GB2456930A - method of providing resources for a multicast service - Google Patents

Abstract

A method of providing resources for a multicast service comprises counting the number of user terminals interested in using the service. The counting is performed using a probability factor (which determines the fraction of terminals responding to an interrogation merssage from the network) and an additional parameter.

Description

Mobile Communications This invention relates to a broadcast or multicast service in a telecommunications system. More explicitly, but not exclusively, the invention relates to the realisation of a Multicast Broadcast services in a radio access network (RAN) such as in the Universal Mobile Telecommunications Service (UIvITS) radio access network. UMTS concerns a third generation radio network using wideband code division multiple access (W-CDMA) technology.

A cellular communications system includes mobile user equipment (UEs), a radio access network (RAN) and one or more core networks (CNs), as illustrated in Figure 1 for the UMTS case. A detailed overview over the architecture of a cellular telecommunications system of the third generation may be found in the 3GPP specification "UTRAN Overall Description" 3GPP TS25.401 and related specifications. Communication between the UEs and the UTRAN is provided via the Uu interface (Uu), whereas the communication between the UTRAN and the core networks is done via the lu interface (lu).

A radio access network includes base stations and radio network controllers or base station controllers (RNC/ BSC). The base stations handle the actual communication across the radio interface, covering a specific geographical area also referred to as a cell. The radio network controllers control the base stations connected to it, but in addition perform other functionality like for example the allocation of radio resources and the control of local mobility. An RNC connects to one or more core networks via the lu interface, to a number of base stations (node B's for the case of UTRAN) via the lub interface and possibly to one or more other RNCs via the lur interface.

The core network includes a serving GPRS (General Packet Radio Service) support node (SGSN) and a broadcastlmulticast -service centre (BM-SC).

The BM-SC controls the distribution of the data related to MBMS service(s).

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Communications Networks of the third generation (3G) such as the UMTS network provide Multimedia Broadcast Multicast Services (MBMS).

MBMS is a point-to-multipoint service in which multimedia data such as audio, images or video data is transmitted from a single source entity to multiple recipients by using an uni-directional bearer service. The MBMS bearer service offers both a broadcast mode and a multicast mode. In the broadcast mode, the data are broadcasted to all users. In contrast, a user needs to subscribe to a particular MBMS service or a group of MEMS services with a service provider in order to receive multicast services. The operator may then announce the service or use a service discovery mechanism to inform users about the range of MBMS services available. If the user is interested in a particular MBMS service, the user joins the service, i.e. the user activates the MBMS multicast service. In this way the user becomes a member of a particular multicast group and indicates to the network that he or she wants to receive the MBMS data of a particular MBMS service.

The area in which a specific multicast MBMS Bearer Service is available is referred to as the MBMS Service Area, or the MBMS Multicast (MC)-service area It is defined individually per MBMS Bearer Service, i.e. the MBMS service area is MBMS service specific. Such a MBMS service area consists of a number of cells. An MBMS MC-service area might not have any relation to other area of the network, such as the UTRAN Registration Areas (tJRA's), Routing Areas (RAs) or Location Areas (LAs).

Thus, an MBMS MC-service area may consist of some cells of a URA, RA or LA, without necessarily including all cells of that URA, RA or LA.

More information regarding MBMS realisation in the UTRAN may be found in the corresponding stage-2 document "Introduction of Multimedia Broadcast Multicast Service (MBMS) in the Radio Access Network (RAN)", IS 25.346.v2.6.0.

Transmitting the same data to multiple recipients allows network resources to be shared. In this way the MBMS architecture is designed to enable an efficient usage of radio-network and core-network resources.

In order to initiate a MBMS session, the CN sends a session start command to the RNC. The Session Start command is used to indicate that the core network is ready to send data relating to a particular MBMS service. The Session Start command triggers the establishment of a bearer resource for MBMS data transfer. It is noted that the Session Start occurs independently of activation of the service by the user. This means that a user may activate a particular service either before or after a Session Start.

After receiving the Session Start command, the RNC send MBMS notifications to the UE in order to inform the IJEs about forthcoming or even ongoing MBMS multicast data transfers. The RNC manages the use of the radio resources and decides whether the MBMS data will be transmitted using point to multipoint or point-to-point transfer mode on the radio interface. If there are sufficient UEs in a cell, the point-to-multipoint transfer mode is most efficient. If however the number of users in a cell is low, the point-to-point transfer mode may be most efficient. To decide which transfer mode to use, the RNC may perform a counting operation. Subsequently multimedia data relating to a particular MBMS service are transmitted from the CN via the RNC to the UEs during the data transfer phase.

When the BM-SC determines that there will be no more data to send, the CN sends a Session Stop command to the RNC and the bearer resources are released If a user is no longer interested in a particular MBMS service, the user deactivates the service. Accordingly, the user leaves the multicast group if he or she does no longer wants to receive Multicast mode data of a specific MBMS bearer service.

It is noted that the phase subscription, joining and leaving are performed individually per user. The other phases, such as the notification and the data transfer, are performed for a particular service, i.e. for all users interested in the related service.

As of 3GPP RAN2 meeting number 41 (16-20 February 2004), the situation regarding how MBMS will be handled on the Uu interface has again become clearer. The current status of MBMS realisation in RAN2 is described

in 3GPP specification TS25.401.

Although several contributions have already been submitted to RAN2 related to how to handle liEs in URA_PCH / CELL_PCH states in combination with MBMS reception, see for example: document R2-040068: "MBMS Common Paging versus MBMS Dedicated Paging Samsung", available from htlp://www.3 gpp.or/ftp/1sg ran/WG2 RL2/TSGR2 40/Does; document R2- 040087: "Requirements for responding to MBMS Notification for liEs in URA_PCH Vodafone Group", available from http://www.3gpp.org/ftp/tsg ran/WG2_R L2/TS G R2_40/Docs; document R2-040505: "Handling of Ues in CELL_PCH and URA_PCH Ericsson", available from http://www.3gpp.org/f1p/tsgjan/WG2_RL2/TSGR2_4 1/Does; and document R2-040532: "Tracking and MBMS bearer establishment for UEs in LJRA_PCH Vodafone Group", available from htlp://www.3 pp.org/fip/tsg_ranfWG2_RL2/TSGR24 I/Does.

It is an aim of the present invention to improve the method and system described above.

A terminal having an RRC connection is referred to as an RRC connected UE, whereas a terminal with no RRC connection established is referred to as a liE in idle mode. A terminal in RRC connected mode can be classified into the CELL_PCH, URA PCH, CELL_FACH or CELL_DCH mode, depending on the channels the UE can currently receive.

For a terminal in the CELL_DCH state, a dedicated logical channel and a transport channel DCH are established, and data on the DCH can always be received. For a terminal in the CELL_FACH state, a dedicated logical channel and a transport channel FACH are established and the FACH data can always be received, but the DCH and the DSCH cannot be received. For a terminal in the CELL_PCH or the URA_PCH state, a dedicated logical channel has not been established. However, a paging message can be received via the PCH or a cell broadcast service (CBS) message can be received via the FACH.

According to one embodiment of the present invention, there is provided a method of providing information related to a multicast service in a mobile telecommunications network, wherein information related to the multicast service one or more user terminals have joined is stored in a radio network controller while the user terminal has joined said one or more multicast services.

Another aspect is the handling of tiEs in the URA_PCH state in relation toMBMS.

In this application a solution is described which allows to support point-to-multipoint (ptm) MBMS radio bearer (RB) reception in URA_PCH state.

However, since the UTRAN is not informed about the mobility of the HE in a URA in URA_PCH state, this type of solution implies that the network has to provide consistent service availability information in all cells across the IRA. In this way this solution is considerably different to the first approach.

In references R2-040087 and R2-040532 some aspects of the second approach are discussed. However, both papers do not describe the problem of having to indicate the service availability correctly in the whole URA.

Reference R2-040505 proposes to handle CELL_PCH and URA_PCH in the same manner. However, the problem of providing consistent service availability in the whole URA is not discussed According to another aspect of the present invention, there is provided a method of transmitting information related to a multicast service in a mobile telecommunications network, wherein information related to the presence of cells in a registration area which are part of a multicast service area for said multicast service is transmitted between radio network controllers.

According to another aspect of the present invention, there is provided a method of providing a multicast service in a mobile telecommunications network, wherein a first radio network controller keeps information related to mobile terminals in a particular state within a registration area which are interested in one or more multicast services.

The alternative approach to support ptm MBMS RB reception in IJRA_PCH state, is described in the 3GPP specification TS 25.423 v6.4.0 (see section 8.2.2.2). However, the specification is unclear about how the counting procedure is carried out to decide whether the MBMS service is distributed in ptp or ptm mode.

As has been described above the counting or recounting procedure used for tJEs in RRCIdle state is carried out using a probability factor which requires a certain percentage of the UE to wake up out of the RRC-Idle state to respond to the counting request. After the number of responding UEs has been determined, it is then decided whether the MBMS service is distributed in the ptp or ptm mode.

In the document R2-040532 it is proposed that the same probability factor as is used for UEs in RRC Idle mode is also used for UEs in URA_PCH state.

When an RRC-Idle mode UE needs to be counted, it will transit from RRC_Idle state to RRC-Connected state in order to respond to the counting request. On the other hand, when a lIE in the IJRA_PCH state needs to be counted, it has to perform a cell update procedure.

However, it is usually simpler and quicker to bring a lIE from the URA_PCH state to any other RRC-connected state using a cell update procedure, than to bring a UE in an RRC-Idle mode to RRC-Connected state using an RRC connection establishment procedure. Therefore, the solution proposed in R2-040532 appears not to be a very efficient solution.

According to another aspect of the present invention, there is provided a method of providing a multicast service in a mobile telecommunications network, wherein counting of user terminals in RRC- idle state and/or IJRAPCH state interested in a particular multicast service is performed using a first and a second probability factor, respectively, and wherein said first probability factor is different to said second probability factor.

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According to yet another aspect of the present invention, there is provided a method of providing a multicast service in a mobile telecommunications network, wherein counting of user terminals interested in a particular multicast service is performed using a probability factor and an additional parameter.

Embodiments of the present invention will now be described, by example only, with reference to the accompanying figures, whereby Figs. I and 2 are schematic outlines of a mobile communications network, in which the present invention can be incorporated, Fig. 3 is a schematic illustration of a messaging timeline of a MBMS session; Fig. 4 is a schematic diagram illustrating signalling sequences according to one embodiment of the present invention; and Fig.5A and B are schematic illustrations of probability factor signalling according to two embodiments of the present invention.

Figure 2 illustrates the architecture of a radio access network. The RAN comprises base stations 2, such as the so-called Node B's for the UTRAN, and radio network controllers 4 (RNC) , also referred to as base station controllers (BSC). The base stations 2 handle the actual communication across the radio interface, covering a specific geographical area also referred to as a cell. The RNCs 4 control the base stations 2 connected to it, and also include other functionality for tasks such as the allocation of radio resources, i.e. the local mobility. An RNC 4 is connected to one or more core networks 8 via the lu interface 12, to a number of base stations 2 via the Tub interface 10 and possibly to one or more other RNCs 4 via the lur interface 14.

In a LJMTS network, the Radio Resource Control (RRC) protocol is used across the radio interface, i.e. between the UE and UTRAN. These protocol end points interact by exchanging protocol parameters, by sending messages comprising of one or more information elements.

In order to set up a MBMS session, the RNC receives a respective request from the CN. This MBMS Session Start Request contains a MBMS Service Identification, specifies the MBMS Bearer Service Type and MBMS Session Attributes such as the MBMS Service Area Information or Quality of Service parameters. After the RNC receives the MBMS Session Start Request, it notifies the UEs which are interested in and have activated the particular MBMS service.

The MBMS Session Start Request contains all information necessary to set up an MBMS Radio Access Bearer (RAB). Upon reception of the Session Start message, the RNC executes an MBMS data bearer set up over the lu interface, and subsequently informs the sending CN of the outcome of the set up in a MBMS Session Start response message.

For a particular MBMS service, data is then transferred via an MBMS RAB between the network and the TilE.

In order to set up the connections between the RNC and the UE, the existing transport channel mechanism of the Forward Access Channel (FACH) over lub is used in case of a point-to-multipoint (ptm) MBMS transmission. A ptm connection is established if the number of counted MBMS users in a cell exceeds a certain operator-defined threshold.

Otherwise, a point-to-point (ptp) connection is established over the DTCH as defined for other dedicated services.

The CN sends the MBMS Session Stop command in a similar way to the RNC, and the RNC then notifies the interested and activated TiEs of the end of the MBMS session. When the RNC receives an MBMS Session Stop Request, it releases the associated MBMS RAB resource.

Referring now to Figure 3, the sequence of main events that take place during a MBMS session is described. More details may be found in the 3GPP specification TS 25.346. The session is started when a SESSION START message 101 is received by the UTRAN over lu, and terminated when the SESSION STOP message is received over Iii.

After the SESSION START message 101, the UTRAN sends out MBMS notification indicators (NI's) 103 in order to wake-up UE's in RRC_Idle, CELL_PCH, URA_PCH and CELL FACH states. The MBMS notification indicators 103 are sent on the MBMS notification Indicator channel (MICH). UE's only need to wake-up and look for the MBMS NI's at their normal paging occasions, i.e. the paging occasion for the normal UE DR.X cycle used for conventional (R99) paging. As a result, the MBMS notification indicators 103 sent by the network have to be repeated continuously during one or more UE DRX cycles.

If a UE detects that an MBMS NI 103 is set for an MBMS service in which it is interested, the Ut listens to the MBMS point-to-multipoint Control Channel (MCCH). It has been agreed that transmissions on MCCH will be scheduled, although this is not specifically described in the 3GPP specification 25.346. Thus, all UEs receiving the MBMS NIs 103 during a certain specified period will all listen to the MCCH at one specific instance, in this document referred to as the MCCH notification occasion. The specified period is typically the largest UE DRX cycle. It is assumed that the MCCH notification occasion configuration is broadcast on BCCH or MCCH.

The message sent every DRX cycle at the MCCH notification occasion is the MBMS NOTIFICATION message 105. This message 105 will at session start typically first trigger a counting procedure by indicating that a certain percentage, the so-called "counting probability", of UEs interested in the session being started should respond by establishing an RRC connection.

It is noted that the MBMS Notification message has not yet been described in

3GPP specification 25.346.

After the Ut receives the MBMS notification message 105a, it sends a request 113 to establish an RRC connection to the core network to allow for the counting process. The request 113 includes a Service identification (ID), which identifies the MBMS service the Ut is interested in. As a response, the CN identifies the MBMS service the Ut is interested in and sends a MBMS Linking Request Message 115 over the lu interface.

As soon as the UEs receive an "interesting" MBMS NI 103, the UE shall listen to the MCCH at the MCCH notification occasions. An interesting" MBMS NI in this respect means that the NI relates to any of the MBMS services the UE has joined. After the first MBMS Notification message 105a has been sent, the one or more subsequent MBMS Notification messagesl05b may contain different counting probabilities. In this way the UTRAN determines whether the MBMS service should be provided by point-to-point or point-to-multipoint (ptp/ptm). By having higher counting probability at subsequent counting cycles, the UTRAN is able to obtain a gradual idea about how many UEs in the cell are interested in a specific MBMS service, and can then decide whether the MBMS service shall be provided ptp or ptm.

When the UTRAN has taken the ptp/ptm decision, the counting process will be stopped. In case ptp is selected, the interested UE's will receive a RADIO BEARER SETUP message. Figure 3 illustrates the case that the service is provide by ptm. In this case the UTRAN configures the MBMS point-to-multipoint Traffic Channel (MTCH) and updates the MCCH by sending the MBMS SERVICE INFORMATION message 107 and MBMS RADIO BEARER INFORMATION message 109. The two messages include the service identification and radio bearer information for the MBMS service.

After the UE has read the MBMS SERVICE INFORMATION messagesl07 and MBMS RADIO BEARER INFORMATION messages 109, it is able to read the MBMS data transmissions 111 on the corresponding MTCH.

When transmission of the MBMS session is finalised and the SESSION STOP message 117 is received over lu, the UE will be infonned about the session stop by a RADIO BEARER RELEASE message in case of ptp or a SESSION STOP notification 121 for ptm transmission. In order to ensure that all IJEs detect the SESSION STOP notification, the UTRAN send again MBMS NIs 119, such that the interested liE listens to the MCCH.

Handling of CELL_Pd state UEs One issue which has to be solved is how UEs in CELL_PCH state are to be handled if MBMS reception is to be envisaged.

In the following two solutions are proposed which allow to reduce the signalling, particularly between the serving radio network controller (SRNC) and the drift radio network controller (DRNC).

Solution 1 The signalling involved in providing a MBMS service can be significantly reduced if the DRNC stores information about which of the Ues have joined a particular MBMS service. In this way the SRNC does not have to submit this information to the DRNC if one of the IJEs change from one cell to another cell within the DRNC.

When, for example one of the UEs served by a DRNC moves to another cell within the DRNC, the DRNC will be able to correctly count the number of UEs in CELL_PCH state which are interested in a specific service by using the information stored.

For example, if a UE in CELL_PCH state which is interested in two different MBMS services, denoted as MBMS service S and MBMS service 10, moves from cell 1 to cell 2 under a DRNC, the DRNC subtracts 1 from the number of interested UEs interested in services S and 10 in cell 1, and adds Ito the number of interested UEs for MBMS services S and 10 in cell 2.

In this way the DRNC is always informed about the number of UEs in CELL_PCH state interested in the different MBMS services without the SRNC being required to inform the DRNC about every change from one cell to another Proposals so far for the handing of CELL_PCH UEs with respect to MBMS reception seem to assume the usage of a new type of context (e.g. "MBMS-context") for storing MBMS information related to tilEs in CELL_PCH state.

The above described solution can thus be implemented into this MBMS-context. This means that the information about the number of TilEs in CELL_PCH state which are interested in the different MBMS services is stored in this MBMS-context.

Solution 2 Another way of implementing the above described solution of storing information about the UEs in CELL_PCH state in the DRNC is to use a so-called UE-context rather than a MBMS-context Such a UE-context has been described in particular for UEs in CELL_DCH or CELL_FACH state. When a UE is in CELL_DCH or CELL_FACH state under a DRNC, the DRNC will have a UE-specific context for storing all liE specific information, such as the Uu channel configuration, or the radio network temporary identities (RNTI).

Although it is probably not common to have a liE-context in a DRNC for a UE in CELL_PCH state, already from R99 the SRNC can require the DRNC to maintain a UE-context also for CELL PCH state UEs.

Having a UE-context in the DRNC for CELL_PCH UE's allows the DRNC to store information with respect to MBMS services the liE has joined and the SRNC will not have to inform the DRNC about this information at every cell change.

It is noted that the SRNC may require the DRNC to have a liE-context for a UE in CELL_PCH while the liE has joined one or more MBMS services and one or more of these services has a session ongoing.

Alternatively, the SRNC may request the DRNC in addition to have a UE-context for a UE in CELL_PCH state while the liE has joined one or more MBMS services even if none of these services has a session ongoing.

The advantages of solution 2 compared to solution 1 is that by using the existing liE-context, creation of additional contexts in the DRNC for CELL_PCH state UEs is prevented; and that by using the existing IJE-context, existing R99 signalling can be re-used. If for example a UE moves to from a first to a second RNC, the existing COMMON TRANSPORT CHANNEL RESOURCE RELEASE request can be used to inform the DRNC about the change. The DRNC can then adjust the number of liEs interested in a particular service accordingly.

Support PTM MBMS RB reception in URA_PCH state Currently it is envisaged that UEs can receive a PTM MBMS RB in RRC-Idle state, as well as in states where the TJE is located up to cell level.

What are the implications of enabling PTM MBMS RB reception by UEs in IJRA_PCH state as well? When an RRC-Idle mode IJE moves within a certain RA, it will not perform any RA updates (RAUs). As a result, when such a LIE enters a particular RA, the network needs to ensure that all cells which are part of the RA need to transmit consistent information regarding whether an MBMS service is available or not in that cell, i.e. whether the cell is part of the multicast service area or not.

For UEs in RRC-Idle mode, keeping the information consistent across all cells in the RA is responsibility which is shared between the CN and UTRAN.

The CN sends a SESSION START to all RNC's which are part of the RA, and the UTRAN updates the MBMS service availability in all cells which are part of the RA and arc part of the multicast area.

An approach similar to the RRC-Idle approach will be needed for UE's in URA_PCH state which do not perform any URA-updates when moving within the JJRA. Therefore the network needs to ensure that all cells which are part of the URA need to transmit consistent information regarding whether an MBMS service is available or not in that cell.

Since the CN does not know about URAs, keeping the information consistent across all cells in a URA is the responsibility of the UTRAN, and can be achieved in the following way.

When the 1fF enters a TJRA, the serving RNC (SRNC) sends a message to all drift RNCs (DRNCs) which handle cells which are part of the IJRA.

With this message, the SRNC requests the DRNC to provide correct service availability information in all cells of the LJRA for one or more services. This message is referred to as URA_ATTACH message in the following.

When the DRNC has received at least one URA-ATTACH for a cell in the DRNS, it has to make sure it will receive the SESSION START message for this MBMS service, and provide the correct MBMS service availability during the session on the MCCH.

When the UE leaves the URA, or leaves the IJRA_PCH state, the SRNC can indicate the end of the requirement by using another message to all DRNCs which handle cells which are part of this TJRA. This message is referred to as "IJRA-DETACH" message in the following.

The SR.NC updates its request to the DRNC whenever a first UP (from an SRNC point of view) which has joined a specific MBMS service enters the URA in URA_PCH state; or when a last UE (again from an SRNC point of view) which has joined a specific MBMS service leaves the URA in URA_PCH state.

It is noted that entering/leaving the URA can be due to mobility (e.g. a UE in URA_PCH state moving into a new URA) as well as due to a state transition (e.g. a UE already present in cells belonging to a certain IJRA in CELL_DCH state, now transiting to URA_PCH state).

The DRNC will accumulate the requests from the different SRNCs (including itself) and if at least one SRNC wants the MBMS service availability information in a certain IJRA, the DRNC shall provide the information across Uu.

Referring now to Figure 4, a signalling sequence is shown to illustrate this embodiment of the present invention.

Consider a UE which is interested in two different MBMS services, denoted as MBMS services 4 and 172 in the following. This UB now moves from a first URA (URA-37) to another IJRA (URA-5) under the control of a DRNC (DRNC1). Both DRNCI and DRNC2 have cells belonging to URA-5.

In step 201 the UE sends a IJRA UPDATE message to the DRNCI, and the DRNC1 sends a uplink (Ut) signalling transfer message to the SRNC, including a RRC URA UPDATE message and UIRA information, specifying the liE moved to URA-5 under the control of DRNC2 (step 203).The SRNC then send back a downlink (DL) signalling transfer message to DRNCI, confirming the RRC URA UPDATE (step 205).In step 207 the DRNC1 sends an URA UPDATE confirmation message to the UE.

In addition to the above described normal signalling required for handling the URA update procedure, the SRNC also informs DRNC 1 that from now on the service availability for MBMS services 4 and 172 shall be signalled correctly in URA-5 (step 209). In addition, the availability signalling for MBMS services 4 and 172 is no longer required in URA-37, and the SRNC sends an URA_DETACH message to the DRNC 1 including this information (step 211).

Since URA-5 spans two RNCs, also DRNC2 is informed about the request to keep the service availability information for MBMS Services 4 and 172 accurate in URA-5 in an UIRA_ATTACH message (step 213).

It is noted that for efficiency reasons, several of the flows shown above might be combined in one message.

At a first glance it might appear as if the new functionality adds a lot of additional signalling. However, taking into account the following aspects, the additional signalling is limited.

An update of the URA-ATTACH information is only required when the first lIE of an SRNC enters the URA in the DRNC, or the last UE of an SRNC leaves the URA. Thus, and TJRA-ATTACH message is not required for every lIE entering a URA.

Several of the information flows in the above example may be combined in one message.

In the typical case of mobility within the SRNC in which the UIRAs are confined to one RNC only, the requirement of having consistent service availability signalling across a URA will not cause any significant signalling load.

By providing additional signalling over Jur in the way described above, the network is enabled to provide consistent MBMS service availability in all cells of a URA, thus enabling PTM MBMS RB reception in URA_PCH state.

In the above described embodiment "common signalling" (i.e. not UE specific signalling) is used over lur. In this way the signalling load over lur is reduced compared to solution using "dedicated signalling" (i.e. TIE specific signalling).

By enabling the UE to stay in URA_PCH state while receiving a PTM MBMS RB transmission the impact on existing R99-R5 RRM procedures is limited, and additional UE power consumption, which wouid be required in other RRC-Connected states, is prevented.

Counting procedure for UEs in URA_PCH state Below there are two alternative solutions proposed to improve the counting procedure for UEs in the URA_PCH state.

Solution I One possibility is to use a separate probability factor for URA_PCH state TJE. This probability is different to the probability factor for UEs in RRC_Idle state. In this way the probability factor for UEs in URA_PCH state can be chosen to be, for example, greater than the probability factor for TIEs in RRC_ldle state.

In this way the solution provides added flexibility in deciding which percentage of UEs in the different states (RRC-Jdle or!JRA PCH) are to be counted or recounted.

Figure 4A illustrates the use of two separate probability factors. The signalling message part 301 and 302 include the probability factors for UEs in URA_PCH and RRC Idle state, respectively. If the counting process for UEs in IJRA_PCH state is carried out, the UTRAN network signals to the lIE the probability factor p_URA_PCH, so that the signalled percentage as determined by this probability factor 301 of the UEs in UIRA_PCH state respond to the request. If, on the other hand, the counting process for UEs in RRC_Idle state is carried out, the UTRAN network signals to the UE the

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probability factor p_RRC_Idle, so that the signalled percentage as determined by probability factor 302 of the UEs in RRC_Idle state respond to the request.

Solution 2 An alternative solution is to use only one probability factor to determined which percentage of UEs are to be counted, and this probability factor appies both to UEs in the RRC_Idle mode and URA_PCH state.

However, the RRC_Idle mode UEs are not counted on every instance the URAPCH state UEs are counted, but at every instance the IJRA_PCH state UBs are counted, the RRC_Id]e mode UEs are also counted.

This solution considers the fact that it is easier to bring URA_PCH TJEs to another RRC-Connected mode state, then to bring RRC-Idle mode liEs to RRC-Connected state.

This solution can for example be implemented by adding a 1-bit flag to the probability factor when signalling it over the Uu interface to the UE: This flag specifies the states for which the counting procedure is carried out. The flag distinguished between two possibilities, referred to as "URA-PCH" and "RRC_Idle" in the following.

If the flag indicates "URA-PCH", the counting procedure is only carried out for liEs in the URA_PCH mode. For these UEs the signalled probability factor applies. liEs in RRC_Idle mode are not required to establish an RRC_Connection in relation to the counting, i.e. the probability factor in this case does not apply to UEs currently in RRC_Idle state.

If the flag indicates "RRC-Idle", all UE's in URA_PCH state that have joined the MBMS service are required to establish an RRC connection in order to respond to the counting request. In addition, the probability factor is also applicable to liEs in RRC-Idle state, and these liEs are also requested to move to an RRC_Connected state.

Figure 4B illustrates a message from the UTRAN network to the liE including a common probability factor 311 and a flag 312.

The advantage of solution 2 compared to solution I is that the additional signalling over Uu can be reduced, as solution I requires the signalling of two probability factors rather than only one. Instead, for solution 2 only an additional 1-bit flag or the like needs to be signalled to the UE. At the same time the counting procedure can be significantly improved compared to the

solution described in the prior art.

Thus, solution 2 provides most of the functionality provided by solution 1 when one assumes that RRC-Idle mode UEs are only requested to go to RRC-Connected mode in case all URA-PCH liEs have been counted.

Solution 2 is more efficient from a signalling point of view, and is still assumed to provide sufficient flexibility.

The advantage of solution I is that it provides the greatest flexibility for the counting procedure.

It is to be understood that the embodiments described above are preferred embodiments only. Namely, various features may be omitted, modified or substituted by equivalents without departing from the scope of the present invention, which is defined in the accompanying claims.

Claims (13)

1. CLAIMS: 1. A method of providing a multicast service in a mobile telecommunication network, the method comprising the steps of: acquiring a first probability factor and an additional parameter; and counting of user terminals interested in a particular multicast service is performed using the probability factor and the additional parameter.
2. 2. The method according to claim 1, wherein the additional parameter determines whether the probability factor is applicable as follows: i) user terminals in the state indicated by the parameter apply the probability factor; or ii) user terminals in states corresponding to a higher activity level respond to counting irrespective of the probability factor.
3. 3. The method according to claim 1, wherein the additional parameter is a second probability factor.
4. 4. The method according to claim 3, wherein the first probability factor is different to the second probability factor.
5. 5. The method according to claim 4, wherein said counting of user terminals further comprises counting of user terminals in RRC-idle state and UIRA_PCH state interested in said particular multicast service using the first and the second probability factors respectively.
6. 6. A network element adapted to implement the method of any of claims 1 to 5.
7. 7. A network element according to claim 6, wherein the network element is a radio network controller.
8. 8. A mobile communication network adapted to implement the method of any of claims Ito 5.
9. 9. A method of transmitting information related to a multicast service in a mobile telecommunications network, wherein information related to the presence of cells in a registration area which are part of a multicast service area for said multicast service is transmitted between radio network controllers.
10. 10. A method of providing a multicast service in a mobile telecommunications network, wherein a first radio network controller keeps information related to mobile terminals in a particular state within a registration area which are interested in one or more multicast services.
11. ii. A method of providing information related to a multicast service in a mobile telecommunications network, wherein information related to the multicast service one or more user terminals have joined is stored in a radio network controller while the user terminal has joined said one or more multicast services.
12. 12. A method of providing a multicast service in a mobile telecommunications network, wherein counting of user terminals in RRC- idle state and/or URA_PCH state interested in a particular multicast service is performed using a first and a second probability factor, respectively, and wherein said first probability factor is different to said second probability factor.
13. 13. A method of providing a multicast service in a mobile telecommunications network, wherein counting of user terminals interested in a particular multicast service is performed using a probability factor and an additional parameter.