ASMS-TF Meeting Toulouse April 24 th - 26 th 2001 vernucci@space.it

1. ASMS-TF MeetingToulouse April 24th - 26th 2001vernucci@space.it ASMS-TF Technical GroupS-UMTS Operational Models forPoint-to-Point Services

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. 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. 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. 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. 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. 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. Embedded System Model

9. Self-Standing System Model

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. 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. 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. 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