Mobile Satellite Systems

2

Mobile Satellite Systems

2.1 Introduction

2.1.1 Current Status

Satellites have been used to provide telecommunication services since the mid-1960s. Since

then, key developments in satellite payload technology, transmission techniques, antennas

and launch capabilities have enabled a new generation of services to be made available to the

public and private sectors. For example, satellite television is currently available in both

digital and analogue formats, while global positioning system (GPS) navigation reception is

now being incorporated into new car systems [DIA-99].

In a similar time frame to that of terrestrial cellular development, mobile-satellite services

have been around since the start of the 1980s, when they were first used to provide commu￾nications to the maritime sector. Since then, aeronautical, land-mobile and personal commu￾nication services have been introduced.

Satellites are categorised by their orbital type. Specifically, there are four types of orbits

that need to be considered: geostationary orbit, highly elliptical orbit, low Earth orbit (LEO)

and medium Earth orbit (MEO) (sometimes referred to as intermediate circular orbit). Up

until very recently, geostationary satellites had been used as the sole basis for the provision of

such services. Over the years, as a geostationary satellite’s power and antenna gain charac￾teristics have increased, combined with improvements in receiver technology, it has been

possible to decrease the size of the user’s terminal to something approaching the dimensions

of a briefcase, a small portable computer or a hand-held device.

Significantly, it is now possible to receive via satellite a telephone call virtually anywhere

in the world using a hand-held mobile receiver, of roughly a similar dimension to existing

cellular mobile phones. In addition to stand-alone satellite receivers, it is also possible to buy

dual-mode phones that also operate with a cellular network, such as GSM; simple, alphanu￾meric pagers are also on the market. These latest developments were initially made possible

through the launch of satellite personal communication services (S-PCS), which make use of

non-geostationary satellites. This class of satellite can be placed in LEO, at between 750 and

2000 km above the Earth; or MEO at between 10 000 and 20 000 km above the Earth.

GLOBALSTAR is a system that exploits the low Earth orbit, while NEW ICO is a MEO

system. Recent advances in geostationary satellite payload technology, in particular the use

Mobile Satellite Communication Networks. Ray E. Sheriff and Y. Fun Hu

Copyright q 2001 John Wiley & Sons Ltd

ISBNs: 0-471-72047-X (Hardback); 0-470-845562 (Electronic)

of multi-spot-beam coverage, has enabled this category of orbit to provide hand-held commu￾nication facilities.

2.1.2 Network Architecture

2.1.2.1 Overview

The basic network architecture of a mobile-satellite access network is shown Figure 2.1.

In its simplest form, the network architecture consists of the three entities: user segment,

ground segment and space segment. The roles of each segment are discussed in the following.

2.1.2.2 The User Segment

The user segment comprises of user terminal units. A terminal’s characteristics are highly

related to its application and operational environment. Terminals can be categorised into two

main classes.

† Mobile terminals – Mobile terminals are those that support full mobility during operation.

They can be further divided into two categories: mobile personal terminals and mobile

group terminals.

– Mobile personal terminals often refer to hand-held and palm-top devices. Other mobile

44 Mobile Satellite Communication Networks

Figure 2.1 Mobile-satellite network architecture.

personal terminal categories include those situated on board a mobile platform, such as a

car.

– Mobile group terminals are designed for group usage and for installation on board a

collective transport system such as a ship, cruise liner, train, bus or aircraft.

† Portable terminals – Portable terminals are typically of a dimension similar to that of a

briefcase or lap-top computer. As the name implies, these terminals can be transported

from one site to another, however, operation while mobile will not normally be

supported.

2.1.2.3 The Ground Segment

The ground segment consists of three main network elements: gateways, sometimes called

fixed Earth stations (FES), the network control centre (NCC) and the satellite control centre

(SCC).

Gateways provide fixed entry points to the satellite access network by furnishing a connec￾tion to existing core networks (CN), such as the public switched telephone network (PSTN)

and public land mobile network (PLMN), through local exchanges. A single gateway can be

associated with a particular spot-beam or alternatively, a number of gateways can be located

within a spot-beam in the case where the satellite coverage transcends national borders, for

example. Similarly, a gateway could provide access to more than one spot-beam in cases

where the coverage of beams overlap. Hence, gateways allow user terminals to gain access to

the fixed network within their own particular coverage region.

Integrating with a mobile network, such as GSM, introduces a number of additional

considerations that need to be implemented in the gateway. From a functional point of

view, gateways provide the radio modem functions of a terrestrial base transceiver system

(BTS), the radio resource management functions of a base station controller (BSC) and the

switching functions of a mobile switching centre (MSC) [ETS-99], the latter being connected

to the local mobility registers (visitor location registration (VLR)/home location registration

(HLR)). Figure 2.2 shows a gateway’s internal structure as defined in Ref. [ETS-99]. The RF/

IF components and the traffic channel equipment (TCE) together form the gateway transcei￾ver subsystem (GTS). The gateway subsystem (GWS) consists of both the GTS and the

gateway station control (GSC).

The NCC, also known as the network management station (NMS) is connected to the

Mobile Satellite Systems 45

Figure 2.2 Gateway internal structure.

Customer Information Management System (CIMS) to co-ordinate access to the satellite

resource and performs the logical functions associated with network management and

control. The role of the these two logical functions can be summarised as follows.

† Network management functions: The network management functions include [ETS￾99a]:

– Development of call traffic profiles

– System resource management and network synchronisation

– Operation and maintenance (OAM) functions

– Management of inter-station signalling links

– Congestion control

– Provision of support in user terminal commissioning

† Call control functions include:

– Common channel signalling functions

– Gateway selection for mobile origination

– Definition of gateway configurations

The SCC monitors the performance of the satellite constellation and controls a satellite’s

position in the sky. Call control functions specifically associated with the satellite payload

may also be performed by the SCC. The following summarises the functions associated with

the SCC.

† Satellite control functions, including:

– Generation and distribution of satellite ephemera

– Generation and transmission of commands for satellite payload and bus

– Reception and processing of telemetry

– Transmission of beam pointing commands

– Generation and transmission of commands for inclined orbit operations

– Performance of range calibration

† Call control functions including provision of real-time switching for mobile-to-mobile

calls.

The CIMS is responsible for maintaining gateway configuration data; performing system

billing and accounting and processing call detail records.

The NCC, SCC and CIMS can be collectively grouped together into what is known as the

control segment.

2.1.2.4 The Space Segment

The space segment provides the connection between the users of the network and gateways.

Direct connections between users via the space segment is also achievable using the latest

generation of satellites. The space segment consists of one or more constellations of satellites,

each with an associated set of orbital and individual satellite parameters. Satellite constella￾tions are usually formed by a particular orbital type; hybrid satellite constellations may also

be deployed in the space segment. One such example is the planned ELLIPSO network (see

46 Mobile Satellite Communication Networks