Mobility Support for LTE Broadcast Service in 3GPP StandardStandard

Carrier Aggregation (CA) is a feature of LTE-Advanced that allows the aggregation of multiple component carriers to attain maximum transmission bandwidth (up to 100 MHz, corresponding to five 20 MHz carriers in the current LTE network). CA combines carriers at the mobile device to increase the user data rate, giving a better quality for high-bitrate services such as HD video streaming application. In case of eMBMS service, CA can offer the same service over multiple carriers/frequencies in order to increase the service’s coverage. Another possibility is one service may be diffused on different component carriers in different quality (resolution of the video) to provide the diversity of media content and adapt to the need of the mobile users.

In the high mobility environment nowadays, a circumstance most likely happen is thats when receiving eMBMS service, a client moves out of the coverage of serving cell or frequency. Realizing the need of supporting the mobility for eMBMS user in LTE-Advanced network, especially with CA deployment, the 3GPP standardization body has introduced a supplement to provide the service continuity for broadcast services in the release 11 of its specifications [1]. With this supplement, the mobile terminals can achieve the service continuity when changing cell within one MBSFN area. Thanks to the guidance of the serving eNB, the UEs are able to switch to the frequency that supports their interested eMBMS services.

According to the 3GPP standard, the network is responsible for informing LTE ter-minals (in both RRC-Connected and RRC-Idle mode) about the eMBMS services that are supported in the current frequency as well as in the neighboring frequencies. This in-formation is given in the User Service Description (USD) and an MBMS specific System Information Block - SIB15 [41]. In the USD, each service will be associated with its own Service Identity which is a part of the Temporary Mobile Group Identity (TMGI), the frequencies on which it is transferred and the MBMS Service Area Identities (SAIs). The SIB15 contains two elements :

— A list of all MBMS Service Area Identifications (SAIs) that are supported in the

current cell/frequency.

— A list of neighboring carrier frequencies together with the corresponding SAIs they are supporting.

In order to get the SAIs of neighboring frequencies/cells, the serving cell can demand directly from its neighbors over the X2 interface or from the Operations, Administration, Maintenance (OAM).

Figure 3.3 – System Information Block Type 15 Structure.

Combining the information of the SAIs listed in SIB15 and the SAI of the MBMS service indicated in USD, the mobile terminal can determine which frequency provides the services it is receiving or interested in receiving. For LTE terminals in RRC-Idle state, identifying the appropriate frequency is very important because it could help them to prioritize candidates for the cell re-selection procedure in a right order. If an eMBMS UE is looking for a cell to camp and interested in eMBMS services, it would assign a higher priority order to the frequencies that offer its services. With this mechanism, the eMBMS users can choose the right candidate to camp and receive their desired broadcast services.

As the result, the eMBMS reception will be maintained for the idle UEs if at least one neighbor frequency provides the required services.

If none of neighboring frequencies provides the appropriate eMBMS services, the UE

Figure 3.4 – eMBMS Mobility Support for Idle Users in LTE standard.

will choose the candidate with the best signal as usual. After camping to the new cell, the terminal will listen to the control information related to eMBMS service in SIB13 and MCCH message. Fig. 3.4 illustrates how the eMBMS service continuity is done for RRC-Idle users in LTE network.

Unlike in the idle mode, the serving cell will choose the target cell to hand the RRC-connected users over. The mobile users who are getting or wish to get eMBMS service will assist the current cell in handover procedure by giving the information regarding the eMBMS interest besides the normal measurement report. This extra information is enclosed in one RRC message and is sent to the serving cell as a response to the SIB15. The message is called MBMS Interest Indicator and it consists of the frequencies on which the UE is receiving or interested in receiving eMBMS services. The user may send the MBMS Interest Indicator message as soon as the RRC connection is established or when it changes the interest. And this information can be sent by the serving cell to the target cell during the Handover procedure. In addition, this message also contains one bit to indicate whether the UE prefers the eMBMS reception to normal unicast reception. The current cell will use these informations in choosing the suitable cell for handover decision. The candidate on the frequency providing the required eMBMS services will be in first priority and when the UE switches to that frequency, it can continue to receive its interested service. The eMBMS mobility support procedures for RRC-Connected UE in 3GPP standard release 11 is depicted in Fig. 3.5.

With the enhancement in Rel-11, LTE standard is now supporting the service continuity for eMBMS. However, the mobility support is still restricted in changing among frequencies within one MBSFN area. There is no information provided to help the UE in case of switching from one MBSFN Area to another. Therefore, if none of neighbor frequencies

Figure 3.5 – eMBMS Mobility Support for Connected Users in LTE standard.

provides the desired broadcast service or there is no CA deployment in the network, the eMBMS users may not get the interested services in target cell (even though it is available in another MBSFN area and the UE should have connected to the neighboring cell belongs to that MBSFN area).