1. 1 ASMS-TF Meeting Toulouse April 24 th - 26 th 2001 vernucci@space.it ASMS-TF Technical Group S-UMTS Operational Models for Point-to-Point Services

2. 2 Background Viable operational models for point-to-point S- UMTS shall be well conceived and assessed before proposing solutions to UMTS operators Operational models cover issues such as: degree of satellite system connectivity (GW-to-beam) landing-GW flexibility ways of sharing satellite capacity among GWs ways to fit S-UMTS within the overall UMTS The above issues yield a remarkable impact on S-UMTS design and on attainable efficiency

3. 3 Examples of Existing Systems IRIDIUM: full GW-to-beam connectivity, free landing GW selection system capacity is a common pool for all GWs Inmarsat: full GW-to-beam connectivity just limited by absence of ISLs satellite capacity is a common pool among visible GWs Globalstar: despite the LEO constellation with no ISLs, a certain GW-to- beam connectivity is in principle available but landing-GW is anyway fixed due to operators agreements capacity is shared among GWs on pre-assigned basis

4. 4 Ability To Select The Landing-GW Landing-GW should be selected on a call-by-call basis. High GW-to-beam connectivity is required With MEO / GEO constellations, landing-GW selection is more appealing, especially if the number of deployed GWs is significant: terrestrial tails cost reduction (more important for circuit-based services), though bulk rates are often offered to operators better GW traffic-load balancing Other advantages (w.r.t. transparent LEO non-ISL system): allows using satellites with highest elevation angle higher number of satellites available for diversity lower average number of satellite handoffs during a call (lower dropped calls rate)

5. 5 Guidelines An S-UMTS system should ideally: be designed for maximum GW-to-beam connectivity and capacity pooling but also be operated such that the built-in flexibility is actually exploited Achieving high GW-to-beam connectivity may be a challenge under typical S-UMTS scenarios: number of beams is great (e.g. > 100) access allowed to quite a great number of GWs: to maximize advantages deriving from landing-GW selection to allow more operators joining the system

6. 6 S-UMTS System Design Issues W-CDMA offers moderate resources assignment granularity (5-MHz modules). With many GWs and beams, GW-to-beam connectivity is constrained : risks of reduced bandwidth efficiency (BIG potential problem): CDMA modules fill-factor may become low if a module is fully assigned to a GW, but sharing CDMA modules among multiple GWs may hardly be feasible on-board processing (OBP) would help. Protocol adaptations likely required risks of reduced power efficiency: control channels yield a significant overhead in terms of power sharing control channels among multiple GWs hardly possible again OBP could help

7. 7 Impact of Integration Strategy Other constraints arise from integration strategy Two alternatives are considered (VIRTUOUS): embedded system: S-UMTS implements a set of USRANs, each attached to the core network of a service provider : satellite system mostly relies on T-UMTS mobility functions MT is bound to land at the GW owned by his service provider satellite resources are typically pre-assigned to GWs self-standing system: S-UMTS having its own core network MT is allowed to land at each GW, unless when unfeasible satellite resources flexibly shared among GWs, even on a call- by-call basis

8. 8 Embedded System Model

9. 9 Self-Standing System Model

10. 10 System Design Example Reference is here made to an hypothetical gap-filler S- UMTS system: based on GSO satellites generating beams-clusters on earth regions not adequately covered by T-UMTS (e.g. developing countries) a beams-cluster will eventually be moved to another earth region when T-UMTS is starting to take place in the previous one GSO satellites permit to largely bypass terrestrial networks. Landing-GW flexibility should then be pursued as much possible

11. 11 Approach With Transparent Satellites With a self-standing system: despite the GSO satellites wide-area coverage, each beams- cluster will only be served by just a few GWs (not to impair system efficiency) in developing countries most telephone calls may be local; the GW should then be located not too far away for packet- services (e.g. Internet) the GW should be connected to a backbone, possibly only available at great distances With an embedded system: further flexibility decrease if, as expected, MT will only be allowed to connect the GW owned by his service provider sharing a GW among multiple operators should be encouraged (adaptations required, e.g. BCCH), though capacity sharing will still be on pre-assigment basis

12. 12 Approach With OBP Satellites On-board regeneration and switching can solve a great deal of problems: full GW-to-beam connectivity becomes viable resources can be flexibly shared among GWs deviation from standard protocols probably unavoidable but OBP advantages may not fully be exploited is system is embedded Regenerative return-link (harder to implement than forward-link) may not be required: with a global-coverage down-link, GWs can exhaustively demodulate all codes and only take those of their concern

13. 13 Conclusions Tight liason with UMTS operators will be needed: for better chances of being endorsed, S-UMTS shall have those features and behave the way UMTS operators like, but the overall S-UMTS + T-UMTS operational model that UMTS operators have in mind may not result in optimally exploiting the satellite system resources and/or flexibility An in-depth appreciation of S-UMTS is crucial for: trading-off conflicting requirements within S-UMTS understanding all technical and economic implications of the solutions that UMTS operators may propose at this regard, some studies are already being carried out (e.g. the ESA funded S-UMTS Bridging Phase) but more detail activities are required, specifically focused on the S-UMTS operational models