1. Introduction

1.1 Scope

IMT-2000 is the ITU defined third generation mobile system concept. IMT-2000 is scheduled by ITU to start service around the year 2001 subject to market needs considerations. They will provide access, by means of one or more radio links, to a wide range of telecommunication services supported by the fixed telecommunication networks (e.g. PSTN/ISDN) and Internet, and to other services which are specific to mobile users.

The IMT-2000 terrestrial component will not be able to provide global coverage, and large geographic areas will remain without terrestrial coverage. The satellite component will provide IMT-2000 services in these geographic areas. Of the population within terrestrial coverage, some IMT-2000 users will travel for both business and pleasure, to areas without terrestrial coverage. The high penetration expected for IMT-2000 means that some users will want communications with high mobility at anytime and everywhere they travel. It is only through the combination of IMT?2000 terrestrial and satellite components that true global coverage can be accomplished for key features.

Therefore, the required key features of IMT-2000 satellite components are high degree of commonality with IMT-2000 terrestrial components, compatibility of services with IMT-2000 terrestrial components and with fixed network. In addition, use of a small sized terminal with world-wide roaming capability using multi-mode function, high quality and capability for multimedia applications and a wide range of services are also expected for key features.

At WARC-92, 60 MHz of spectrum was identified for IMT-2000 MSS in Radio Regulations. However, the expected demand for additional IMT-2000 MSS spectrum after initial deployment have to be considered due to the growth in satellite mobile communications, has been discussed at ITU-R TG8/1 meetings. The TG8/1 made the proposed text for CPM report to WRC-2000 at 16th meeting(8-9 March 1999, Fortaleza, Brazil) of TG8/1, which indicates that an additional allocation for MSS of 2 x 67 MHz would be required by 2010.

An important factor when looking at spectrum for mobile satellite systems is the system replacement and evolution. Satellite systems equipment needs to be replaced after for ages, however, replacement of this equipment is inherently a long-term process and can be difficult to accomplish. Therefore, satellite's RTT for future satellite IMT-2000 systems will require substantial planning, investment and long-term vision with the operators.

Ideally one RTT, encompassing all environments (satellite, cellular, cordless , WILL) and all regions should be considered. In ITU-R TG8/1, the majority of opinions regarding the harmonisation for RTTs of IMT-2000 MSS is that there is no need to agree on a single RTT. Global/regional and transparent/regenerative systems may favour different solutions.

However, there are needs for which, open (not proprietary) RTT to ease competition/access in/to the satellite component, open standard to ensure efficient exploitation of spectrum resources, ensure the commonality with terrestrial system to maximise spin-off and to make cheaper/smaller dual-mode terminals.

Therefore, from the above view points, the scope of this document covers requirements and objectives, satellite operating environments, overview of radio interface, guideline of satellite radio interface layer 1, consideration to upper layer radio interface of satellite components and network interface and coordination to other systems of a IMT-2000 Mobile System.