Multicast Broadcast Services (MBS) may be required when multiple MSs connected to a BS receive the same information or when multiple BSs transmit the same information. Indeed, this allows resource to be saved by allocating a single radio pipe for all users registered to the same service instead of allocating as many pipes as there are users. This is of particular interest for the broadcast TV type of application where, at the same time, several users under the same coverage area are connected to the same service (in this case, a TV channel).
The mobile WiMAX system supports MBS as an optional feature for the BS. When the MBS feature is supported, the Multi-BSs Access mode is implemented, as defined in the IEEE 802.16e standard .
In a Multi-BS MBS system, several BSs in the same geographical area transmit the same broadcast/multicast messages at the same time on the same frequency channel. These BSs actually belong to MBS_ZONE. An MBS_ZONE is a unique identifier, which is transmitted from each BS of the set on the DCD message. An example of an MBS_ZONE allocation is provided in Figure 12.11. It has to be noted that a BS may belong to different MBS_ZONEs.
A Multi-BS MBS operation requires from the BSs belonging to the same MBS_ZONE:
time synchronisation (frame number and symbol level);
use of the same CID for broadcast/multicast messages;
use of the same Security Association (SA) for the encryption of the broadcast/multicast messages.
Using a Multi-BS MBS solution provides two major advantages. First, the MSs that successfully registered to a MBS service can receive MBS information from any BS of the MBS_ZONE without needing to register to a specific BS of that zone (even MS previously in the Idle mode). Secondly, the MS receives MBS signals from multiple BSs simultaneously. This provides a macrodiversity gain and performance enhancement for the MBS signals. This is actually similar to the Single Frequency Network (SFN) concept that can be found in DVB-H systems.
When the MBS is activated on a BS, it can use a dedicated frequency or it can use only a dedicated zone in the frame, as illustrated in Figure 12.12. The descriptions of existing MBS zones in a frame are indicated using the DL-MAP in the extended DIUC 2 field (DUIC = 14) by MBS_MAP_IE, which includes (among others) the MBS_ZONE identifier, the symbol offset where the MBS zone starts, the permutation to be used (PUSC or other) and the size of the MBS zone (in symbols and subchannels).
Each MBS zone starts with MBS_MAP, which describes the MBS connections available in the MBS zone. In particular, it contains, among others, the CIDs used by the multicast/broadcast connections, the channel configurations (modulation, coding and eventually repetition coding) and the logical channel ID. Logical channel IDs are used to distinguish the different MBS connections inside a MBS zone that belong to the same MBS CID (for instance, this may be used to differentiate between different TV channels with different subscription rights).
The MBS PDUs in the MBS region follow the order of the combination of multicast CID/logical channel CID, as described by the service association between the MS and the BS. Of course, from the time when the MS goes into the Idle mode or when the MS moves across different BS areas of the same MBS_ZONE there is no need for the MS to be registered to the BS to receive the MBS content. The mapping between the multicast CID and the logical channel CID must be the same in the cells belonging to the same MBS_ZONE. The procedure to provide consistency of the identities used by the MBS channels is beyond the scope of the IEEE 802.16 standard.