Content for  TS 23.434  Word version:  19.3.0

In this scenario, upon triggered by VAL server, the NRM server pre-establishes MBMS bearer(s) in certain pre-configured areas before the initiation of the VAL service group communication session. When a user originates a request for a VAL service group communication session for one of these areas, the pre-established MBMS bearer(s) is used for the DL VAL service communication. The following steps need to be performed prior to the start of the VAL service group communication session over pre-established MBMS bearer:

• Pre-establish MBMS bearer(s)
• Announce the pre-established MBMS bearer to the NRM clients

When these preparation steps have been done the VAL service group communication session using MBMS bearer can start. The vertical application level communications are sent on the MBMS bearer. Optionally a separate MBMS bearer could be used for the application level control messages, due to different bearer characteristic requirements.

The procedure Figure 14.3.4.2.2-1 shows only one of the receiving VAL clients using an MBMS bearer. There might also be VAL clients in the same VAL service group communication session that receive the communication on unicast bearers.

In this scenario, the VAL server uses a unicast bearer for communication with the UE on the DL at the start of the group communication session. When the VAL server triggers to use an MBMS bearer in EPS for the DL VAL service communication, the NRM server decides to establish an MBMS bearer in EPS using the procedures defined in TS 23.468. The NRM server provides MBMS service description information associated with MBMS bearer(s), obtained from the BM-SC, to the UE. The UE starts using the MBMS bearer(s) to receive DL VAL service and stops using the unicast bearer for the DL VAL service communication.

Figure 14.3.4.3.2-1 illustrates the use of dynamic MBMS bearer establishment.

The MBMS announcement may be done on either a unicast bearer or a MBMS bearer. Using a unicast bearer for MBMS bearer announcement provides an interactive way of doing announcement. The NRM server will send the MBMS bearer announcement message to the NRM client regardless if there is an MBMS bearer active or the VAL client can receive the data on the MBMS bearer with sufficient quality. The benefit of the existing procedure is that it gives a secure way to inform the NRM client about the MBMS bearer and how to retrieve the data on the MBMS bearer. When there is more than one MBMS bearer active in the same service area for VAL service, there are not the same reasons to use unicast bearer for additional MBMS bearer announcement. Instead a MBMS bearer for application level control signalling can be used to announce additional MBMS bearers. The MBMS bearer announcement messages are sent on an MBMS bearer used for application control messages. This bearer will have a different QoS setting compared to an MBMS bearer used for VAL service communication, since application signalling messages are more sensitive to packet loss.

Figure 14.3.4.4.2-1 illustrates a procedure that enables the NRM server to announce a new MBMS bearer. Pre-conditions:

1. An MBMS bearer used for VAL service application control messages must have been pre-established and announced to the NRM client.
2. Additional MBMS bearer information may have already been announced to the NRM client.

The same procedure can also be used to modify existing MBMS bearer announcement information. Example of such modification could be addition of UDP ports or modification of codec in the SDP.

The NRM client and NRM server use this procedure to report and take action on the MBMS bearer quality towards VAL service communications. A NRM client monitors an MBMS bearer to enable receiving VAL service communication. Based on the received quality (e.g. radio level quality, transport level quality), the NRM client needs to inform the NRM server that the VAL UE is able to receive the VAL service communication on the MBMS bearer with sufficient quality or not able to receive the VAL service communication on the MBMS bearer with sufficient quality. Furthermore, based on the received quality, the NRM client may notify the NRM server at which MBMS reception quality level it has received the VAL service communication on the MBMS bearer. The issue can be more complex since the NRM client needs to estimate the quality of the bearer even in the scenario when there are no data currently transmitted on the MBMS bearer. The reason for this is that an NRM client that has entered an area with significantly degraded MBMS quality, might not even notice that a VAL service communication is ongoing, meanwhile the NRM server still assumes that the VAL UE can receive the VAL service communication being broadcasted. To estimate the MBMS bearer quality, for example as an equivalent BLER (Block Error Rate), when no data is sent is implementation specific. This estimation can be dependent on for example the modulation and coding scheme (MCS) and measurements from the reference signals from the eNB(s). Other metrics (e.g. RTP packet loss) may be used to estimate the MBMS bearer quality. Based on the MBMS bearer quality reported from the NRM clients, the NRM server may decide to use the MBMS bearer for a group communication if a certain number of NRM clients located in the MBMS service area are able to receive the VAL service communication. And if a NRM client is not able to receive the VAL service communication on the MBMS bearer, the NRM server may decide to switch the user plane deliver mode for the NRM client from MBMS bearer to unicast bearer.

The NRM client shall indicate the ability of the NRM client to receive the MBMS bearer. Pre-conditions:

1. There is an MBMS bearer activated and the MBMS bearer information is announced to the NRM client
2. The NRM client is located in the MBMS broadcasting area
3. The VAL UE monitors SIB-13 (or SIB-20) and (SC-)MCCH to receive the modulation and coding scheme
4. The VAL UE monitors the cell specific reference signal and when MBSFN transmission is used, the MBSFN specific reference signals