Content for  TS 23.434  Word version:  19.5.0

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

This subclause specifies service continuity scenarios when MBMS bearers are used. There are different solutions for different scenarios.

The service continuity solution described in this subclause is suitable in the scenario when multiple MBMS bearers are used with the purpose to cover a larger area. In VAL communications several VAL service communication streams may be multiplexed in one MBMS bearer. Furthermore, one VAL service communication stream may be sent on more than one MBMS bearer if the receiving users are distributed over more than one MBMS service area. A VAL UE that is interested in receiving a VAL service communication stream that is broadcasted in both MBMS bearers is a candidate for this service continuity procedure. Figure 14.3.4.6.2-1 illustrates a deployment scenario that provides service continuity between two MBSFN areas. Two different MBMS bearers are activated (TMGI 1 and TMGI 2), the activation of the bearers is done in the two MBSFN areas (MBSFN 1 and MBSFN 2). The MBSFN areas 1 and 2 are partially overlapping, meaning that some transmitting cells belong to both MBSFN area 1 and MBSFN area 2.

Figure 14.3.4.6.2-2 illustrates the procedure:

This service continuity procedure mitigates the risk of packet loss that may occur if the VAL UE would request to transfer the VAL service communication stream to a unicast bearer when moving into the new area and then back to a multicast bearer when the UE can listen to TMGI 2. However, it is still required that the NRM client sends a location report (and MBMS listening report), as described in steps 4-6 above. To send the location report and the MBMS listening report by the NRM client to the NRM server a unicast bearer is needed. The location report from the NRM client is required, since the NRM server must know that the VAL UE has entered a new area and can only listen to MBMS bearer active in that area. If this is not done the VAL server might send a VAL service communication stream that the VAL UE is required to listen to on the MBMS bearer 1, since the NRM server still assumes that the VAL:UE is located in the MBSFN area 1. The solution can be improved as illustrated in Figure 14.3.4.6.2-3. In this case two different MBMS bearers are activated (TMGI 1 and TMGI 2), these MBMS bearers are used only for VAL service communication. An application level signalling bearer is activated (TMGI 9), in both MBSFN areas. This bearer is used for application level signalling messages that are sent on the MBMS bearer TMGI 9. By using an application level signalling bearer (e.g. TMGI 9) the VAL UEs can receive application control messages for all VAL service communication going on in the areas of both TMGI 1 and TMGI 2. A VAL UE that is located in the area of TMGI 2 and is interested in a VAL service group transmission (e.g. V2X) only going on in TMGI 1, can with the information received in TMGI 9 initiate a unicast bearer and request to receive that specific VAL service communication over a unicast instead. Without the information received over TMGI 9 the NRM client must immediately report that the VAL UE has left the broadcast area that the NRM server assumes that the VAL UE is located in. With the use of TMGI 9 there is no immediate need for the NRM client to inform the NRM server of a location change.

The procedural steps in this scenario will be the same as described above in this subclause. However, in this scenario the NRM client is not required to initiate a unicast bearer to send location report (or MBMS listening report). The VAL UE may move between the two MBMS bearers (TMGI 1 and TMGI 2) without the need to report an area change. A condition for this to work is that there is an application level signalling bearer (TMGI 9) activated in the full area (i.e. the area of both TMGI 1 and TMGI 2). The TMGI 9 will broadcast all application control messages for all VAL service communications ongoing in both areas. If the VAL UE is in coverage of one of the two MBMS bearers that does not transmit the VAL service communication of interest the VAL UE can report to the NRM server that it is not able to listen to the VAL service communication over the MBMS bearer, which triggers the NRM server to switch to a unicast bearer instead.

In this procedure the NRM client is requested by the NRM server to send a MBMS suspension report. This request for MBMS suspension report can be included in the MBMS bearer announcement and the NRM server may choose to only send this request for MBMS suspension report to a subset of all NRM clients.

Figure 14.3.4.7.2-1 illustrates a procedure in which the NRM client notifies the NRM server about an MBMS suspension decision in RAN. The NRM server can decide on a subset of all VAL UEs in the MBMS broadcast area that shall report on MBMS bearer suspension. When the NRM server makes the decision of the VAL UE subset, consideration shall be taken to the location of the VAL UEs, since VAL UEs' location is dynamically changed. This means that the MBMS suspension reporting instruction may need to be updated regularly based on the VAL UEs mobility. Pre-condition:

• It is assumed that there is at least one active MBMS bearer

The NRM client that is not instructed to send an MBMS suspension report shall still detect the MBMS suspension indication from RAN (step 3). A NRM client shall in this case not send other types of report (e.g. MBMS listening reports). The same procedure can be applied at MBMS resumption or other MBMS events that may be detected by the NRM client.

The NRM server is an instantiation of a GCS AS. For the NRM server to know the status of the MBMS bearer, and thus know the network's ability to deliver the VAL service, it is required that the network provides MBMS bearer event notifications to the NRM server. The different events notified to the NRM server include the MBMS bearer start result (e.g. when the first cell successfully allocated MBMS resources), including information if any cells fail to allocate MBMS resources to a specific MBMS bearer.

The procedure in Figure 14.3.4.8.2-1 shows notification information flows from NRM server to BM-SC.

The NRM server monitors the bearers used for VAL service communications and decides to switch between MBMS and unicast bearers.

Figure 14.3.4.9.2-1 shows the procedure for service continuity when a UE is about to move out of MBMS coverage or getting into good MBMS coverage by switching between MBMS bearer and unicast bearer. Pre-condition:

• It is assumed that a bearer (unicast or MBMS) has been activated by the VAL server for downlink delivery.