Adaptation of MBMS Architecture and Protocols for SDMB *V. Y. H. Kueh, hB. Martin, jO. Peyrusse, iM. Cole, kE. Bourdin, hC. Selier, *B. G. Evans, jY. Bertrand, kD. Braun *University of Surrey, iLogicaCMG - UK; hAlcatel Space, jMotorola Toulouse SAS, kErcom - France Email: [email protected] ABSTRACT Although achieving maximum hardware/software reuse on the network side and at the terminal side is the main design The Satellite Digital Multimedia Broadcast (SDMB) aim of the SDMB system, there are some differences system implements a satellite based broadcast layer to foreseen with respect to UMTS in general, and MBMS in enhance the 3G and beyond 3G systems in the efficient particular, due to the specific requirements and delivery of the Multimedia Broadcast Multicast Service characteristics of SDMB to implement its specific features. (MBMS). Designed to accommodate 3G standardised Hence this paper describes these differences and the handsets with low cost impact and no form factor adaptations made in SDMB with respect to MBMS, modifications, the SDMB system is based on the 3GPP whereby their impact on the MBMS architecture and standardised technology and utilizes the IMT2000 protocols and procedures as defined in 3GPP is also frequency band. This paper describes the differences and highlighted. The work presented here is based on the the adaptations made in SDMB with respect to MBMS, current output from the IST MAESTRO (Mobile and highlights their impact on the MBMS architecture and Applications & sErvices based on Satellite and Terrestrial protocols. inteRwOrking) integrated project [3]. Keywords – MBMS, SDMB, UMTS, WCDMA The paper is organised as follows. In section II, after I INTRODUCTION briefly illustrating the overall SDMB architecture, the respective SDMB entities and components are described in Multimedia Broadcast Multicast Service (MBMS) is more detail with regard to their modifications and/or add- currently under standardization in various groups within ons with the impact foreseen to the MBMS architecture. the 3rd Generation Partnership Project (3GPP) with the first This is followed by a description of the differences in phase set to be finalised as part of the Release 6 standard SDMB with respect to the MBMS protocols and [1]. By enhancing the UMTS system with broadcast and procedures and their expected impact. Section IV multicast mode capability, the main aim of MBMS is to concludes the paper. efficiently support, in terms of radio and network resources, simultaneous distribution of identical rich II IMPACT OF SDMB ON MBMS ARCHITECTURE multimedia content (streaming and download-and-play The SDMB system architecture is depicted in Figure 1, type services) to large user groups. whereby the satellite link is unidirectional, with the To complement this effort, a hybrid mobile satellite interactive link provided by the standard terrestrial 2G/3G broadcast system, also known as the Satellite Digital mobile network (note that a direct satellite return link is Multimedia Broadcast (SDMB) system [2], has been envisaged in the future evolution of the SDMB system). defined to provide a dependable and cost effective The system is compliant with the broadcast mode of the broadband transmission capacity for multimedia services 3GPP defined MBMS standard, allowing the support of such as mobile TV and video services for mobile services open services platforms as defined in the Open Mobile over a pan-European coverage. In addition to utilizing the Alliance (OMA). 3GPP standardised technology (where possible), the Example of umbrella cells coverage SDMB system makes use of IMT2000 satellite frequency over Europe bands and high power geostationary (GEO) satellite(s) Satellite High power IMT2000 distribution link Geo- combined with terrestrials gap-fillers in order to mobile stationary satellite band satellite accommodate low cost 3G handsets with outdoor as well (S-band) 3G Air interface as indoor reception. Based on this concept of a hybrid FSS (Ka) Direct link frequency satellite/terrestrial architecture, SDMB takes advantage of band Terrestrial Indirect link the natural assets of satellite systems, while ensuring Repeaters Hub based (IMR) on 3G equipment maximum interoperability with terrestrial UMTS (T- 2G/3G standard UMTS) standards in order to encourage multimedia usage Interactive link Content providers take-up in Europe and contribute to the successful 3G handset 2G/3G Mobile Network Content + satellite Network deployment of 3G. Such hybrid architectures have proven features to be business-successful in USA, as demonstrated by the 2G/3G Base MBMS Broadcast/Multicast station Service Centre popular XM Radio/Sirius mobile satellite broadcasting systems. Figure 1: SDMB system architecture and its interworking with terrestrial network Each of the SDMB entities and components shown in re-combine it. The cost of either change is expected to be Figure 1 is described in more detail in the following minimal. subsections. B. Intermediate Module Repeater (IMR) A. User equipment (UE) The IMR receives signal from the space segment and re- The UE is a 3GPP compliant dual mode GSM/UMTS amplifies it. Basically, IMRs will be deployed in terminal, with frequency band extension. It is able to urban/suburban areas, to offer deep indoor coverage and perform parallel idle mode, i.e. maintaining either GSM increase throughput where the satellite signal is not 100% activity or UMTS activity during SDMB reception. The available. They can be co-located with T-UMTS Node Bs. basic type does not have a dedicated receiver for SDMB The SDMB architecture allows the deployment of several and is then required to switch from UMTS terrestrial to types of IMRs : SDMB satellite reception. The enhanced type has a dedicated reception chain for SDMB and is then able to Frequency conversion repeater: receives the SDMB receive SDMB data flow without interruption. signal from the SDMB satellite in High Density Fixed Satellite Systems (HDFSS) frequency bands, amplifies One of the goals of the SDMB system is to have the lowest and retransmits in Mobile Satellite Service (MSS) possible cost impact on the UE, starting with an UE that is band. Tx antenna gain is typically 15 dB (sectored compliant with the UMTS Terrestrial Radio Access antenna), total Tx power ranges from 30 to 35 dBm. Network (UTRAN) standards. This implies that the UE must have little hardware and software changes to be able On-channel repeater: receives the SDMB carriers from to receive contents through the SDMB system. So SDMB the SDMB satellite in MSS band, amplifies them and re-uses as much as possible the existing protocols and air retransmit them on the same frequency slot(s). Tx interfaces defined in UTRAN. antenna gain is typically 15 dB (sectored antenna), while total Tx power is 30 dBm. Due to required Nevertheless remaining adaptations of the UE are needed. isolation between Rx and Tx antenna, this solution One element of SDMB with regards to MBMS is the use of targets limited coverage such as in-building, tunnel, the downlink only: during the attach procedure and the and underground. transfer of data for SDMB, in order to save current drain in the UE, no uplink transfer to the satellite is recommended. Node B-based repeater: receives the SDMB signal One possible solution is to use the 3G terrestrial networks from the SDMB satellite in HDFSS frequency bands, to perform a “classical” (as defined in 3GPP) attachment amplifies and retransmits in MSS band. Tx antenna procedure, then handover to the satellite in order to receive gain is typically 15 dB (sectored antenna), while total the data transfer from the SDMB system. The drawback to Tx power is up to 43 dBm. It is built with 3GPP this solution is the need for cooperation between terrestrial standardised Node B. IMR is managed by the Radio and satellite operators. An alternative solution would be to Network Controller (RNC) of the Hub for radio modify the attachment procedure to avoid the terminal resource allocation and transmission synchronisation. sending information to the satellite and thus receive It is under the control of the RNC of the Hub, via Iub directly all the required information to establish a data interface over the satellite radio link. This link is transfer. unidirectional, thus adaptation of Iub protocols must be implemented. This type of IMR can be integrated During the data transfer, the UE should not send a request inside a T-UMTS Node B. for missing blocks to the satellite directly. This could be avoided by using the terrestrial networks (through requests IMRs are equipped with O&M modules and a wireline/less for missing packets at higher level, IP for instance) or by modem for site supervision and equipment monitoring. using techniques such as data carousels or adapted Forward Error Correction (FEC) algorithm. When IMRs repeat the satellite signal with the same scrambling code, UE Rake receiver maximum window Another change is due to the difference in the channel length introduces high synchronisation constraints on IMR propagation properties between the satellite and terrestrial signal transmission and also fixes IMR coverage radius networks. To ensure optimum reception quality for SDMB, limitations. more fingers (12) should be allowed for the recombination The 3rd type of IMR, Node B based repeater, will of the signal compared to the number of fingers used in the terrestrial environment (usually up to 8). Since mobile eventually allow implementation of signal repetition with a manufacturers mainly implement rake receivers by using a scrambling code different from the one on the satellite signal. In this way, soft and/or selective combining as hardware accelerator, this would imply a hardware change in the UE to increase the number of fingers. specified by 3GPP can be introduced in SDMB. Another advantage is the re-use of 3GPP flexibility such as The last constraint considered herein is the need of a wider enlarged UE reception window, thus decreasing window for re-combining the received signals at the rake synchronisation constraints on the IMR signal receiver level in the case of SDMB.