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WCDMA MBMS SolutionBy ZTE Corporation

MBMS (Multimedia Broadcast/Multicast Service) is introduced into 3GPP R6

to send multimedia data from a single source to multiple destinations. With

MBMS, mobile users can receive the same data of wide bandwidth applica-

tions on the same transmission path. Thus, consumption of network resources

depends on content number instead of user number. When there are more

users, network resources, especially radio resources, are more efficiently

used.

Figure 1 and Figure 2 compare unicast and broadcast/multicast.

Figure 2: Broadcast/Multicast

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Figure 3: MBMS network architecture

MBMS supports streaming and file download services, with data rate up to256 kbps. Typical services can be mobile TV, video and audio streaming,video and audio distribution, text and image distribution, etc.

Network ArchitectureMBMS network architecture is based on WCDMA PS domain network. AnMBMS specific functional entity, BM-SC (Broadcast/Multicast Service Center),is introduced for MBMS user service provisioning and delivery. Gmb interface,which is also new, provides control plane functions between the BM-SC andthe GGSN, while Gi interface provides user plane functions. The GGSN, theSGSN, the UTRAN and the UE are enhanced to support MBMS.

MBMS network architecture is shown in Figure 3.

The BM-SC consists of five sub-functions:

• Membership function;• Session and transmission function;• Proxy and transport function;• Service announcement function;• Security function.

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The UE shall perform MBMS related functions:

• Activation/deactivation of the MBMS bearer service;• Security function;• Reception of MBMS user service announcement, paging information and

simultaneous services while receiving MBMS data.

The UTRAN is responsible for delivering MBMS data to the designated MBMSservice area with appropriate radio bearers. Both intra-RNC and inter-RNCmobility of MBMS receivers shall be supported by the UTRAN with minimumMBMS data loss.

The SGSN performs MBMS bearer service control functions for each UE andprovide MBMS transmissions to the UTRAN. The SGSN also supports intra-SGSN and inter-SGSN mobility procedures. The SGSN shall be able to gener-ate charging data per multicast MBMS bearer service for each user.

The GGSN acts as an entry point for IP multicast as MBMS data. It establishesor releases a bearer plane for MBMS transmission upon the notification fromthe BM-SC.

The Cell Broadcast Centre (CBC) may be used to announce MBMS user ser-vices to the users. It is optional for MBMS.

Service ProvisionThe MBMS bearer service can be provided in two modes, i.e. broadcast modeand multicast mode. In case of broadcast mode, the MBMS bearer servicecan be received by all the MBMS capable UEs. Meanwhile, only the regis-tered UEs can receive the MBMS bearer service in case of multicast mode.

As depicted in Figure 4, the phases subscription, joining and leaving onlyappear in multicast mode. They are user specific and initiated by each multicastuser. Other phases are same in both modes and started by the BM-SC.

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Radio InterfaceMinimum change to the R99/R5 system is requested when MBMS relatedspecifications are made in 3GPP, especially on the radio interface. Therefore,the physical layer of the R99 Uu interface is inherited in MBMS. Layer 2 andlayer 3 are enhanced to support MBMS.

Two kinds of radio bearers are defined to transmit MBMS data. One is point topoint (P-t-P) and the other is point to multipoint (P-t-M). Dedicated radiobearers are used in P-t-P transmission. Shared radio bearers are used inP-t-M transmission.

For MBMS P-t-P transmission, the exactly same channel structure is usedas for traditional unicast transmission, as illustrated in Figure 5. Meanwhile,Figure 6 gives out the channel structure for MBMS P-t-M transmission. MCCH,

Figure 4: MBMS service provision

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RK MTCH and MSCH are MBMS specific logical channels. The MCCH is used for

control information transmission and the MTCH for user data transmission.The MSCH carries MBMS data schedule on the corresponding MTCH so thatthe receivers get to know when to receive data from the MTCH in advance.These channels are mapped to different FACHs with different data rate throughMAC-m, which is integrated into MAC-c/sh. The UEs in either RRC connectedmode or RRC idle mode can receive P-t-M data. This is quite important fornetwork resource efficiency and receiver power saving.

To improve the service performance of MBMS P-t-M transmission, severalenhancements have been applied. 40 ms TTI and 80 ms TTI are used inFACH transmission. Downlink power consumption can be greatly decreasedwith soft combining or selective combining of service data from neighboringcells. Soft combining and selective combining are also helpful to minimizedata loss in case of user mobility between cells. Frequency layer conver-gence and frequency layer dispersion are introduced to make it possible tooffer MBMS user services on a designated carrier.

MBMS Prospect, Pros and ConsAmong all the mobile broadcast services, mobile TV has aroused the greatest

Figure 5: MBMS P-t-Ptransmission channels

Figure 6: MBMS P-t-Mtransmission channels

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concern. Whether mobile TV based on MBMS proves successful or not isvery important for the application development of MBMS.

Nowadays there emerge kinds of technology to carry out mobile TV, such asDVB-H from European continent, Media FLO from Qualcomm and S-DMBfrom Korean companies. These systems are specifically designed for mobileterminals to receive TV services. They are featured of numerous programchannels, high spectrum efficiency, wide coverage area, short constructionperiod, etc. As to MBMS, the spectrum efficiency is comparatively low andprogram channels are less because of the limitation of WCDMA physical layerperformance. If an operator does not own a WCDMA network, it will take highcost and long period to offer MBMS user services.

However, MBMS has its advantages too. It is a good choice for mobile opera-tors where they have or are going to have WCDMA networks. Firstly, theyneed not invest a lot especially for mobile TV. Instead, they need only up-grade their WCDMA system without hardware change. Secondly, with MBMS,they have strong control of the network and service. Thirdly, technical advan-tages of MBMS can be maximized, e.g. user discovery, local area services,multiple service forms, service priority, adaptive service data rate. What’s more,for WCDMA terminals, it is also easier to support MBMS than other technology.

ZTE ProgressAs an important member of 3GPP, ZTE has been engaged in MBMS studyever since the kickoff of the specifications. A lot of contributions on MBMShave been offered by ZTE and adopted in 3GPP meetings. At the same time,ZTE has long investigated the market requirements of MBMS and listed it inproduct roadmap for years.

Based on ZTE WCDMA R99/R5 system, the MBMS system will be ready at theend of year 2006. The present CN and RNC are structured with large IPcapacity. The Node B is capable of high FACH rate. Only software upgrade isneeded for these network elements to support MBMS. ZTE is working hard topush MBMS onwards and to provide the customers with best products andservices.

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