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Tuesday, April 19, 2011

3G Digital Content Delivery: The concept of Multimedia Broadcast Multicast Service (MBMS)


Wideband Code Division Multiple Access (WCDMA) Release 99 and High-Speed Downlink Packet Access (HSDPA) Release 5 are used in commercial networks to deliver multimedia content like Mobile-TV and streaming videos. The point-to-point data delivery in WCDMA Release 99 and HSDPA Release 5 allows full flexibility when delivering user-specific and on-demand content to many users at any time. The point-to-point delivery is also spectrum efficient when the number of users is not too large. When the number of users increases and they would like to receive the same content at the same time, it is more efficient to use point-to-multipoint delivery. The 3rd Generation Partnership Project (3GPP) Release 6 Multimedia Broadcast Multicast Service (MBMS) specification introduces point-to- multipoint support. Cellular networks with MBMS can provide the same content efficiently to a large number of subscribers at the same time. In this article I explain the concept MBMS.


MBMS reuses the radio and core network elements in the legacy 3GPP architecture to make the introduction of MBMS cost efficient as illustrated below;

In addition to the legacy elements, MBMS introduces the multimedia Broadcast and Multicast Service Centre (BM-SC). The new BM-SC element supports a number of functions, e.g. service scheduling, service authentication, content encryption, key management and charging. In the legacy 3GPP elements, MBMS requires additional software functionality and possibly additional hardware due to capacity reasons. Adding MBMS functionality to the Universal Mobile Telecommunication Services (UMTS) terminal platforms is similarly straightforward, since, except for minor additions, existing transport channels and physical layer channels are re-used and the MBMS data rates are low compared with WCDMA or HSDPA peak data rates. Therefore, it is expected that MBMS functionality can be part of almost all 3G terminals in the future as long as the terminal form factor allows the use of the services introduced with MBMS. Hence, a promising key strength of MBMS is a large terminal base and a large number of MBMS-capable networks using the same standard globally. Having a large terminal base is a prerequisite for all broadcast technologies.

Since MBMS is designed on top of the UMTS network, it can automatically benefit from the existing WCDMA point-to-point channels to carry any potential service-level feedback information as illustrated below;
The feedback information can be used for chatting, to track how many users that watch certain content, by the content provider for interactive applications, etc. The MBMS user-plane bearers support both point-to-point (unicast) and point-to-multipoint (broadcast) connections, the concepts of which are illustrated below;
This is because if the number of users per cell is low it is most efficient to deliver the MBMS user-plane data over ordinary point-to-point connections. If, on the other hand, the number of users increases and they all would like to receive the same content at the same time, it is more efficient to use point-to- multipoint connections. The point-to-point channels are more efficient for a low number of users because with point-to-point connections it is possible to utilise feedback techniques, including closed-loop power control, link adaptation and retransmissions. Those techniques reduce the required transmission power and interference and they cannot be applied efficiently to point-to-multipoint broadcast transmission. The MBMS standard allows the radio network to select dynamically between point-to-point and point-to-multipoint transmission depending on the number of simultaneous users receiving the same content. The most popular services could be carried over point-to-multipoint bearers for efficient mass delivery and other less popular services could use point-to-point delivery to provide a large selection of different channels. The point-to-multipoint delivery is also beneficial from the terminal power consumption point of view, since no uplink transmission is required for the radio feedback.

MBMS contains two modes of operation: broadcast and multicast. In the case of broadcast the 3GPP core network (Gateway General Packet Radio System (GPRS) Support Node (GGSN) and Serving GPRS Support Node (SGSN) elements) are mainly unaware of when and how individual users view certain content and channels. The broadcast mode supports both point-to-point and point-to-multipoint connections and the transmission in the radio cell is on or off depending on the number of users listening to a certain content channel in this cell. Note, though, that as long as the broadcast bearer is active, the user data is always transmitted from the BM-SC all the way to all Radio Network Controllers (RNCs), i.e. in the core network the transmission is always on from the BM-SC to all RNCs as long as the MBMS session is active. The key difference between the multicast mode and the broadcast mode is that in the multicast mode a user needs to register to each service that the user is interested in receiving, i.e. the user needs to join a multicast group. This is done by the mobile station transmitting a joining command to the network. Hence, in this case the core network is aware of what mobiles actually listen or are interested in listening to each service. Because the core network is aware of the users in the case of multicast, it is possible to activate the MBMS bearers only to those SGSNs and RNCs where there are users listening to the content. In addition, the awareness of individual users can allow for additional charging possibilities, as well as the introduction of group-oriented services. From a radio resource point of view the multicast mode works similar to the broadcast mode. The differences between broadcast and multicast modes are illustrated below;
The dedicated broadcast solutions, like traditional TV and radio broadcasting, or Digital Video Broadcasting–Terrestrial (DVB-T) and –Handheld (DVB-H), use high-power transmission from high masts. The same content is transmitted over a large area. There may be just a few tens of large broadcast towers per country. The large broadcast towers are then also typically complemented with smaller sites to provide coverage throughout the continuous area indoors. The dedicated broadcast solutions do not need to have an uplink connection from the user equipment to the base station. Hence, the link budget for a broadcast solution can be improved by increasing the output power in the base station without worrying about the limited power available in the mobile station in the uplink direction. This enables dedicated broadcast solutions to have both high capacity and high coverage. The MBMS radio transmission uses existing cellular sites and equipment. There can be thousands of base station sites per operator per country. The large number of sites and the limited broadcast capacity can make the MBMS activation more costly than the high-power broadcast solutions. On the other hand, the base station sites are in many cases already available, and with software upgrades the cost may be limited, as long as extra capacity is not needed.MBMS also allows localised content to be delivered. When it comes to the deployment costs, MBMS can then compete well with dedicated broadcast solutions as long as the usage is not too large, while the high capacity offered by the broadcast solutions makes them more cost efficient for heavy usage. The broadcast service withMBMS can combine nationwide channels with local





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