5.9 MBS Broadcast

38.3313GPPNRProtocol specificationRadio Resource Control (RRC)Release 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

5.9.1 Introduction

5.9.1.1 General

UE receiving or interested to receive MBS broadcast service(s) applies MBS broadcast procedures described in this clause as well as the MBS Interest Indication procedure as specified in clause 5.9.4.

MBS broadcast configuration information is provided on MCCH logical channel. MCCH carries the MBSBroadcastConfiguration message which indicates the MBS broadcast sessions that are provided in the cell as well as the corresponding scheduling related information for these sessions. Optionally, the MBSBroadcastConfiguration message may also contain a list of neighbour cells providing the same broadcast MBS service(s) as provided in the current cell. The configuration information required by the UE to receive MCCH is provided in SIB20. Additionally, System Information provides also an information related to service continuity of MBS broadcast in SIB21.

5.9.1.2 MCCH scheduling

The MCCH information (i.e. information transmitted in messages sent over MCCH) is transmitted periodically, using a configurable repetition period and within a configured transmission window. MCCH transmissions (and the associated radio resources and MCS) are indicated via the PDCCH addressed to MCCH-RNTI. PDCCH monitoring occasion(s) for MCCH transmission are determined according to the common search space indicated by searchspaceMCCH. If searchspaceMCCH is set to zero, PDCCH monitoring occasions for MCCH message reception in the MCCH transmission window are the same as PDCCH monitoring occasions for SIB1 where the mapping between PDCCH monitoring occasions and SSBs is specified in TS 38.213[13]. If searchspaceMCCH is not set to zero, PDCCH monitoring occasions for MCCH message are determined based on search space indicated by searchspaceMCCH. PDCCH monitoring occasions for MCCH message which are not overlapping with UL symbols (determined according to tdd-UL-DL-ConfigurationCommon) are sequentially numbered from one in the MCCH transmission window. The [x×N+K]^th PDCCH monitoring occasion for MCCH message in MCCH transmission window corresponds to the K^th transmitted SSB, where x = 0, 1, …X-1, K = 1, 2, …N, N is the number of actual transmitted SSBs determined according to ssb-PositionsInBurst in SIB1 and X is equal to CEIL(number of PDCCH monitoring occasions in MCCH transmission window/N). The actual transmitted SSBs are sequentially numbered from one in ascending order of their SSB indexes. The UE assumes that, in the MCCH transmission window, PDCCH for an MCCH message is transmitted in at least one PDCCH monitoring occasion corresponding to each transmitted SSB and thus the selection of SSB for the reception MCCH messages is up to UE implementation.

5.9.1.3 MCCH information validity and notification of changes

Change of MCCH information only occurs at specific radio frames, i.e. the concept of a modification period is used. Within a modification period, the same MCCH information may be transmitted a number of times, as defined by its scheduling (which is based on a repetition period).

When the network changes (some of) the MCCH information, it notifies the UEs about the change starting from the beginning of the MCCH modification period via PDCCH which schedules the MCCH in every repetition in that modification period.

Upon receiving a change notification, a UE receiving or interested to receive MBS services transmitted using MBS broadcast acquires the new MCCH information starting from the same slot. The UE applies the previously acquired MCCH information until the UE acquires the new MCCH information. The notification is transmitted with a 2-bit bitmap, see TS 38.212 [17] clause 7.3.1.5.1. The MSB in the 2-bit bitmap, when set to ‘1’, indicates the start of new MBS service(s). The LSB in the 2-bit bitmap, when set to ‘1’, indicates modification of MCCH information other than the change caused by start of new MBS service(s), e.g. modification of a configuration of an on-going MBS session(s), MBS session(s) stop or neighbouring cell information modification.

5.9.2 MCCH information acquisition

5.9.2.1 General

Figure 5.9.2.1-1: MCCH information acquisition

The UE applies the MCCH information acquisition procedure to acquire the MBS broadcast configuration information broadcasted by the network. The procedure applies to MBS capable UEs interested to receive or that are receiving MBS broadcast services that are in RRC_IDLE, RRC_INACTIVE or RRC_CONNECTED with an active BWP with common search space configured by searchSpaceMCCH.

5.9.2.2 Initiation

A UE shall apply the MCCH information acquisition procedure upon becoming interested to receive MBS broadcast services. A UE interested to receive MBS broadcast services shall apply the MCCH information acquisition procedure upon entering the cell providing SIB20 (e.g. upon power on, following UE mobility), upon receiving SIB20 of an SCell via dedicated signalling and upon receiving a notification that the MCCH information has changed due to the start of new MBS service(s). A UE that is receiving data via broadcast MRB shall apply the MCCH information acquisition procedure upon receiving a notification that the MCCH information has changed due to MCCH information modification other than the change caused by the start of new MBS service(s).

NOTE: It is up to UE implementation how to address a possibility of the UE missing an MCCH change notification.

Unless explicitly stated otherwise in the procedural specification, the MCCH information acquisition procedure overwrites any stored MCCH information, i.e. delta configuration is not applicable for MCCH information and the UE discontinues using a field if it is absent in MCCH information.

5.9.2.3 MCCH information acquisition by the UE

An MBS capable UE interested to receive or receiving an MBS broadcast service shall:

1> if the procedure is triggered by an MCCH information change notification:

2> start acquiring the MBSBroadcastConfiguration message on MCCH in the concerned cell from the slot in which the change notification was received;

1> if the UE enters a cell broadcasting SIB20; or

1> if the UE receives sCellSIB20:

2> acquire the MBSBroadcastConfiguration message on MCCH in the concerned cell at the next repetition period.

5.9.2.4 Actions upon reception of the MBSBroadcastConfiguration message

No UE requirements related to the contents of the MBSBroadcastConfiguration message apply other than those specified elsewhere e.g. within the corresponding field descriptions.

5.9.3 Broadcast MRB configuration

5.9.3.1 General

The broadcast MRB configuration procedure is used by the UE to configure PDCP, RLC, MAC and the physical layer upon starting and/or stopping to receive a broadcast MRB transmitted on MTCH, or upon modification of a configuration of a broadcast MRB received by the UE. The procedure applies to MBS capable UEs that are interested to receive or that are receiving an MBS broadcast service that are in RRC_IDLE, RRC_INACTIVE or RRC_CONNECTED with an active BWP with common search space configured by searchSpaceMTCH.

NOTE: How to perform a modification of a broadcast MRB which is already configured in the UE is left to UE implementation.

5.9.3.2 Initiation

The UE applies the broadcast MRB establishment procedure to start receiving an MBS session of an MBS broadcast service it is interested in. The procedure may be initiated e.g. upon start of the MBS session, upon entering a cell providing an MBS broadcast service the UE is interested in, upon becoming interested in the ongoing MBS broadcast service, upon removal of the UE capability limitations inhibiting reception of the ongoing MBS broadcast service UE is interested in.

The UE applies the broadcast MRB release procedure to stop receiving a session of an MBS broadcast service. The procedure may be initiated e.g. upon stop of the MBS session, upon leaving the cell broadcasting the MBS service the UE is interested in, upon losing interest in the MBS service, when capability limitations start inhibiting reception of the concerned service.

5.9.3.3 Broadcast MRB establishment

Upon a broadcast MRB establishment, the UE shall:

1> establish a PDCP entity and an RLC entity in accordance with MRB-InfoBroadcast for this broadcast MRB included in the MBSBroadcastConfiguration message and the configuration specified in 9.1.1.7;

1> configure the MAC layer in accordance with the mtch-SchedulingInfo (if included);

1> configure the physical layer in accordance with the mbs-SessionInfoList, searchSpaceMTCH, and pdsch-ConfigMTCH, applicable for the broadcast MRB;

1> receive DL-SCH on the cell where the MBSBroadcastConfiguration message was received for the established broadcast MRB using g-RNTI and mtch-SchedulingInfo (if included) in this message for this MBS broadcast service;

1> if an SDAP entity with the received mbs-SessionId does not exist:

2> establish an SDAP entity as specified in TS 37.324 [24] clause 5.1.1.

2> indicate the establishment of the user plane resources for the mbs-SessionId to upper layers.

5.9.3.4 Broadcast MRB release

Upon broadcast MRB release for MBS broadcast service, the UE shall:

1> release the PDCP entity, RLC entity as well as the related MAC and physical layer configuration;

1> if the SDAP entity associated with the corresponding tmgi has no associated MRB:

2> release the SDAP entity, as specified in TS 37.324 [24] clause 5.1.2;

2> indicate the release of the user plane resources for the tmgi to upper layers.

5.9.4 MBS Interest Indication

5.9.4.1 General

Figure 5.9.4.1-1: MBS Interest Indication

The purpose of this procedure is to inform the network that the UE in RRC_CONNECTED state is receiving or is interested to receive MBS broadcast service(s) and to inform the network about the priority of MBS broadcast versus unicast and multicast MRB reception. MBS Interest Indication can only be sent after AS security activation.

5.9.4.2 Initiation

An MBS capable UE in RRC_CONNECTED may initiate the procedure in several cases including upon successful connection establishment/resume, upon entering or leaving the broadcast service area, upon MBS broadcast session start or stop, upon change of interest, upon change of priority between MBS broadcast reception and unicast/multicast reception, upon change to a PCell providing SIB21 (i.e. where the SIB1 scheduling information contains SIB21), upon receiving SIB20 of an SCell via dedicated signalling, upon handover.

Upon initiating the procedure, the UE shall:

1> if SIB21 is provided by the PCell:

2> ensure having a valid version of SIB21 for the PCell;

2> if the UE did not transmit MBS Interest Indication since last entering RRC_CONNECTED state; or

2> if since the last time the UE transmitted an MBS Interest Indication, the UE connected to a PCell not providing SIB21:

3> if the set of MBS broadcast frequencies of interest, determined in accordance with 5.9.4.3, is not empty:

4> set the contents of MBS Interest Indication according to 5.9.4.5 and initiate transmission of the MBSInterestIndication message;

2> else:

3> if the set of MBS broadcast frequencies of interest, determined in accordance with 5.9.4.3, is different from mbs-FreqList included in the last transmission of the MBS Interest Indication; or

3> if the prioritisation of reception of all indicated MBS broadcast frequencies compared to reception of any of the established unicast bearers and multicast MRBs has changed since the last transmission of the MBS Interest Indication:

4> set the contents of MBS Interest Indication according to 5.9.4.5 and initiate transmission of the MBSInterestIndication message;

NOTE: The UE may send MBS Interest Indication even when it is able to receive the MBS services it is interested in i.e. to avoid that the network allocates a configuration inhibiting MBS broadcast reception.

3> else if SIB20 is provided for the PCell or for the SCell:

4> if since the last time the UE transmitted the MBS Interest Indication, the UE connected to a PCell not providing SIB20 and the UE was not provided with SIB20 for an SCell; or

4> if the set of MBS broadcast services of interest determined in accordance with 5.9.4.4 is different from mbs-ServiceList included in the last transmission of the MBS Interest Indication:

5> set the contents of MBS Interest Indication according to 5.9.4.5 and initiate the transmission of MBSInterestIndication message.

5.9.4.3 MBS frequencies of interest determination

The UE shall:

1> consider a frequency to be part of the MBS frequencies of interest if the following conditions are met:

2> at least one MBS session the UE is receiving or interested to receive via a broadcast MRB is ongoing or about to start; and

NOTE 1: The UE may determine whether the session is ongoing from the start and stop time indicated in the User Service Description (USD), see TS 38.300 [2] or TS 23.247 [67].

2> for at least one of these MBS sessions, SIB21 acquired from the PCell includes mapping between the concerned frequency and one or more MBS FSAIs indicated in the USD for this session, or for at least one of these MBS sessions, the concerned frequency is not included in SIB21 but is indicated in the USD for this session; and

NOTE 2: The UE considers a frequency to be part of the MBS frequencies of interest even though NG-RAN may (temporarily) not employ a broadcast MRB for the concerned session, i.e., the UE does not verify if the session is indicated on MCCH.

2> the supportedBandCombinationList the UE included in UE-NR-Capability contains at least one band combination including the concerned MBS frequency.

NOTE 3: When evaluating which frequencies the UE is capable of receiving, the UE does not take into account whether they are currently configured as serving frequencies.

5.9.4.4 MBS services of interest determination

The UE shall:

1> consider an MBS service to be part of the MBS services of interest if the following conditions are met:

2> the UE is receiving or interested to receive this service via a broadcast MRB; and

2> the session of this service is ongoing or about to start; and

2> one or more MBS FSAIs in the USD for this service is included in SIB21 acquired from the PCell for a frequency belonging to the set of MBS frequencies of interest, determined according to 5.9.4.3 or SIB21 acquired from the PCell does not provide the frequency mapping for the concerned service but that frequency is included in the USD of this service.

NOTE: The UE may determine whether the session is ongoing from the start and stop time indicated in the User Service Description (USD), see TS 38.300 [2] or TS 23.247 [67].

5.9.4.5 Setting of the contents of MBS Interest Indication

The UE shall set the contents of the MBS Interest Indication as follows:

1> if the set of MBS frequencies of interest, determined in accordance with 5.9.4.3, is not empty:

2> include mbs-FreqList and set it to include the MBS frequencies of interest sorted by decreasing order of interest, using the absoluteFrequencySSB for serving frequency, if applicable, and the ARFCN-ValueNR(s) as included in SIB21 or in USD (for neighbouring frequencies);

2> include mbs-Priority if the UE prioritises reception of all indicated MBS frequencies above reception of any of the unicast bearers and multicast MRBs;

NOTE: If the UE prioritises MBS broadcast reception and unicast/multicast data cannot be supported because of congestion on the MBS carrier(s), NG-RAN may for example initiate release of unicast bearers/multicast MRBs.

2> if SIB20 is provided for the PCell or for the SCell:

3> include mbs-ServiceList and set it to indicate the set of MBS services of interest sorted by decreasing order of interest determined in accordance with 5.9.4.4.