG. 11  is a flowchart illustrating an operation in an RNC for periodically transmitting an MBMS Service Availability message during an MBMS service according to an embodiment of the present invention. Referring to  FIG. 11 , the RNC determines if a particular MBMS service is NSA in each cell under its control in step  801 . If the MBMS service is NSA, the RNC proceeds to step  851 . If the MBMS service is not NSA, the RNC determines if the cell has sufficient resources for the MBMS service in step  811 .  
      If there is a lack of resources for the MBMS service, the RNC sets the ID of the MBMS service, mRNTI mapping information, and Resource Availability as false in an MBMS Service Availability message in step  812 .  
      If there are sufficient resources for the MBMS service, the RNC determines if any UE is to receive the MBMS service in the cell in step  821 . If no UEs are to receive the MBMS service, the RNC sets the ID of the MBMS service, the mRNTI mapping information, Resource Availability set as true, and Current Service Type as no UE in the MBMS Service Availability message in step  822 . If at least one UE is to receive the MBMS service in the cell, the RNC determines if the service type of the MBMS service is PtP according to the number of UEs to receive the MBMS service in step  831 .  
      In the case of PtP, the RNC sets the ID of the MBMS service, the mRNTI mapping information, Resource Availability as true, and Current Service Type as PtP in the MBMS Service Availability message in step  832 . If the service type of the MBMS service is not PtP, the RNC sets the ID of the MBMS service, the mRNTI mapping information, Resource Availability as true, and Current Service Type as PtM in the MBMS Service Availability message in step  841 .  
      The RNC determines if the availability information of all available MBMS services in the cell is completely set in the MBMS Service Availability message in step  851 , because the MBMS Service Availability message includes as many pieces of availability information as the number of available MBMS services in the cell. In step  861 , the RNC transmits the MBMS Service Availability message.  
      As described above, according to the present invention, in an MBMS mobile communication system, a UE determines from MCCH control messages if a desired MBMS service is serviceable and resources are available for a desired MBMS service in a current cell. Also, the continuity of reception of the MBMS service is improved during roaming of the UE. The UE selectively establishes an RRC connection only when the MBMS service is available in the cell, thereby reducing signaling overhead caused by an unnecessary RRC connection.  
      While the present invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.