16.10.5.2 Configuration

A UE can be configured to receive data of MBS multicast session only in RRC_CONNECTED state or RRC_INACTIVE state. To receive the multicast service, the UE needs to perform MBS Session Join procedure as specified in TS 23.247 [45]. It is up to gNB to decide whether the UE receives data of MBS multicast session in RRC_CONNECTED state or RRC_ INACTIVE state. The gNB moves the UE from RRC_CONNECTED state to RRC_INACTIVE state via RRCRelease message, and moves the UE from RRC_INACTIVE state to RRC_CONNECTED state via group notification or UE-specific paging.

If the UE which joined a multicast session is in RRC_CONNECTED state and when the multicast session is activated, the gNB may send RRCReconfiguration message with relevant MBS configuration for the multicast session to the UE.

If the gNB configures the UE to receive the MBS multicast session in RRC_INACTIVE state, the gNB may provide the PTM configuration via RRCRelease message for the MBS multicast session as well as information about which multicast service(s) can be continued to be received in RRC_INACTIVE state. The UE does not suspend MRBs of the multicast session indicated to be continued to be received in RRC_INACTIVE state. Multicast MCCH is used in case a cell supports updating PTM configuration or providing PTM configuration to UEs in RRC_INACTIVE state moved from other cells. Otherwise, multicast MCCH can be optionally present.

A notification mechanism is used to announce the change of the multicast MCCH contents due to multicast session modification or session deactivation or due to neighbouring cell information modification. The scheduling information for multicast MCCH reception is provided via SIB24 and optionally via RRCRelease message.

When there is temporarily no data to be sent to the UEs for a multicast session that is active, the gNB may move the UE to RRC_INACTIVE state. When an MBS multicast session is deactivated, the gNB may move the UE in RRC_CONNECTED state to RRC_IDLE or RRC_INACTIVE state. For UEs receiving data of MBS multicast session in RRC_INACTIVE state, the gNB notifies the UE to stop monitoring PDCCH addressed by corresponding G-RNTI via RRCRelease message or multicast MCCH when there is temporarily no data to be sent or when the session is deactivated. gNBs supporting MBS use a group notification mechanism to notify the UEs in RRC_IDLE or RRC_INACTIVE state when a multicast session has been activated by the CN. gNBs supporting MBS use a group notification mechanism to notify the UEs in RRC_INACTIVE state when the session is already activated and the gNB has multicast session data to deliver. If the UE receiving data of MBS multicast session in RRC_INACTIVE state in a cell is notified to stop monitoring PDCCH addressed by G-RNTI for all the joined multicast sessions, the UE does not monitor PDCCH addressed by Multicast MCCH-RNTI until the group notification is received. Upon reception of the group notification that does not indicate multicast reception in RRC_INACTIVE state, the UEs reconnect to the network or resume the connection and transition to RRC_CONNECTED state from either RRC_IDLE state or RRC_INACTIVE state. Upon reception of the group notification that indicates to allow the multicast reception in RRC_INACTIVE state, the UE stays in RRC_INACTIVE state and behaves as specified in TS 38.331 [12]. If the UE is notified by both group notification and the UE-specific paging, the UE follows the UE-specific paging and goes to RRC_CONNECTED state.

The group notification is addressed with P-RNTI on PDCCH, and the paging channels are monitored by the UE as described in clause 9.2.5. Paging message for group notification contains MBS session ID which is utilized to page all UEs in RRC_IDLE and RRC_INACTIVE states that joined the associated MBS multicast session, i.e., UEs are not paged individually. The UE stops monitoring for group notifications related to a specific multicast session, i.e., stops checking for the MBS session ID in the Paging message, when the UE enters RRC_CONNECTED state. The UE does not monitor for group notifications for these cases, i.e., once this UE leaves this multicast session or the network requests the UE to leave, or the network releases the multicast session.

NOTE: The gNB's decision to keep a UE in RRC_CONNECTED state (e.g., to meet latency requirements for mission critical service) or move the UE to RRC_INACTIVE or RRC_IDLE state (e.g., when there is temporarily no data to be sent to the UE or to address congestion in the cell) may consider 5QI value(s) or other QoS parameters for mission critical and non-mission critical UEs.

If the UE in RRC_IDLE state that joined an MBS multicast session is camping on the gNB not supporting MBS, the UE may be notified by CN-initiated paging where CN pages each UE individually due to session activation or data availability, as described in clause 9.2.5. If the UE in RRC_INACTIVE state that joined MBS multicast session is camping on the gNB not supporting MBS, the UE may be notified individually by RAN-initiated paging due to session activation or data availability, as described in clause 9.2.5.