1.1 Scope and organisation
This book has been written to promote an in depth understanding of technical concepts bearing in mind the business aspects of mobile satellite communication systems, a branch of telecommunications rapidly becoming a part of our daily lives. Its relevance to the modern world is best demonstrated by a few illustrative real-life situations:
· A live satellite coverage from the summit camp brings a realism in people’s home never experienced before – a nation rejoices as the summit is conquered;
· A passenger ship cruising in high sea hits a rock, the crew in a desperate attempt for survival, send a distress call; a rescue party arrives shortly;
· An old man’s wizened face comes alive as he talks to his son from his thatched village home, tucked away, where time stands still.
· An executive, preparing for a meeting on a trans-oceanic flight contacts headquarters and receives vital documents within minutes.
Mobile satellite service or MSS systems provide communications to terminals which may be in motion, or moved at will anywhere within the service area. Terminals may be mounted on an aeroplane, a ship or carried by individuals; alternatively, the terminal may be a portable communicator set up at a convenient location. The vital elements are user mobility with minimal regulatory restrictions in the service area. Mobility is achieved by the use of a radio link for communication and incorporating network intelligence to manage mobility. Regulatory restrictions are minimised through appropriate spectrum selection and operating licenses. Service areas of a MSS can span a country, a region, or indeed, the world.
There was a steady and gradual growth in mobile satellite services until early 1990, at which juncture demands began to accelerate as new services were introduced and public awareness heightened due to the success of terrestrial mobile systems. A further spurt in aggregate world-wide demand occurred when satellite phones were introduced in the third quarter of the decade, despite business failure of some MSS operators. The failures were caused by differences in user expectations in terms of cost and quality and operator expectations in terms of market size and rate of penetration dented by advances in roaming arrangements of terrestrial operators. However, there is an enhancing public and commercial awareness of the role of satellite systems in supporting mobile and personal communications seamlessly, while capabilities of MSS systems continue to increase with shrinking terminal size, increasing throughput and integration with terrestrial mobile systems.
As happens in rapidly evolving technologies, collating cohesive information can be time-consuming and difficult, as useful material lie buried and scattered in specialist reference books, journals, conference proceedings, trade magazines, etc. This book attempts to bridge the gap through a structured presentation of such knowledge to assist understanding of system architectures, their components, applications, and trends. A comprehensive list of references are included for those interested.
The International Telecommunications Union (ITU), a United Nations body which regulates world-wide allocation of radio spectrum world-wide, has for spectrum planning categorised radio services according to their broad application – Broadcast Satellite Service (BSS), Fixed Satellite Service (FSS) or Mobile Satellite Service (MSS), etc. Personal satellite communications services such as voice, facsimile and multi-media services require a radically different system design with a commercial approach quite new to the satellite community. The ITU has termed such services as Global Mobile Personal Communications Services (GMPCS). This book deals with system level technical issues of the Mobile Satellite Service and the Global Mobile Personal Communications Services. We will refer to these services together as Mobile Satellite Service, unless a specific distinction is essential.
This chapter introduces the topic, beginning with a review of the evolution of mobile communications. A subsequent section presents basic concepts of MSS architecture to familiarise the reader with the topic early in the book. A plethora of telecommunications product often leaves users uncertain regarding the most suitable solution. To enable users and prospective operators to make informed decisions, salient features of satellite and terrestrial systems are compared, followed by an overview of applications typical of a mobile satellite service. In recent years satellite navigation systems, direct sound broadcast systems and fixed satellite service products have entered the domain of personal communications. There is a certain degree of competition as well as synergy between various types of personal communication products. The principles and salient features of such systems are summarised, and finally, emerging trends reviewed
Chapter 2 discusses an extensively reviewed topic of satellite communications – satellite constellations. After introducing the basics of orbital mechanics for the benefit of readers unfamiliar with this rather specialised topic, the chapter discusses characteristics of various types of constellations and compares them for a number of well known recent constellation designs.
Chapters 3-6 introduce components and concepts applicable to mobile satellite communication systems. Some are applicable generally to satellite communication system, but the treatment here is slanted towards an MSS perspective. Chapters 3 and 4 discuss various MSS air interface – spectrum issues, radio frequency (RF) propagation characteristics, modulation methods, coding schemes, link analysis and multiple access techniques. Chapter 5 addresses the main characteristics and technology of mobile terminals which have profound implications to the success of MSS operators. Considerable public interest has arisen recently regarding radio frequency radiation effects on humans; a section has been devoted to this topic where the current state of understanding is summarised. Gateways provide radio connection between a fixed network and the space segment; a section highlights MSS-specific features of gateways, which are otherwise identical to medium earth stations of the fixed satellite service. Satellites are undoubtedly the most vital node of an MSS. With improvements in spacecraft device technology, there is a gradual shift from the traditional transparent transponder to the regenerative transponder because of advantages offered by the latter. Chapter 6 discusses the main features and emerging spacecraft and inter-satellite link technologies..
This book views the MSS in a commercial perspective as most modern systems are, and in this context, business and technology are intricately entwined. Hence, chapter 7 changes the emphasis to MSS economics illustrating its inter-relationship with technology.
In Chapter 8 we explore MSS in a systems context by collating concepts discussed in previous chapters. Topics include system synthesis methodology with influences, constraints and trade-offs; networking issues; and operational considerations such as radio resource management and quality of service.
A number of technically interesting systems have been proposed recently each with some novel feature and representative examples are discussed in chapter 9 to illustrate how concepts have been translated to practice. The choice of system examples are made purely on the basis of their technical variety and merit with little bearing to commercial performance or affiliations.
In chapter 10 we discuss systems which offer services akin to mobile satellite system but not formally a part of it. In the past few years, the terminal size of the fixed satellite service have shrunk to an extent that they are portable, and hence the distinction between fixed and mobile services have become blurred in applications where mobility is either not essential or restricted. There is a significant interest in providing satellite sound broadcasts directly to small portable receivers mounted on cars or carried by individuals. Several applications, such as fleet management, are combining MSS communications with position fixes available through GPS. Terrestrial mobile communications have now entered the third generation, and it is recognised that the third generation mobile systems will be generic with a satellite component, implying that there be better appreciation of terrestrial mobile systems by the satellite community and vice-versa. In such an environment, a treatment of mobile satellite communications cannot be taken in isolation. We will, therefore, also introduce salient features of terrestrial cellular systems in this chapter.
While first generation satellite phone service are barely established, plans for establishing second generation services are progressing in a number of international forum and the commercial world. The final chapter discusses various new concepts under investigation – integration of fixed and mobile networks, the concept of broad-band satellite mobile systems, advanced space segment concepts and others.
An appendix summarises the characteristics of several mobile satellite communication proposals.