Multicast Broadcast Services (MBS) | WiMAX

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).

1 Multi-BS Access MBS

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 Figure1. It has to be noted that a BS may belong to different MBS_ZONEs.

Figure: Example of MBS zone deployment and MBS_ZONE allocations. All the MS registered to an MBS_ZONE can receive MBS signals from any BS of the MBS_ZONE

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.

2 MBS Frame

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 2. 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).

Figure 2: Example of the 802.16 frame with the MBS service zone. The presence of the MBS zone is indicated in the DL-MAP message (in a MBS_MAP_IE field). The exact details of the MBS zone are then described in the MBS_MAP message at the beginning of the MBS zone

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.

MIMO (Multiple-Input Multiple-Output) Solution

MIMO Basics

MIMO systems use multiple input and multiple output antennas operating on a single channel (frequency). At the transmitter side, the signal is space–time encoded and transmitted from N_T antennas. At the receiving side, the signals are received from N_R antennas (see Figure 1). The space–time decoder combines the signal received by the N_R antennas and transmitted from the N_T antennas after having estimated the channel matrix (N_T×N_R).

Figure 1: Generic MIMO block diagram for the downlink

The objective of the MIMO solution is to exploit the space and time diversity of the channels on the different radio paths between each combination of transmit/receive antennas to improve the reception sensitivity and/or to improve the channel capacity. There are several families of MIMO solutions. The two extreme ones are the spatial diversity MIMO schemes and the spatial multiplexing MIMO schemes.

In addition, several MIMO schemes exist that are a mix between SM and spatial diversity schemes. The diversity order and capacity increase depends on the space–time code and number of antennas.

More recently, MIMO schemes using pre-coding have been defined. In these cases, the space–time code depends on a feedback from the receiver on the channel states. Indeed, this solution requires a closed-loop operation and additional signalling between the receiver and the transmitter.

Finally, MIMO can also be generated from signals transmitted from different BSs (virtual MIMO). This requires time synchronisation of the BS but also a synchronisation of the scheduler of the BS involved in the transmission. 

System Design Aspects of BS and MS

As indicated in Figure 1, MIMO operation has a significant impact on the design of the BS and the MS. Indeed, in addition to the MIMO codes, the algorithms for encoding and decoding MIMO signals, there is the requirement to implement several transmitter and several receivers both at the BS and MS sides. This may be critical for the MS side. Indeed, implementing several receiver chains implies several antennas on a small device and additional hardware and processing power. Moreover, several transmitters at the MS side also mean significant additional power consumption. Efficient implementation of MIMO at the MS side has then some technological challenges to be solved.

The preferred antenna configuration for MIMO is when the multiple antennas are perfectly uncorrelated. In that case, the performances of MIMO are optimal. Good correlation is obtained if at the BS side the antennas are separated by 10 to 20 λ and at the MS side by at least λ. The latter requirement makes solutions with more than two antennas at the MS impractical.

In some cases, the additional effort needed for the space–time decoder is significant. This also imposes some limits on the number of transmit antennas at the BS side.

Support of Beamforming in the IEEE 801.16 Standards

Beamforming is defined in IEEE 802.16-2004 and in 802.16e. This feature is not in the set of the fixed WiMAX profiles. For the mobile WiMAX profiles, this feature is mandatory to be supported by the MS and optional for the BS. For the mobile WiMAX, several mechanisms that enhance the performance and operation of beamforming are provisioned.

In the downlink, in order to be able to beamform several users at the same and on different subchannels, a zone is dedicated (indicated in the DL-MAP). This region, labelled (2) in Figure 1, contains permutations with dedicated pilot channels. This means that an MS receiving a burst in this region only takes as valid pilots the pilots associated with the subchannel it has been allocated.

Figure 1: Example of a frame with regions supporting AAS operation

In the uplink, as mentioned previously, the beamforming mechanisms can be applied on any MS. However, to improve the performance and to help the BS in detecting/measuring the interference experienced by the users it wants to serve, a specific signalling zone may be allocated: the uplink sounding zone (indicated by UIUC=13). Actually, the BS may ask some MS to transmit a signal in this zone so that the BS can evaluate the interference on some subcarriers for those MSs.

Finally, in order to limit the signalling overhead, the WiMAX solution employing beamforming may use the compressed maps and submaps to transmit the common signalling messages (DL-MAP/UL-MAP). Indeed, this permits different modulation, coding and repetition schemes to be applied to several zones in the DL-MAP/UL-MAP message. This solution can also be employed in the case of MIMO or the support of the HARQ.