A DSP Based Diversity Modem for FDMA and TDMA Mobile Satellite Applications

Abstract

Development of mobile Communications user terminal equipment is being driven by the aim of providing personal communications services (PCS) via a handheld unit. Current terrestrial (mobile cellular and cordless phone) handsets are restricted to operation within a single system, e.g. GSM, IS-95, DECT. With the currently available geostationary mobile satellite systems, (e.g. Mobilesat in Australia, MSAT in North America, and Inmarsat-M with global coverage), handheld terminals are not feasible due to power limitations and the large link path losses. These units are usually transportable or vehicle mounted. With the planned integration of mobile services with FPLMTS and PCS, there is growing interest in the development of dual mode or multimode terminals which can support access to more than one system. The user can then select between different services based on cost, availability and performance. For example, a dual mode GSM/DECT telephone user might wish to use DECT access when available, and GSM otherwise for cost and power usage. Similarly, a Mobilesat/GSM handset user would face a similar choice. This scenario is of particular interest as the availability of an overlayed mobile satellite system provides very wide area coverage over, for example, remote areas where cellular coverage may not be available. This is an important part of the strategy for next generation mobile satellite systems, such as Iridium, ICO and Globalstar. Although they will support handheld terminals from almost any geographic location in the world, they are limited to working with a direct line of sight signal, which essentially means out door usage only. Needless to say, the mobile satellite access charges will also be higher than those for terrestrial systems, and so it is important to provide users with the option of selecting either satellite or terrestrial access, depending on availability.