Capabilities of Mobile Satellite Systems (MSS)
The most significant attribute of any satellite communication system is the wide area coverage that can be provided with very high guarantees of availability and consistency of service. The satellite component of UMTS can potentially provide the terrestrial service user with a global service without regard to incompatible terrestrial standards used elsewhere. Existing satellite mobile services have proved very attractive to the maritime and aeronautical sectors and they have also been of great benefit to emergency services, relief agencies, journalists, and expeditions over recent years.
Services are now extending to the land mobile market where hand-portable voice terminals are now technically feasible. The next subclauses address the key attributes of wide area coverage and types of services appropriate for satellite UMTS.
6.1 Large area coverage
A single satellite can see very large areas of the Earth: a single LEO can illuminate an area of 6 000 km diameter and a GSO can illuminate about 1/3rd of the globe. Within these areas, the spacecraft antenna can be designed to maintain a near-constant power flux density on the Earth's surface irrespective of range. However for the GSO and HEO (and possibly the LEO or MEO), the spacecraft antenna may need to be arranged as a cluster of spot beams (1 000 to 2 000 km diameter) in order to make hand-held terminals feasible and to achieve spectrum efficiency. Such spot beams require large spacecraft antennas for either GSO or HEO systems. The advantages of HEO and GSO are that it is possible to deploy a satellite system to fulfil a regional requirement rather than a global one, and frequency planning and co-ordination may be relatively straightforward. Furthermore, the ground infrastructure to support the satellites could follow traditional Land Earth Station (LES) approaches.The only satellite system that cannot provide polar coverage is GSO. With this restriction, any satellite constellation can provide assured line-of-sight global coverage unaffected by weather. Operation to shadowed or in-building terminals would require an additional link margin in the order of 20 dB or more, depending on the coverage required. Note that in cities, the terrestrial UMTS service is likely to be available and therefore in-building and city coverage may not be essential.
The line-of-sight case requires polarisation matching between the satellite and the mobile terminal. To avoid the need for polarisation tracking, mobile communications have traditionally used circular polarisation.
6.2 Flexible networks and services
A feature of most present day satellites is the use of "transparent transponders". Compared to conventional cellular base stations, the satellite transponder is little more than a frequency shifting amplifier. This does have drawbacks with regard to some aspects of system design but it also means that any one satellite is reasonably independent of modulation system or access method, or of service data rate or networking. This has led to satellites being used for a variety of applications, each with different terrestrial architectures. Provided the basic satellite parameters are satisfactory, these services can be introduced long after launch. Future satellites may not be quite so flexible as some studies propose to use on-board processing to improve capacity, spectrum efficiency and satellite payload performance. The transparency concept has however proved extremely costeffective and any on-board processing function is likely to be at least re-configurable and re-programmable. Another feature that might be introduced for MEO or LEO is the inter-satellite link to simplify terrestrial networking between satellites during handover.
The transparency concept has enabled mobile satellite systems to efficiently support a range of services beyond that of voice telephony:
- high data rate services (up to 64 kbit/s) to larger antenna (0,15 m ～ 1,0 m, 8 dBi ～ 20 dBi) mobile or fixed
terminals;
- group call and broadcasting;
- low data rate paging, alerting and two-way messaging;
- terminal location finding.
Some current satellite systems are designed so that extra services can be provided at very little additional cost. This is particularly effective when services are offered as a package to perhaps offset the requirement for line-of-sight paths for low-cost voice telephony.
