1. Field of the Invention
The present invention relates to a method for providing a Multimedia Broadcast/Multicast Service (hereinafter referred to as “MBMS”) in a mobile communication system, and more particularly to a method for transmitting/receiving service availability information for the MBMS in order to support the mobility of user equipment.
2. Description of the Related Art
Recent progress in communication technology and increases in users' need have caused Code Division Multiple Access (CDMA) mobile communication systems to support a multimedia service, in addition to voice service. Multimedia service transmits a large capacity of data using circuit communication, packet communication, and the like. In order to support multimedia communication, 3rd generation standardization organizations provide various standards for providing MBMS to a number of user equipments (hereinafter referred to as “UE”) with the same data from a single data source.
FIG. 1 shows the construction of a conventional mobile communication system for providing MBMS. The system is a 3rd Generation Partnership Project (3GPP( ) system, which is the standard of the 3rd generation asynchronous mobile communication mode based on Global System for Mobile communications (GSM) and General Packet Radio Service (GPRS) and which is adapted to provide MBMS.
Referring to FIG. 1, UEs 101, 102, 103, 104, 105, 106, 107, and 108 refer to apparatuses capable of receiving MBMS data and cells 111, 112, and 113 refer to cell areas of Node Bs in which the UEs 101 to 108 are positioned. A Node B 121 performs an exchange between radio signals and wired signals according to the 3GPP standard and provides the UEs 101 to 108 in the cells 111 to 113 with MBMS.
A Radio Network Controller (hereinafter referred to as RNC) 131 manages a number of Node Bs and controls radio channels which are established between the Node Bs and the UEs in order to provide the Node B 121, as well as a number of other Node Bs (not shown) which support MBMS, with MBMS data. The RNC 131 and the Node B 121 constitute a UMTS Terrestrial Radio Access Network (hereinafter referred to as UTRAN), which plays the role of connecting the UEs 101 to 108 to a Core Network (hereinafter referred to as CN).
A Serving GPRS Support Node (SGSN) 141 controls the service of respective MBMS subscribers. As a representative example, the SGSN manages the service charging data of respective subscribers and selectively provides a specific RNC with multimedia data. A Home Location Register (HLR) 151 is connected to the SGSN 141 and authenticates subscribers. A Gateway GPRS Support Node (GGSN) 161 receives MBMS data, which is to be supplied to the UEs 101 to 108, from a content provider 191 and multicast/broadcast sources 192 and 193 via a Broadcast/Multicast Service Center (BM-SC) 181 and a Border Gateway (BG) 171 and transmits it to the SGSN 141. The GGSN 161 also manages the movement of the UEs 101 to 108 and the service quality of the MBMS provided to the UEs 101 to 108. The content provider 191 and the multicast/broadcast sources 192 and 193 are the source site of MBMS data transmission.
The BG 171 receives MBMS data from the multicast/broadcast source 192, which belongs to a network that is not currently managed by the service carrier, and delivers it to the GGSN 161. The BM-SC 181 receives MBMS data from the content provider 191 and delivers it to the GGSN 161. The BM-SC 181 performs authentication on the content provider 191; decision on MBMS service quality; error correction on MBMS data loss; charging on the content provider 191; and the like. The BM-SC 181 also notifies the UEs 101 to 108 of MBMS services which are currently provided.
An MBMS data stream is transmitted to the GSSN 161 from the content provider 191 via the BM-SC 181, from the multicast/broadcast source 192 of an external network via the BG 171, or directly from the multicast/broadcast source 193 of an internal network. The MBMS data stream is then delivered to the UEs 101 to 108 in the cells 111 to 113 via the Node B 121 under the control of the SGSN 141 and the RNC 131.
Although not shown in the drawings, it is understood that a number of SGSNs and a number of RNCs connected to the respective SGSNs may exist for a single MBMS. The SGSN and the RNC perform selective data transmission to the RNC and the Node B, respectively, and store the list of nodes to which data is to be delivered (the list of RNC in the case of SGSN and the list of Node B in the case of RNC) to this end.
FIG. 2 is a message flowchart showing a conventional MBMS procedure. In the drawing, only the signal flow among a UE 201 to which MBMS is provided, an RNC 211 for MBMS, and an SGSN 221 for MBMS is shown in order to clearly illustrate the procedure for providing MBMS in a specific cell.
Referring to FIG. 2, the SGSN 221 announces information on MBMSs, e.g. MBMS identifiers and whether MBMSs are provided, to the UE 201 via the RNC 211 in step 231. When the UE 201 finds a desired one among the MBMSs recognized in the announcement step 231, it announces that it will join the desired MBMS to the SGSN 221 via the RNC 211 in step 241. In the joining step 241, the UE 201 notifies the SGSN 221 of the identifier of an MBMS it wants to receive and the SGSN 221 authenticates the UE 201 and notifies the UE 201 of whether it can receive the MBMS. The SGSN 221 stores a list and location of UEs which want to receive a specific MBMS in the joining step 241.
If a BM-SC announces the beginning of MBMS which the UE 201 has joined, the SGSN 221 transmits a “Session Start” message to the RNC 211 in which the UE 201 is positioned in step 251. The RNC 211 transmits an MBMS paging message to the UE 201 in step 252 in order to notify that the MBMS will start. The step 252 is referred to as “group paging”, in contrast to existing paging procedure, because the MBMS paging message is transmitted to a plurality of UE which have joined the MBMS.
The UE 201 transmits a notification response regarding the paging in step 261. The RNC 211 counts the number of UEs that want to receive MBMS, of respective cells and decides the kind of radio channel of the corresponding cell, i.e. the service type, in step 261. If the number of UEs which want to receive the MBMS is not smaller than a predetermined threshold value, the RNC 211 provides the MBMS via a common channel in a Point-to-Multipoint (hereinafter referred to as PtM) mode. If the number is smaller than the predetermined threshold value, the RNC 211 provides the MBMS by setting up Radio Resource Control (RRC) connection of respective UEs and by constituting an exclusive channel in a Point-to-Point (hereinafter referred to as PtP) mode.
The RNC 211 transmits MBMS Radio Bearer (RB) information to the UE 201 in step 271. The MBMS RB information includes information on the radio channel to which MBMS is to be provided, such as Orthogonal Variable Spreading Factor (OVSF) code information; transport format information; Radio Link Control (hereinafter referred to as RLC) information; and Packet Data Convergence Protocol (hereinafter referred to as PDCP) information. The UE 201 receives the MBMS according to the MBMS RB information.
When MBMS RB information is used to provide a specific MBMS in a specific cell, it is available to all UEs which want to receive the MBMS in the cell. Accordingly, it is effective to transmit a control signal, which is related to MBMS, via a common control channel which is provided in a cell. According to the 3GPP, an MBMS control signal is transmitted periodically via an MBMS Control Channel (hereinafter referred to as MCCH), which is a logical channel.
A plurality of cells, in which UEs can be positioned, may provide or fail to provide a specific MBMS according to an agreement with the service carrier or the availability of radio resources. However, a UE cannot determine whether MBMS which the UE wants to receive is available. In other words, the UE cannot determine whether the MBMS has Service Availability (SA) or Non-SA (NSA) in the cell in which the UE is positioned. As a result, a UE may needlessly perform the superfluous procedure of setting up RRC connection for MBMS having NSA and requesting the service. This causes the problem of generation of unnecessary signaling overhead and increasing service delay time.