SATELLITE AND TERRESTRIAL INTEGRATED UMTS COMPONENTS. P. Angeletti, M. Lisi, F.Mini and G. Olivieri

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1 SATELLITE AND TERRESTRIAL INTEGRATED UMTS COMPONENTS P. Angeletti, M. Lisi, F.Mini and G. Olivieri Alenia Spazio SpA Via Saccomuro 24, 00131, Rome, Italy ABSTRACT Mobile satellite systems are usually regarded as complementary to terrestrial systems, as they can serve geographical areas not covered by cellular networks. However, next killer application in the Internet may be represented by multicasting applications which, by their nature, are particularly well matched with the satellite characteristics. The satellite cost-sharing advantage can be, in fact, sufficiently high to make use of satellites more convenient with respect to the alternative of flooding the terrestrial mobile networks with replicas of the same data sets. An integrated Satellite and Terrestrial UMTS network allows network operators to improve their terrestrial cellular network with satellite component which not represent only a coverage gap-filler but a cost effective means for implementing multicasting services. In particular, for these kind of services it might be the optimal solution to reserve the satellite segment for the down-link to the customers and use the terrestrial segment for the return direction. Terrestrial UMTS networks will serve the indoor, urban, suburban and some rural locations with a limited geographic coverage. Thus the use of satellite based networks will be essential to realise the UMTS goal of ubiquitous universal multimedia communications services by the International Astronautical Federation or the International Academy of Astronautics. All rights reserved. Satellite components well integrated with terrestrial UMTS networks Universal Mobile Telecommunications System (UMTS), as part of the ITU s IMT-2000 global family of third generation (3G) mobile communications systems, is the most suitable mobile platform for tomorrow s multimedia services and applications. It represents a unique opportunity to create a mass market for highly personalised and user friendly mobile access to the Information Society. The terrestrial deployment of the 3rd generation mobile services system is expected to be rolled-in the near future and it is of paramount urgency to define the role of satellites in one of the fastest growing telecommunications markets of the future. Mobile satellite systems are usually regarded as complementary to terrestrial systems, as they can cover geographical areas unserved by cellular networks. However, with the expected advent of wideband mobile satellite systems namely, i.e. the UMTS satellite component, one can envision another promising market segment, i.e. that of services which, by their nature, are particularly well suited to the inherent multicasting feature of satellites. Examples of such services, mostly non-real-time but possibly also real-time, are news (e.g. general, financial, sport, weather, road traffic), audio / video streams, games, navigation maps, city information, pointcast, etc. The rationale behind such services is that the same data set will simultaneously be delivered to a multitude of users having requested them, thus sharing the satellite cost per delivered bit across many users, and so making the satellite system 1

2 utilisation more attractive from the economic standpoint. The demand for the above mentioned satellite multicast services may not be limited to regions uncovered by cellular systems; as a matter of fact a market for satellite multicasting services can expectedly be found also in areas served by terrestrial systems, provided that the above mentioned satellite cost-sharing advantage is sufficiently high to make use of satellites more convenient with respect to the alternative of flooding the terrestrial mobile networks (namely T-UMTS) with replicas of the same data sets (as required to offer a service equivalent to satellite multicast). Based on the above the main objective should be the definition of a satellite component well integrated with the terrestrial UMTS component and conceived to improve the coverage of the terrestrial cellular network as well as its capability to support new and advanced mobile services. To achieve such ambitious objective the proposed approach is based on the design and development of a proper Demonstrator which should allow the validation of an integrated terrestrial/satellite wideband communication infrastructure for the support of mobile and portable Internet Services. Proposed Demonstrator Approach The future target system deployment which the demonstrator aims to represent should be visioned as a satellite system capable of providing multicast services and conventional services (i.e. point-to-point). In operating multicast services is expected that, most commonly, users will forward service requests via terrestrial facilities, this hybrid approach (i.e. down-link via satellite, and up-link via terrestrial means) opening the road to cheaper user terminals which need not having the capability of transmitting signals to satellites. Moreover by having a similar, or even equal, air interface in both the satellite segment and terrestrial segment of UMTS, the same terminal (with minimum or no upgrading at all) may operate in both segments. This inter-segment operation is also favoured by the fact that T-UMTS and bands are adjacent thus giving the possibility to use the T-UMTS RF-front also in the S- UMTS bands with minimum impact on performance. This is particularly true if the is only used in Rx mode. The above approach would open the road to a truly integrated T- and network. In that case a network operator may integrate its terrestrial cellular network with a satellite overlay which does not represent only a coverage gap-filler but may be exploited for those services, having broadcasting /multicasting nature, where the use of the satellite is more cost-effective than the use of the terrestrial segment. This may be done in a way which can be actually transparent to the user which shall be forced neither to subscribe to any ad-hoc satellite operator nor to buy a costly multi-mode terminal. The demonstrator project objectives can be summarised as follows: Definition of an integrated Satellite and Terrestrial UMTS network architecture for the provision of wireless wideband mobile services exploiting the broadcasting capability of a GEO satellite for the forward link and terrestrial UMTS (or GPRS as far as this project demonstrator is concerned) for the return link. Design and development of prototype user terminals which are able to operate with the set of wireless networks listed above. Design and development of the prototypal equipment part of the terrestrial infrastructure required to operate with the above mentioned environment Validation of the system concept and test of the service performances with an extensive trial campaign involving complementary satellite and terrestrial components. Moreover, as far as the satellite component is concerned, the possibility to exploit existing transponders, for example a LLM payload should be used to test important aspects and in particular: to assess the technical feasibility of providing services through UMTS satellite components, by experimental campaign; to demonstrate the feasibility of high rate multimedia services via satellite, for mobile users; to validate interworking with terrestrial components; to validate the Satellite Air Interface; to consolidate UMTS technology; to identify critical aspects for low cost terminals and to manufacture prototypes aiming at validating critical technologies. to demonstrate the capability of high data rates mobile communication service provision, by a comprehensive field trials. 2

3 Finally, knowing the link behaviour in a real world could greatly improve the accuracy in calculating the contributory impairments in equipment and propagation characteristics. This could lead to a reduction of the link margin in the theoretical link budget and hence to a decrease of system building costs and operating tariffs (note that a 3 db excess margin would almost double user charges).the first generation of wireless communication system still works today but has low capacity when compared to more modern systems. It also cannot accommodate advanced features such as Internet access and multimedia and has a tendency to drop calls. The second generation, based on digital transmission methods, allows for easier implementation of advanced features, dramatically better sound quality and increased battery life, but operates at speeds hardly adequate for data transfer, and the number of user on the system is creating capacity problems. Hence the evolution towards the third-generation (3G) of wireless communications is a fundamental step towards the world of multimedia mobile communications where users will have access not just to voice but to video, image, text, graphic and data communications. The Universal Mobile Telecommunications System (UMTS) represents the third generation of mobile technology, aiming to provide communications services, to any one, any where, at any time, in a Multimedia environment with a quality comparable with that provided by fixed Networks such as ISDN. It is expected that the satellite will play an important role in the extension of UMTS coverage worldwide: providing an extended set of high data rate services at reasonable cost, particularly to support Internet applications, while enhancing terminal mobility. Figure 1: Proposed Architecture 3

4 providing conventional voice and low data-rate services (e.g. 8 Kbit/s, or so) with higher quality of service and superior in-building penetration capabilities. A first proposed system architecture suited to provide the required services is shown in Figure 1. Moreover further trade-offs should be done (e.g. the kind of satellite constellation, regenerative versus transparent payload, etc) in order to identify the best architecture for the satellite component of UMTS. The innovative aspects of the proposed system architecture with respect to the state of the art are: the definition of a really integrated satelliteterrestrial architecture exploiting for Broadcast like or point-to-multipoint applications the unbeatable satellite broadcasting capability for the forward link and the terrestrial network for the return link. the development of user breadboards able to provide UMTS services via hybrid satellite/terrestrial facilities. the possibility to check on the field multicasting application performances in the developed hybrid S- and T-UMTS network. Implementation Aspects UMTS will be a global system comprising both terrestrial and satellite components. The target system architecture which the proposed demonstrator aims to represent is depicted in Figure 2 and is conceived as an hybrid terrestrial / satellite system. The proposed demonstrator should help to convince terrestrial operators to develop e common study work on satellite terrestrial UMTS network integration especially as far as the multicasting applications. Hence the main objective of the demonstrator should address the design and implementation of high performance Internet services (with particularly emphasis on multicasting applications) over networks consisting of interconnected high data rate satellites, and near future terrestrial network such as T-UMTS or GPRS Terrestrial network, with various data capability (with rates from 64Kbps to 400Kbps). The satellite segment according to IMT 2000 system vision is an system able to operate either in a gap filler role with respect to the T-UMTS component (even providing seamless inter-segment roaming and handover) or, and this constitutes the novelty of this project, in a much more integrated way with the terrestrial component allowing simultaneous usage of both satellite and terrestrial resources for the same communication session if such simultaneous usage is advantageous to reduce the service cost. A typical situation in which this scenario may be appropriate is related to the provision of multicasting services. For this service category it is expected that the satellite segment represents the most suitable technology for the delivery of multicasting data (obviously assuming that multicasting is done in the outbound direction, i.e. from the network to the mobile users) whilst the terrestrial one is more cost effective for the transmission of inbound reply packets. The most compelling disadvantage of such a scenario is the need for the user to equip itself with a costly dual-mode terminal. However by having similar air interfaces in both the satellite segment and terrestrial segment of UMTS, the same terminal (with minimum adaptation) may operate in both segments, this interoperation also being favoured by the fact that T- UMTS and bands are adjacent thus opening the possibility to also share the same RF-front with minimum impact on performances. Further, a class of terminals which are only able to receive from the satellite segment may actually present no significant difference with respect to terrestrial only terminals (given the downlink air interface is appropriately selected to match as much as possible the down-link T-UMTS one, this being a task which seems within reach,being most of the differences between S- and T-UMTS localised to the up-link) and would be fully appropriate for exploiting the satellite segment advantage in a multicasting scenario. In a truly integrated S- and T- UMTS network it would be only the network operator which based on economic consideration would decide on the routing of multicasting or broadcasting data without any involvement of the user in such decision process. The demonstrator is intended to validate the performance of such an integrated, hybrid terrestrial / satellite multicast target system by means of an European common experimental set-up sufficiently representative of the system, though necessarily simplified to reduce total costs. To achieve this goal the demonstrator will feature: a limited number of User Terminals; the likely use of a GPRS (General Packet Radio Service) network during the first phase of the project and successively the utilisation of a T- UMTS island ; 4

5 the adoption of the so-called half-rate mode (1.92 Mchip/s) of the Satellite Wideband CDMA (SW-CDMA) standard, to comply with the limited bandwidth (4 MHz) made available for example by the Artemis LLM payload which may expected to be used for the demonstration. At this last regard it should be noted that, should Artemis not be available in useful time for the demonstration, recourse will be made to the EMS payload which is already operational aboard the Italsat II satellite, and which also offers a similar bandwidth; demonstration will be limited to the multicast services ( point-to-point services are known to be demonstrated in the course of other projects) The overall system demonstrator block diagram is shown in Figure 2. The Demonstrator is designed to support generic TCP/IP applications, thus allowing any Application Client (connected to the User Terminal) to interact with a remote Application Server (connected to the Demonstrator Server Facility via the global Internet). In particular, two specific multimedia applications which are considered to be particularly well matched to the satellite multicasting capability will be developed and validated by the demonstrator. As already mentioned, the demonstrator exploits satellite resources for efficient multicasting of packet data to multiple users and terrestrial resources (GPRS as baseline, but possibly T-UMTS if available) for uplink information delivery back to the transmitter (e.g. TCP/IP acknowledgement or satellite access return signalling). To this end, the user terminal comprises the essential elements of both an mobile station (requiring ad-hoc developments) and a GPRS mobile station (a commercial test set). L-band mobile link Feeder Link Gateway Station RF & IF User Terminal #2 APPLICATION CLIENT PHYSICAL LAYER TX RADIO CUSTOM RX RADIO PROXY RF & IF PHYSICAL LAYER RX RADIO TX RADIO CUSTOM TCP IP STACK tunneling (*) User Terminal #1 GPRS MOBILE STATION CHIP SET GPRS BASE STATION SGSN (*) GPRS NETWORK CORE NETWOR IP WAN (*) SGSN = SERVING GPRS SUPPORT NODE (**) GGSN = GATEWAY GPRS SUPPORT NODE GGSN (**) LAN INTERNET SERVER Demonstrator Server Facilit APPLICATION CLIENT (*) both for application and for radio access APPLICATION SERVER Figure 2 : Detailed Block Diagram of the Proposed demonstrator 5

6 The section of the User Terminal is receiveonly. Hence closure of the radio access protocols is tunnelled on a TCP/IP connection established over the GPRS mobile link. Similarly, the Application Client operates on an IP stack which is suitably modified to tunnel outbound traffic on another TCP/IP connection also established through the same GPRS link. The GPRS Base Station is connected to the GPRS core network IP-based WAN through a SGSN (Serving GPRS Support Node). On the same IP WAN a GGSN (Gateway GPRS Support Node) connects to the LAN provided as part of Demonstrator Server Facility. On this LAN an Access Server is provided which on the one hand terminates the IP tunnel related to application(s) and reroutes them to the appropriate destination (e.g. the Application Server in the case of the applications specifically developed for the demonstration), and on the other hand relays the radio access feedback tunnel to the remote Gateway (probably via an Internet base Virtual Private Network, as shown in Fig. 1, to spare a dedicated interconnection link between the Gateway and the Demonstrator Server Facility) and in particular to a Proxy which implements a custom TCP/IP stack matched to that operating at the User Terminal. The Access Server also tunnels the Internet data to be delivered to the User Terminal to the Gateway through another IP tunnel, which is also terminated on the Gateway Proxy. Said Proxy interfaces with the Transmit Radio Access protocol stack, which is in turn interfaced to the physical layer. Also it interfaces the receive Radio Access protocol stack for terminating the peer-to-peer radio access protocols. These protocols shall be custom designed and optimised upon the topology here proposed. This implies that no tight interaction between the Gateway and the User Terminal shall be required at the radio access layer, due to the considerable delay which can be experienced on a link which is actually closed via an IP WAN. This delay would adversely impact the performance of typical procedures such as power control or beam handoff, therefore such procedures should be carefully re-assessed. At this regard it should be noted that the need for power control in a satellite mobile system based on a GEO satellite constellation could not be so stringent, the channel dynamic being quite limited; moreover, for multicast applications support, power control appears be to probably useless, the same signal being simultaneously addressed to multiple Mobile Terminals. Finally, the need for beam- (and obviously satellite-) handoff can be less important due to the relatively wide beams and their stationarity with respect to ground. The Demonstrator will avail itself of an existing Gateway, most likely interconnected to the Demonstrator Server Facility via a Virtual Private Network over Internet, the Gateway could be a transportable Ku band station. Conclusions Summarising the work to be undertaken on a possible design and development of a satellite-terrestial integrated UMTS demonstrator, following main activities can be mentioned. A first activity will have the aim to identify all the parameters characterising the services that the system intends to provide and to define the requirements for the target system. In the frame of this activity, an important effort will be assigned for a marketing analysis with the scope to give an accurate portrait of the current situation, and then to give a market forecast. Furthermore, this activity will have the scope to identify the key elements of the user terminal and their cost forecast. A second important activity will be devoted to define the system architecture (in terms of Space segment, Ground segment including the user terminals and network interfaces) best suited to provide the identified services whilst maintaining compatibility with T-UMTS networks. The selection of the satellite constellation and transponder selection for the demonstration will be the result of a trade-off based on different service criteria, such as low cost, high availability, low risk of call interruption and unimpaired interactive communications. The need for small, portable and low cost User Terminals will be a driving factor in the overall system definition. In the frame of this activity, due to the importance of efficient bandwidth utilisation, satellite air interface trade-offs analyses will be carried out. Moreover, standardisation and regulatory aspects will be identified. Finally, an experimental phase based on field trials execution will be carried out in order to check the theoretical evaluations and to provide important feedback to the previous system definition phase. Moreover this phase will allow testing of the developed equipment. 6

7 References Preparatory Activities Final Study Report, (ESA Contract 13695/99/NL/4S). A Regulatory Framework for UMTS Report #1 from the UMTS Forum. UMTS Market Forecast Study Final Report for EC DG XIII, Analysys/Intercai Report. Preparatory Activities Early Technological Developments and Service Demonstration, (ESA Contract 13695/99/NL/4S). 7