Mobile Satellite Systems

INTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS

Int. J. Commun. Syst. Network 2010; 28:29–57Published online 11 August 2009 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/sat.941

A survey on mobile satellite systems

Paolo Chini1, Giovanni Giambene1,�,y and Sastri Kota2

1CNIT—Universita degli Studi di Siena, Via Roma, 56, Siena 53100, Italy2Harris Corporation—GCSD, 1134 East Arques Avenue, Sunnyvale, CA 94086, U.S.A.

SUMMARY

Satellite systems represent a significant solution to provide communication services to mobile users inunder-populated regions, in emergency areas, on planes, trains, and ships. In all these cases, satellitesystems have unique capabilities in terms of robustness, wide area coverage, and broadcast/multicastcapabilities. This paper surveys current mobile satellite networks and services from different standpoints,encompassing research issues, recent standardization advances (e.g. mobile extension for DVB-S2/-RCS,DVB-SH) and some operational systems (e.g. Globalstar, Inmarsat BGAN, Iridium, and Thuraya). Thelast part of this paper is devoted to qualitative and quantitative comparisons of the different mobilesatellite systems to understand their characteristics in terms of services, capacity, resource utilizationefficiency, and user mobility degree. Copyright r 2009 John Wiley & Sons, Ltd.

Received 21 July 2008; Revised 13 April 2009; Accepted 17 June 2009

KEY WORDS: satellite communications; mobile satellite systems; design issues; standards

1. INTRODUCTION

Satellite networks are an attractive approach for communication services in areas of the worldnot well served by existing terrestrial infrastructures. There is a vast range of sectors (e.g. land-mobile, aeronautical, maritime, transports, rescue and disaster relief, military, etc.) needingmobile communication services and where the satellite is the only viable option [1]. This is thereason why at present there is a renewed interest and market opportunities for Mobile SatelliteSystems (MSSs). Technologies for multi-spot-beam antennas, low-noise receivers, and on boardprocessing have permitted to achieve the direct access to the satellite for small, portable or evenhandheld terminals by using S, L, and recently Ku and Ka bands. Satellites can also be equippedwith a regenerating payload and inter-satellite links, thus respectively permitting to switch traffic

*Correspondence to: Giovanni Giambene, Dipartimento di Ingegneria dell’Informazione, Universita degli Studi diSiena, Via Roma, 56, 53100 Siena, Italy.yE-mail: [email protected]

Contract/grant sponsor: European SatNEx II; contract/grant number: IST-027393

Copyright r 2009 John Wiley & Sons, Ltd.

flows from different beams of a satellite and traffic forwarding/routing in the sky throughsatellites.

Satellites are on suitable orbits around the earth; on the basis of their altitude, they can becategorized into Geosynchronous Earth Orbit (GEO) and non-GEO, as explained below [2]:

� A GEO satellite is on the earth’s equatorial plane at a height of about 35 800 km, asignificant distance that entails huge signal propagation delay and attenuation. TypicalGEO satellite communications use high frequencies (e.g. S, L, and even Ku and Kabands), thus exacerbating the path loss experienced by the signal. For these reasons, GEOsatellites are better suited for fixed communication services, where large-size antennas canbe used in the earth station. Nevertheless, there are several GEO systems providingservices to mobile users.

� Non-GEO satellites use two possible orbit types: Low Earth Orbit (LEO), at a heightbetween 500 and 2000 km of altitude, and Medium Earth Orbit (MEO), at a heightbetween 8000 and 12 000 km of altitude. Non-GEO satellites have the advantage to becloser to the earth with respect to GEO ones, thus allowing much lower end-to-endlatency in transferring data as well as better link budget conditions. Unfortunately, non-GEO systems need several satellites (i.e. a constellation) to cover a region or the wholeearth, so that frequent handover procedures are needed to switch a connection from onesatellite antenna beam to another, from one satellite to another or even from a terrestrialgateway to another.

MSSs may suffer from non-Line-of-Sight (non-LoS) propagation conditions due to thepresence of obstacles or return link budget restrictions caused by the low power and smallantenna size available on portable terminals. In order to address these problems, two similar,but distinct, innovative design approaches can be adopted: (i) hybrid networks and (ii) integratednetworks. In the first case, terrestrial gap fillers (repeaters) can be employed to retransmit locallythe satellite signal in non-LoS conditions. Moreover, the return link can be supplied by aterrestrial cellular system to simplify the power management of mobile terminals. Finally, thesatellite coverage can be extended (e.g. indoor or urban cases) by means of a local wirelesssystem where the base station ‘converts’ the satellite signal to the wireless one and viceversa. For what concerns the integrated networks, a terrestrial cellular network can be used asan alternative system to connect the mobile user (both forward and return links) with respect tothe satellite one. Some examples of integrated networks are analyzed in [3] and in [4], referringto the Mobile Applications & sErvices based on Satellite & Terrestrial inteRwOrking(MAESTRO) project. In order to define the terrestrial segment, the European Commissionhas introduced the concept of Complementary Ground Component (CGC); while, FCC in U.S.has used the term Ancillary Terrestrial Component (ATC). These concepts are quiteinterchangeable, even if CGC is more related to hybrid networks and ATC to integratednetworks. In any case, terrestrial systems could be based on 3rd generation (3G), WirelessFidelity (WiFi, IEEE 802.11 a/b/g), orWorldwide Interoperability for Wireless Microwave Access(WiMAX) technologies.

The following MSS projects deal with the challenges and the efforts for providing broadbandmultimedia services to users in land vehicular, aeronautical, and maritime environments:(i) MObile Wideband Global Link sYstem (MOWGLY) [5]; (ii) Mobile Broadband InteractiveSatellite multimedia Access Technology (MoBISAT) by ETRI [6]; and (iii) Broadband Global

P. CHINI, G. GIAMBENE AND S. KOTA30

Copyright r 2009 John Wiley & Sons, Ltd. Int. J. Commun. Syst. Network 2010; 28:29–57

DOI: 10.1002/sat

Area Network—eXtension (BGAN-X) by the European Space Agency, ESA (see Section 4.3) [7].Moreover, the SATellite-based communication systems within IPv6 (SATSIX) project aims,among others, to incorporate the IPv6 protocol inside broadband MSSs [8–10].

The aim of this paper is to provide an overview on several aspects of MSSs, such as maindesign issues, available standards and an analytical approach to compare their capacity, theresource utilization efficiency, and the user mobility. Some operational MSSs for public use aretaken as examples. This paper is organized as follows: Section 2 describes design implications tosupport mobile users in satellite networks. In Section 3, we present five standards suitable forMSS communications. Section 4 describes some MSSs that are compared in Section 5 in termsof several parameters. In Section 6, we present future trends for MSSs. Finally, concludingremarks are drawn in Section 7.

2. DESIGN ISSUES FOR MSS NETWORKS

This section addresses important improvements and special solutions that are needed in thedesign of satellite communication systems in order to support mobile users. Our study is carriedout below by focusing on specific aspects at the different layers of the protocol stack.

2.1. Medium-related issues

2.1.1. Frequency bands and regulations. Frequency bands are assigned at the World Radio-communication Conferences (WRCs), periodically organized by the International Telecommu-nication Union–Radiocommunication sector (ITU-R). While fixed services use high C and Kfrequency bands, mobile services are better suited for lower L and S frequency bands that wereassigned at the World Administrative Radio Conference (WARC) 92 [11]. MSSs have exploitedL/S-band technology for a long time: L/S-band systems permit small on-board antennas due tolower signal attenuation and reduced impact of atmospheric effects. However, the need ofbroadband services and the limited amount of available L/S-band resources (2� 30MHz) havepushed toward the use of Ku and Ka bands for MSSs. ITU-R has assigned Ka band frequencyportions to MSSs and Fixed Satellite Systems (FSSs) on a primary basis in all regions(29.9–30GHz for earth-to-space link and 20.1–21.3GHz for space-to-earth link) and Ku bandfrequency portions to MSS on a secondary basis in all regions (14–14.5GHz for earth-to-spacelink and 10–12GHz for space-to-earth link). At present, Ku-based MSSs are available toprovide broadband services in many mobile environments, such as trains, boats, planes, andcars. However, Ku-band satellites, as opposed to L/S-band satellites, do not provide a goodcoverage over seas, because antenna spot-beams footprints are focused on landmasses [12]. Infact, Ku-band satellites are mainly intended for fixed users, so that there are not enough Ku/Kaband satellites providing coverage over oceans. Hence, a trade-off has to be achieved betweenthe need of increased bandwidth and coverage issues.

The European regulatory framework for the use of L/S band by MSS is becoming obsolete.For this reason, on February 2007 the European Commission decided a public consultationamong the MSS manufacturers and operators for the use of 2� 30MHz bandwidth in L/S bandfor MSSs (i.e. the band (1980–2010)MHz in uplink and (2170–2200)MHz in downlink). Aninteresting issue that such a consultation had to address was how to allocate bandwidth to MSSsonce auctions will be done for the use of available L/S bands [13]. The main outcome was theconfirmation of the interest for these frequency bands by many MSS operators and the

A SURVEY ON MOBILE SATELLITE SYSTEMS 31


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indication of the possible number of operators to be selected with auctions. On December 2008,the European Commission, decided to admit four applicants and, among those, selected on May2009 Inmarsat and Solaris (i.e. a joint venture between Eutelsat and SES-ASTRA).

2.1.2. Mobile terminal antenna. The antenna design is a crucial issue for mobile terminals. Animportant aspect is the antenna size, the cost, and the adopted technology. Moreover, theantenna system should be reliable and efficient in terms of sensitivity, gain, and interference. It isimportant to highlight some differences between fixed and mobile services: fixed terminals usedirectional antennas, while mobile terminals can also use omni-directional antennas (anyway,phased-array directional antennas with fast tracking algorithms could be adopted instead ofomni-directional antennas in order to improve the link budget). Typically, mobile terminals cantransmit in all the directions and receive signals from all the directions as well. For this reason,mobile terminals could interfere with other satellite networks. There are some studies in theliterature that address interference issues between fixed and mobile satellite services. In [14], theauthors describe interference characteristics between a non-GEO MSS and a GEO FSS. In [15],the author analyzes non-GEO fixed and mobile satellite service constellations, providing somesuggestions for regulations (in terms of maximum transmitted power and elevation angles) toavoid interference among them.

Further considerations on terminal antenna design can be done by taking into account thedifferent application environments: for example, the railway scenario is well served by Ku-bandsatellites (coverage over landmasses), but the antenna on trains should be small (low-directivitygain), thus generating higher interference levels for adjacent satellites. In aeronautical andmaritime scenarios, planes and boats could be at the edge of spot-beam coverage, thus requiringa suitable antenna design. However, big antennas could be used in the case of big boats thathave lower design constraints.

The antenna size on the mobile terminal determines the characteristics of interference forboth uplink and downlink transmissions. Moreover, there are off-axis power flow limitations foruplink transmissions in Ku band (there are only secondary allocations for MSSs). This entailsconstraints on the Effective Isotropic Radiated Power (EIRP) for the mobile user. In order tomitigate interference, spread-spectrum schemes can be used. Several spread-spectrum techniquescan be adopted (e.g. Direct Sequence, DS, Frequency Hopping, FH, and burst repetition). Thestandardization for the mobile extension of Digital Video Broadcasting—Satellite version 2/Digital Video Broadcasting—Return Channel via Satellite (DVB-S2/DVB-RCS) has consideredDS spreading for the forward link and burst repetition for the return link (maximum spreadingfactor of 16 with Single Channel Per Carrier, SCPC) [16]. The DS approach is a well-knownscheme where the transmitted bits are multiplied by a sequence of shorter chips; while, the burstrepetition technique consists in transmitting many times the same packet in the same time slotinterval. Further details on DVB-S2/DVB-RCS extension for mobile usage are described inSection 3.

2.1.3. Satellite antenna and frequency reuse. One of the key aspects in realizing MSSs is the useof a high-directivity multi-spot-beam satellite antenna, consisting of a large deployable reflectorand a feeder system. At present, typical big-antennas on GEO satellites can reach a diameter upto 25m, while we can expect a diameter around 2m for LEO systems. Spot-beams are needed inorder to focus the covered area on the earth with a high antenna gain. Current MSSs exploitsatellite antennas with a high number of beams: hundreds of beams for GEO satellite antennas;



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dozens of beams for LEO and MEO satellite antennas. The allocated frequency band is dividedinto some carriers that are distributed among beams in order to avoid interferences amongadjacent beams; carriers can be reused in sufficiently far beams. A cluster is a set of beams whereall the system carriers are used. Some examples of (average) cluster sizes (i.e. number of beamsper cluster) for MSSs (see also Section 5) are [17]:�12 beams/cluster for Iridium, �27 beams/cluster for BGAN, and�21 beams/cluster for Thuraya. We can note that GEO systems, such asBGAN and Thuraya, are characterized by higher values of the cluster size: in GEO systems‘narrower’ (i.e. higher directivity) beams than in non-GEO ones are needed to irradiate the samearea on the earth. Hence, beams are much ‘closer’ each other in antennas on GEO satellites, thusentailing higher levels of mutual interference and the need for a larger frequency reuse cluster.Here, ‘closer beams’ means a greater density of satellite antenna beams per unit of solid anglerelated to the satellite; such density is much higher in GEO cases than in non-GEO ones. On thecontrary, the spot-beam footprints (i.e. cells) irradiated on the earth by non-GEO satellites aresmaller and closer each other than in the case of GEO satellites.

2.1.4. Elevation angle. Another important issue for a good quality of the communication is theminimum elevation angle according to which a mobile terminal can see the satellite in an MSS.While the requirements on this angle are not so stringent for FSSs due to the fact that thelocation and orientation of the user antenna can be optimized (e.g. LoS conditions can beachieved for GEO satellites by selecting appropriate earth station locations), in the MSSscenario (in particular for land-mobile users) we need to avoid a low value of the minimumelevation angle, otherwise we could have frequent shadowing and blockage events for the signaldue to trees, buildings, hills, etc. Such a problem is not much relevant to aeronautic andmaritime users, where there are no blockage events, except for those cases where planes or shipsare close to the borders of satellite coverage. Increasing the elevation angle, the signal qualityimproves (reduction of shadowing/blockage effects), but also system costs increase (highernumber of satellites in the constellation). For this reason, a good choice of the minimumelevation angle for future MSSs is around 201 in the case of land-mobile users [18]. However, infirst-generation MSSs, also elevation angle values p101 have been adopted. The minimumelevation angle requirement entails suitable design constraints for the number of satellites in aconstellation and also entails that GEO satellites cannot service polar regions.

2.1.5. Channel models. ESA has carried out a measurement campaign at Ku and Ka bands thathas permitted to define a channel model for MSSs [19]. In particular, in the land-mobile case, thechannel at both Ku and Ka bands can be characterized by a three-state Markov chain model,featuring LoS, shadowing and blockage (non-LoS) conditions. For what concerns the Ku (Ka)band, a Rice (Loo) distribution characterizes each state. Multipath, shadowing and blockageeffects are also present at lower L frequency bands. However, a typical choice for the L-bandchannel is to consider a two-state (good–bad) Markov model [20]. The parameters of thesemodels depend on the environment (city center, suburban area, and rural area) where the mobileterminal is located as well as the elevation angle.

2.2. Physical (PHY) layer issues

An important aspect for MSSs is to use an adaptive air interface with the possible choice amongseveral modulation and coding techniques to adapt to channel variations due to user movement;



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note that adaptation to channel variations implies the use of a feedback channel to inform thetransmitter about the most suitable physical layer transmission parameters to guarantee acertain quality at the receiver. Such adoption is viable only for land-mobile (low speed) usersand becomes critical for higher frequency bands.

It is important to highlight that signal blockage effects can cause a demodulatorsynchronization loss with a period of unavailability during the resynchronization process.Different solutions may be used to face this non-LoS problem: for example, gap fillers (in thepresence of extended or permanent obstacles), space diversity (e.g. using two receiving antennasthat are distant more than the length of obstacles), and time diversity (e.g. using a timeinterleaver for spreading the errors occurring during a persistent fading event).

2.3. Medium Access Control (MAC) layer issues

According to [21], many handover scenarios can be considered: in a non-GEO case, usermobility is dominated by the satellite constellation mobility; while in a GEO case, mobility ispresent only for users accessing the service from planes, trains, and ships. The resourceassignment at the MAC layer (layer 2) has to provide adequate priorities for handovermanagement: handed-over traffic typically suffers from extra switching delays (and, in somecases, re-routing delays when gateway changes are involved) and, hence, it needs an adequatelayer 2 prioritization in receiving resources in the destination cell, otherwise the related sessioncould be terminated by higher layers.

2.4. Network layer issues

Let us refer to satellites with on-board IP routing capabilities. Hence, Mobile IP (MIP)developed by IETF could be used to support handover procedures.z Unfortunately, MIP hasthe problem of high handover latency. NASA and CISCO have carried out many projects toimprove MIP for handover procedures in IP-based satellite networks [21].

As an alternative to the above MIP approach, Connexion by Boeing (an in-flight GEO-basedInternet connectivity service, not anymore active since 2006) allowed global IP mobility usingthe Border Gateway Protocol (BGP) [22]. In particular, a Class C IP address block is assigned toa mobile platform (i.e. a plane or a ship, having on-board a data transceiver/router box andsome 802.11 a/b/g wireless access points). These addresses are ‘selectively announced’ by thenearest terrestrial gateway, for the period the plane/ship passes through the region where thegateway is located (four gateways have been used to cover North America, Europe, and Asianregions). When the plane/ship leaves the region, the gateway stops advertising the IP addressblock that is advertised by the neighbor gateway.

Finally, the IEEE 802.21 Media Independent Handover (MIH) standard could be adopted tomanage handovers between IP-based satellite networks and other mobile networks in anintegrated system (this functionality is provided through a new layer of the protocol stack that isbetween layer 2 and IP at layer 3) [23].

zMIP solves the IP mobility problem by means of a routing approach, managing a dynamic association between a care-of-address to a home address, called binding.



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3. STANDARDS FOR MSSS

Nowadays, we can consider at least the following five standards that are directly related toMSSs:

� Global System for Mobile Communications (GSM) via satellite,� Satellite—Universal Mobile Telecommunications System (S-UMTS),� Digital Video Broadcasting—Satellite Version 2 (DVB-S2) and related return-link

standard,� Satellite—Digital Multimedia Broadcasting (S-DMB), and� DVB—Satellite to Handheld (DVB-SH).

The main driving force for the definition of the above standards is that satellitecommunication systems should be able to provide to mobile users the same accesscharacteristics of their terrestrial counterparts. These standards are described below.

3.1. GSM

At present, GSM is the most popular standard for cellular communications in the world andsupports packet-switched data with the General Packet Radio Service (GPRS) [24]. GSM is aterrestrial system, but extensions are commercially available that permit a ‘form’ of GSM viasatellite. In particular, we can consider the GEO Mobile Radio (GMR) air interface that is usedfor mobile services via GEO satellite. The European Telecommunications Standards Institute(ETSI) has produced two sets of specifications for GMR derived from GSM. Thesespecifications are called GMR-1 (used by Thuraya, see Section 4.4) and GMR-2 (used byACeS, see Section 4.5) and contain adaptations for the GSM standard to cope with thecharacteristics of GEO systems. GMR allows the access via satellite to the GSM core network.Besides a cellular coverage on the earth and the frequency reuse concept, there are othersimilarities between GSM and GMR in particular for what concerns the protocol layers abovethe physical layer. Mobile terminals can be dual-mode, thus allowing using either the terrestrialGSM interface or the GEO satellite one when there is no terrestrial signal (integrated networkapproach).

3.2. S-UMTS

There has been a standardization activity within ETSI for the extension of the UMTS standardto the satellite context, S-UMTS [2]. UMTS is one of the 3G terrestrial cellular technologies [25];we refer here to the UMTS version based on the Wideband—Code Division Multiple Access(WCDMA) air interface with Frequency Division Duplexing (FDD). The ETSI TC SES grouphas defined the S-UMTS Family G specification set, aiming at achieving the satellite airinterface fully compatible with the terrestrial WCDMA-based UMTS system. S-UMTS is notonly intended to complement the terrestrial UMTS coverage, but it is also conceived to extendUMTS services to areas where the terrestrial coverage would be either technically oreconomically unfeasible. S-UMTS uses frequency bands around 2GHz that are close to thoseused by terrestrial 3G systems. S-UMTS supports user bit-rates up to 144 kbit/s, an acceptablevalue for multimedia services to mobile users, typically having small devices.



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S-UMTS has been largely analyzed in the literature. For instance, the study made in [26]proposes a possible S-UMTS system architecture where the satellite segment is interconnectedwith an IP-based core network.

We refer to S-UMTS ‘phase-1’ as to a WCDMA system implementing a forward path viasatellite at 2GHz for the support of broadcast and multicast services, with the possibility toexploit a return path through a terrestrial 3G segment for interactive services (hybrid networkapproach). Then, the S-UMTS ‘phase-2’ will also allow a return path via satellite with optimizedlink budget for mobile handsets and possibly considering an Orthogonal Frequency DivisionMultiple Access (OFDMA)-based air interface, operating at 5GHz. This is in agreement withthe recently started activity in the working party 4C of ITU-R, focusing on multi-carrier airinterfaces for the satellite component; this is in the aim of the compatibility with terrestrialsystems evolving toward 4th generation (4G) mobile networks, such as UMTS Long TermEvolution (LTE) and WiMAX.

3.3. DVB-S2—mobile extension

DVB-S2 is the second-generation standard for satellite broadcast transmissions [2, 27]. Besidesbroadcast services, DVB-S2 can also be employed for interactive point-to-point applications(e.g. Internet access) by using new operation modes that permit to adapt dynamically theModulation and Coding (ModCod) levels depending on channel conditions at the receiver. DVB-S2 has been conceived for fixed users, but at present there is interest in evolving this standard tosupport mobile users on planes, trains, and landmasses by operating in Ku and Ka bands [28].This extension needs to address many challenging issues, such as stringent frequencyregulations, Doppler Effect, frequent handovers, and impairments in synchronizationacquisition and maintenance. In addition to this, the railway scenario is affected by periodicshadowing, fast fading (due to train mobility, there are deep and frequent fades caused by thepoles of the electrified lines) and long blockages (presence of tunnels and large train stationswith non-LoS propagation conditions to the satellite). A new version of the DVB-RCS standardhas been made available by the DVB organization in order to support mobile users; thisspecification is identified with the acronym DVB-RCS1M [16].

3.4. S-DMB

3G mobile networks providing multicast/broadcast services are assuming an interesting role inSouth Korea, Japan, and Europe, where many mobile entertainment services are provided tousers. The S-DMB standard envisages a satellite-based broadcast component for 3G mobilenetworks; it permits to distribute the Multimedia Broadcast Multicast Service (MBMS) that canalso be offered via GSM or 3G cellular networks [29].

The S-DMB system has been studied in the Mobile Digital broadcast Satellite (MoDiS)project [30]. The S-DMB architecture is composed of a GEO satellite and some terrestrialrepeaters named Intermediate Module Repeaters (IMR) that are co-located with 3G basestations (WCDMA technology) and have the function to cope with heavy shadowing in urbanareas (hybrid network approach). South Korea started S-DMB in 2005 in order to providesatellite digital radio services. Then, such technology has been employed to provide also videoservices. Utilized bandwidths are in the VHF and L bands. From joint S-DMB and DVB-Handheld (DVB-H) experiments, a new technology, i.e. DVB-SH, has been conceived operatingin the S band around 2.2GHz, as detailed below.



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3.5. DVB-SH

DVB-SH is an ETSI mobile broadcast standard based on an Orthogonal Frequency DivisionMultiplexing (OFDM) air interface for the provision of audio and video broadcast services tosmall handheld terminals and to some vehicular devices [31]. DVB-SH achieves a large coverageby combining a satellite component (geographical global coverage) and a CGC system:terrestrial repeaters are envisaged to increase the DVB-SH service availability in zones where itis impossible to have LoS conditions with the satellite (e.g. urban and indoor areas). DVB-SHwill also complement the coverage of DVB-H terrestrial systems: dual-mode terminals areconsidered, with DVB-SH reception in S-band (around 2.2GHz, near the 3G terrestrialfrequencies) and DVB-H reception in UHF-band. DVB specifications for IP DataCasting(DVB-IPDC) allow the two systems being complementary.

The main interest of DVB-SH is on broadcast services, but also data push delivery and IP-based interactive services (via an external return channel, e.g. UMTS) are supported. The usercan access these services when traveling on ships, cars, trains, or while walking.

At present, there is an increasing interest for the DVB-SH standard and related services. Thisis the reason why ICO Global Communications (see Section 4.5) have selected DVB-SH for themobile video service platform supported by the recently launched ICO-G1 GEO satellite,covering the North American region (CONUS area) [32].

4. AN OVERVIEW OF CURRENT MSSS

In this Section, we provide details of some operational MSSs (i.e. the LEO-based systems ofIridium and Globalstar and the GEO-based systems of Inmarsat BGAN and Thuraya). Then,some basic information is provided for additional operational or planned MSSs (i.e. the GEO-based systems of ACeS, Eutelsat & SES Astra (Solaris Mobile), Hispasat, ICO, MSV, andTerreStar). In these systems, the satellite antenna has multiple spot-beams that irradiate cells onthe earth, thus creating a cellular-like coverage.

4.1. Iridium system

The Iridium system has to be considered as one of the ancestors of all the MSSs existingtoday [33]. Iridium is the only satellite system to provide complete earth coverage, includingpolar regions, aeronautical routes, and oceanic regions. Such system (see Figure 1) is a LEO-based wireless communication network designed to support voice and low bit-rate datatransmissions anywhere and anytime. Iridium constellation uses 66 active LEO satellites withOn Board Processing (OBP) capabilities and Inter Satellite Links (ISLs) among satellites. Notethat an end-to-end path is established (switching) in the sky through ISLs for voice calls, whiledata transmissions may need to use ISLs to relay the flow toward the satellite that covers thenearest Iridium terrestrial gateway. Currently, there are five gateways in service. Iridiumprovides communication services also to the U.S. Department of Defense. Recently, the IridiumCompany has announced the planning of second-generation IP-based satellites that should beable to monitor continuously the environment, to take pictures of the earth and to allow highbit-rate data transmissions [34].



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4.2. Globalstar system

Globalstar is the other system (together with Iridium) that represents the precursor of currentMSSs [35]. Globalstar uses 48 bent-pipe LEO satellites. No ISL is used (see Figure 2).Globalstar satellite 8m antenna is composed of panels with a circular structure to create 16spot-beams. Globalstar can provide communication services in an area within 7701 latitudes(i.e. polar regions are not served), in the zones where terrestrial gateways are present. A new call

Figure 1. Pictorial view of the Iridium system with ISLs.

Figure 2. Pictorial view of the Globalstar system without ISL.



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is directly addressed by the satellite to the gateway under its coverage. Currently, there are 25gateways in operation around the globe.y Each gateway covers a radius of approximately2000 km. Globalstar uses a DS-CDMA PHY technique with spreading factor G5 128.Globalstar adopts path diversity combining: in order to mitigate shadowing and blockageeffects, it is possible to combine the signals to/from up to three visible satellites for a single call.The Globalstar system offers real-time voice, data, and fax. Voice is encoded at a variable bit-rate (2.4, 4.8, or 9.6 kbit/s), depending on the background noise level. The maximum supporteddata rate is 9.6 kbit/s. Globalstar is now planning second-generation satellites with improvedcharacteristics.

4.3. Inmarsat systems

Established in 1979 to serve the maritime community, Inmarsat nowadays delivers broadbandcommunication services to enterprise, maritime, and aeronautical users [36]. Inmarsat operates aconstellation of GEO satellites that provide mobile phone, fax, and data communications to theentire world, except polar regions (see Figure 3). In particular, Inmarsat uses 12 GEO satellites:four Inmarsat-2, five Inmarsat-3, and three Inmarsat-4 satellites. The following descriptionfocuses on the most innovative system supported by Inmarsat.

4.3.1. BGAN system. Recently, the Inmarsat Broadband Global Area Network (BGAN) [2]system has acquired momentum to provide several services (e.g. telephony, Internet, messaging,

Figure 3. Inmarsat system components.

yCoverage reduction even within 7701 latitudes is due to a limited number of deployed gateways; at present, there is nocoverage in the sub-Sahara region, oceanic regions, Indian sub-continent, Indonesia, Iran, Thailand, and Singapore.



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and other services) to both fixed and mobile users by using three Inmarsat-4 satellites (built byEADS Astrium). BGAN is intended to be integrated with a terrestrial 3G component (3GPPRelease 4 network). BGAN satellites are bent-pipe: the feeder link uses the C band and has aglobal coverage beam; while, the user link (transmissions to users via satellite) is in the L bandand employs a deployable antenna where up to 256 beams can be used. In a typicalconfiguration, there are 19 wide beams (large coverage), 228 narrow beams (focused coverage),and 1 global beam; only the narrow beams are used for land-mobile communications [37]. Thissystem allows communications (information bit-rate) from 4.5 to about 492 kbit/s [38] to threeclasses of portable user terminals [7]. In particular, class 1 terminals can reach the maximumthroughput of 492 kbit/s (both in downstream and in upstream), class 2 terminals can transmitat 464 kbit/s in downstream and 448 kbit/s in upstream, whereas class 3 terminals can achieve384 kbit/s in downstream and 240 kbit/s in upstream.

BGAN replaces the WCDMA air interface typical of 3G systems with a proprietary airinterface (namely, Inmarsat Air Interface-2, IAI2) where different modulation options areavailable (i.e. QPSK, 16QAM, and p/4-QPSK with variable coding rates obtained by means ofpuncturing); note that class 3 terminals can only transmit in p/4 QPSK and receive in QPSK and16QAM [7]. Moreover, narrow beams use only 16QAM modulation scheme. It is possible toadapt the transmission power, bandwidth, coding rate, and modulation scheme (modulationcannot change in case of narrow beam transmissions) to terminal capabilities and to channelconditions in order to achieve high transmission efficiency.

BGAN supports both circuit-switched and packet-switched voice and data services, asdetailed below:

� IP data (variable bit-rate and background) service: secure virtual private networkconnections for accessing corporate networks and related office applications as well asbrowsing the Internet and transferring files.

� Streaming IP (guaranteed bit-rate and streaming class) service: ‘on-demand’ IP streamingservice for applications where Quality of Service (QoS) is of paramount importance, suchas live video or videoconferencing.

� Voice (circuit switched) service: low-bit-rate phone calls are possible via a standarddesktop phone, a custom handset, or a Bluetooth handset/headset.

� Text service: messages (160 characters) can be sent and received via a laptop or a mobilephone.

BGAN is providing a new service on commercial Airbus and Boeing flights by allowingpassengers to use their mobile phones on board through a base station and related satellite link(hybrid system) [39].

The BGAN-X project is co-sponsored by ESA under the Advanced Research inTelecommunication Systems (ARTES) programme [7]. BGAN-X has extended the terminalclasses to 11 (including three classes for aeronautical, three classes for maritime, and two classesfor land-vehicular categories) with omni-directional and directional antennas. Since BGANoffers point-to-point communications only to land-portable terminals, the BGANenhancements provided by BGAN-X allow extending the service also to maritime(FleetBroadband is the maritime service by BGAN) and aeronautical (SwiftBroadBand is theaeronautical service by BGAN) users. Moreover, a multicast service has also been included.The BGAN core network (3GPP Release 4 network) does not support multicast services; hence,



DOI: 10.1002/sat

BGAN-X has followed the new 3GPP architecture for multimedia broadcast and multicastservices in [40].

Finally, a cooperation between Inmarsat and ESA has permitted to define an enhancedInmarsat payload, namely Inmarsat-XL (or Alphasat I-XL), that should add to the existingBGAN and BGAN-X services, a new class of services (i.e. higher throughput rates, optimizedmulticast/broadcast, and single-hop meshed connectivity) [41]. Such payload will be adopted inthe framework of the Alphasat European platform for telecommunications, planned for launchin 2012.

4.4. Thuraya system

Thuraya was founded in the UAE in 1997 by a consortium of leading nationaltelecommunication operators and international investment houses [42]. Using two GEOsatellites, Thuraya covers more than 110 countries, spanning Europe, North and Central Africa,Middle East, Central Asia, and the Indian sub-continent. The technical project of Thuraya wasmanaged by the U.S. Boeing satellite systems that also supported the realization and the launchof GEO satellites. Thuraya-3 satellite has been recently launched to substitute Thuraya-1satellite, expanding coverage in Asian zones (e.g. China and Japan) as well as in Australia. Atpresent, the Thuraya fleet comprises two operational GEO satellites (i.e. Thuraya-2 andThuraya-3), using GMR-1 air interface in L band, as described in Section 3. Thuraya satellitesare equipped with a 12.25m L-band transmit-receive reflector antenna; 200–300 spot-beams canbe generated per satellite. Thuraya adopts an FDMA/TDMA air interface, where each carrierhas 40ms frame structure with 24 time slots (three time slots per circuit are needed).

OBP is used to support mobile-to-mobile links between any spot-beams in a satellite. Dual-mode handsets integrate the access to either a terrestrial GSM network or the Thuraya system(for underserved and impervious areas), thus allowing customers to roam vast areas withoutservice interruptions. The satellite communication services offered by Thuraya mobile handsetsinclude: GSM-like voice, fax/data at 2.4, 4.8, and 9.6 kbit/s, messaging, etc. Thuraya also allowsInternet connectivity through a small portable terminal (notebook size) with a new high-speedservice up to 144 kbit/s, based on Amplitude Phase Shift Keying (APSK) modulation. Finally,Thuraya has recently launched its high-speed IP-based service at 444 kbit/s.

4.5. Other MSSs

For the sake of completeness, this sub-section provides few details on additional MSSs that areeither operational or in the design/deployment phase.

Intermediate Circular Orbit (ICO) Global Communications have been originally planned toadopt a MEO constellation with 12 satellites at an altitude of 10 390 km [32]; today, only one isoperational, but practically ICO services are supported by a recently launched GEO satellite(ICO-G1): it provides S-band mobile services (voice at 4.8 kbit/s, data at 2.4 kbit/s, fax,messaging, and positioning) for CONUS area.

The Hispasat satellite communication system includes six GEO satellites located at differentorbital positions [43]. Hispasat offers IP-based broadband services, such as mobile services,access to the Internet and distribution of contents, tele-medicine and tele-education, voice overIP, video streaming, and IPTV.



DOI: 10.1002/sat

The Asia Cellular System (ACeS) is composed of a GEO satellite (Garuda 1) and threegateways [44]. ACeS adopts the GMR-2 air interface to allow interoperability with GSMterrestrial networks. ACeS provides voice and optional data services for handheld terminals inAsia.

The Mobile Satellite Ventures (MSV) Company (now re-named SkyTerra) will build anintegrated satellite-terrestrial all-IP network [45]; the MSV system is composed of two GEOsatellites for the provision of mobile broadband services in America (e.g. voice and Internetaccess for public safety, security, and fleet management). The ATC segment is composed ofterrestrial base stations that support the satellite system, providing complementary coverage.

TerreStar is a new mobile network operator that plans to build an IP-based integrated mobilesatellite and terrestrial communication network [46]. TerreStar will provide service overCONUS area using two 10MHz blocks of contiguous spectrum in the 2GHz band.

Solaris Mobile has launched the W2A satellite with S-band payload in April 2009 to providemobile communication services in Europe [47]. This system uses a hybrid satellite/terrestrialnetwork to guarantee uninterrupted coverage, DVB-SH standard to support mobile TV,and smart-antennas (directional antennas) embedded in mobile phones to optimize receptionquality.

5. COMPARISONS OF MSSS

In this section, we compare the above-mentioned MSSs, first of all providing an overview oftheir most relevant characteristics and then giving some numerical evaluations, focusing onBGAN, Globalstar, Iridium, and Thuraya.

5.1. Summary of basic MSS characteristics

Table I summarizes some basic features of the MSSs described in Section 4; these systems aregrouped according to the orbit types (i.e. GEO and non-GEO). In this table, we highlight themodulation adopted, the multiple access technique, the satellite orbit type, the networkcharacteristics, and the most important applications supported. From Table I, it is evident that,among the operational MSSs, there are some systems (e.g. BGAN and Eutelsat & SES-ASTRA)that provide multimedia services, while other older systems (e.g. Globalstar, and Iridium) onlysupport low data rate and the classical phone service.

5.2. Quantitative comparisons

In this sub-section, we provide a quantitative comparison among BGAN, Globalstar, Iridium,and Thuraya in terms of different parameters. Note that the Iridium system uses a Time DivisionDuplexing (TDD) air interface, whereas all the other considered MSSs use FDD air interfaces.Moreover, BGAN and Thuraya are GEO-based systems, while Iridium and Globalstar employLEO satellites.

Figure 4 compares the envisaged four different L/S-band MSSs in terms of some importantparameters, such as number of subscribers, coverage area, system complexity (i.e. switching/routing capabilities, mesh/star networks, ISLs/no ISLs), tariff of the voice service, system cost,IP-based multimedia service provision, broadband capability, multicast capability, and physicallayer efficiency (i.e. Zphy, the efficiency related to the modulation and coding scheme adopted;



DOI: 10.1002/sat

TableI.

Characteristics

ofdifferentMSSs.

System

Satellite

orbit

Frequency

bands

Physicallayer

(PHY)

Multiple

access

(satellite)

Satellite

features

ISL

Standard

Supported

applications

IRID

IUM

LEO

LQPSK

FDMA/

TDMA—TDD

OBP;sw

itching

forvoice,

relaying

fordata

Yes

Dual-mode

(satellite—

GSM)

Real-timevoice,

Web

browsing,

e-mailaccess

GLOBALSTAR

LEO

L/S

QPSK

Combined

FDMA/C

DMA

(uplinkand

downlink)

Bent-pipe

No

Dual-mode

(satellite—

GSM)

Real-timevoice,

Web

browsing,

e-mailaccess

BGAN

GEO

LQPSK,

p/4-Q

PSK,

16QAM

FDMA/TDMA

Bent-pipe

No

Dual-mode

(satellite—

GSM);

proprietary

air

interface

BroadbandInternet

access,VoIP,

Web

browsing,e-mailaccess,livevideo,

videoconferen-cing,andreal-timevoice

HISPASAT

Operator

(AmerHis

transponders

ofthe

Amazonas

satellite)

GEO

Ku

QPSK

MF-TDMA

OBP,beam

switching

No

DVB-S/-RCS

VoIP,p2pfile

exchange,

livevideo,

videocon-ferencing,andreal-timevoice

THURAYA

GEO

Lp/4QPSK

FDMA/TDMA

OBP,beam

switching

No

Dual-mode

(satellite–GSM);

GMR-1

air

interface

p2pfile

exchange,

real-timevoice

AceS

GEO

LGMSK

FDMA/TDMA

OBP,beam

switching

No

Dual-mode

(satellite–GSM);

GMR-2

air

interface

p2pfile

exchange,

real-timevoice,

GSM

supplementary

services,

messaging,high-power

paging

EUTELSAT

&SES-A

STRA

Operators

(S-band

payload,W2A

satellite)

GEO

S—

——

No

DVB-SH

MobileTV,Vehicle

locationtracking,

emergency

communications,

real-timeinform

ationexchange

MSV

GEO

L—

—OBP,beam

switching

Yes

Dual-mode

(satellite—

GSM)

p2pfile

exchange,

real-timevoice,

broadbandInternet

access,

services

forpublicsafety

TERRESTAR

GEO

L—

—OBP,beam

switching

No

Dual-mode

(satellite—

GSM)

p2pfile

exchange,

real-timevoice,

livevideo,broadbandInternet

access



DOI: 10.1002/sat

see Section 5.2.1 for the explanation of this parameter). The parameter values used for obtainingFigure 4 are listed in Table II; moreover, for the computation of the physical layer efficiency inFigure 4, the assumptions on coding and roll-off factor are detailed in the notes for Table III.

Note that, on the basis of the definition of the qualitative comparison parameters in Table II,the greater is the shadowed area for one system in Figure 4, the better is its overall evaluation.Hence, the BGAN system appears to achieve a very interesting evaluation from the resultsshown in Figure 4.

5.2.1. Analytical model. In order to compare the efficiency of different MSSs, we introduce inthis sub-section an analytical approach based on the parameters and the related assumptionsdescribed below:

� B5 bandwidth available for the MSS in downlink.z

� N5 number of simultaneously active beams in the MSS (counting all the satellites and thetransponders in them).

� G5 spreading factor if CDMA is used in the air interface (G5 1, if CDMA is notadopted).

� F5 reuse factor of the MSS (i.e. the number of times that a frequency is reused amongactive beams).

� K5 size of the frequency reuse cluster of the MSS, depending on antenna technology, airinterface type, and tolerance to interference of the multiple access system. Hence, thebandwidth available in each beam is equal to B/K. If a 2-D hexagonal-like cellular layout

Broadband

IP-based Multimedia

System Cost

Tariff of the voice service

System complexityCoverage

Number of subscribers

PHY efficiency

Multicast capability

GEO(BGAN)

Broadband

IP-based Multimedia

System Cost

Tarif f of the voice service



PHY efficiency


GEO(Thuraya)

Broadband

IP-based Multimedia

System Cost

Tariff of the voice service



PHY efficiency


LEO(Globalstar)

Broadband

IP-based Multimedia

System Cost

Tarif f of the voice service



PHY efficiency


LEO(Iridium)

(a) (b)

(c) (d)

Figure 4. Comparison of different MSSs: (a) GEO BGAN (we have considered BGAN-X extension); (b)GEO Thuraya; (c) LEO Globalstar; and (d) LEO Iridium. In Table II, we have listed the range of values

for each parameter.

zAn equivalent study could be done for the uplink case. Note that in the Iridium case (that uses a TDD air interface), thetotal system bandwidth is here considered.



DOI: 10.1002/sat

TableII.Rangeofvalues

fortheparametersin

Figure

4.

Envisaged

score

Number

ofsubscribers

Earth

coverage

(%)

System

complexity

Tariff

of

thevoice

service

($/m

in)

System

cost

(US$billions)

IP-based

multim

edia

Broadband

capability

PHY

efficiency

(bit/s/H

z)Multicast

capability

55Excellent

4100000

100

Bent-pipe1

starnet.1

No

ISL(lowest

complexity)

o0.20

o0.5

YY

42

Y

45Good

(75000–100000)

(76–99)

OBP

1mesh

network

1No

ISL

0.20–0.45

(0.5–1)

N/C

N/C

(1.51–2)

N/C

35Fair

(50000–75000)

(51–75)

OBPwith

switching/

routing1mesh

network

1ISL(highest

complexity)

0.46–0.75

(1.1–3)

N/C

N/C

(1.01–1.5)

N/C

25Poor

(25000–50000)

(26–50)

N/C

0.76–0.99

(3.1–5)

N/C

N/C

(0.51–1)

N/C

15Verypoor

[1–25000)

(1–25)

N/C

41

(5.1–7)

N/C

N/C

(0.1–0.5)

N/C

05Inexistent

00

N/C

c1

47

NN

0N

Y,yes;N,not;andN/C

,notconsidered.



DOI: 10.1002/sat

is used, the possible reuse cluster values belong to the set F�{1}[{i21j21ij}, where i and jare natural numbers, ij 6¼0. Note that K may be variable within a satellite constellation.Hence, an average K value for a given constellation is used in the following numericalevaluations, considering that K5N/F and KAF.

� Zphy 5 efficiency of the modulation and coding scheme adopted: Zphy ¼r�log2 ðMÞ

1þa , where r isthe code rate, M is the number of symbols in the modulation, and a is the roll-off factorthat is equal to 0 for a perfect raised-cosine impulse and 1 for a rectangular impulse (null-to-null band). In the following comparative study, we have used values of r and a that arespecific of the related MSS (see Table III).

� Zguard 5 efficiency parameter that takes into account the presence of guard bands (FDM/

FDMA) and/or guard times (TDM/TDMA): Zguard ¼ ZT � ZB ¼Tframe�Tguard

Tframe� B�Bguard

B ,

where Tframe is the frame duration, Tguard is total time spent in guard times in the frame,

Table III. Traffic engineering parameters and other data for our MSSs of reference.

Parameter GEO BGAN GEO ThurayaLEO

GlobalstarLEO

Iridium

B [MHz/downlink/system] 34 34 16.5 5.15N [] beams/system] �684 (5�228� 3);

see note��600 (5�300� 2);

see note�768 �2150y

F 26 30 768 �180G 1 1 128 1K (] beams/cluster) (average) �27 �21 1 �12Zphy (bit/s/Hz) 2.10z 0.74y 0.83z 1.07J

Zguard 0.82 0.79 0.96 0.55ZTDD 1 1 1 0.5ZMAC 0.97 0.97 0.97 0.97Zenc 0.96 0.96 0.96 0.96Z (bit/s/Hz) 1.60 0.54 0.74 0.27H (km) 35 800 35 800 1414

A (km2) 1.25� 105 1.97� 105 3.7� 106 3.4� 105

D (km) �400 �500 �2200 �660S (] subscr./system) 26 200 250 000 330 000 309 000C (bit/s/user) ref. value (k) 4.8K�� 4.8K�� 4.8K 4.8Kh/24 (Erl/user) 1/24 1/24 1/24 1/24Number of beams/satellite 228 300 16 48Sysc, System cost (US$ billions) 1.5 0.96 2.2 7

�In the calculation of the number of active beams we have considered that in the BGAN system there are three GEOsatellites, whereas in the Thuraya system there are two GEO satellites; each of these satellites presents�228 (BGAN)or �300 (Thuraya) simultaneously active beams.yIn the Iridium system, out of 3168 beams (66� 48) only approximately 2150 beams are simultaneously active, sincesome beams are switched off on polar regions to avoid overlaps.zThis value has been obtained considering that in the BGAN system narrow beams adopt only 16QAM modulationscheme with the following possible code rates r: 0.334, 0.642, 0.775, and 0.882 [48]. We have considered r5 0.642 (anda5 0.22) to be closer to the values of the other compared systems.

yFor the Thuraya system, we have used r5 1/2 and a5 0.35.zFor the Globalstar system, we have used r5 1/2 and a5 0.2.JFor the Iridium system, we have used r5 3/4 and a5 0.4 (note that code rate r5 1/2 is not allowed by Iridium).��Both BGAN and Thuraya systems could support a higher bit-rate value (up to 492 kbit/s for BGAN and up to

444 kbit/s for Thuraya), but we have used the same value (i.e. 4.8 kbit/s, basic-level service type) for all the systems, fora fair comparison.



DOI: 10.1002/sat

and Bguard is the total bandwidth employed in guard bands in B. Guard bands betweenadjacent channels are used to prevent the channels from overlapping and causing cross-talk among modulated signals; since, all the envisaged systems adopt L/S bands, weexpect that ZB should have close values for all these systems. Guard times are used forframe synchronism (preamble) as well as separation between bursts. In GEO systems, ZTvalue is high, since we consider only the contribution of the unique word, whereas, in theIridium LEO case with TDD air interface, significant guard times are employed that entaila low c-T value. The Zguard values have been derived from [49–51]; the related values arelisted in Table III.

� ZTDD 5 efficiency parameter that takes into account if the satellite air interface adoptsTDD or not. ZTDD 5 1/2, if TDD is adopted; ZTDD 5 1, otherwise.

� ZMAC 5MAC-layer packet efficiency. We assume to have a layer 2 encapsulation, wherethe efficiency value is the same of the MPEG2-TS (Motion Picture Experts Group 2—Transport Stream) format, typical of the DVB-S/DVB-RCS standard; such value isZMAC 5 0.97.

� Zenc 5 efficiency of the encapsulation from layer 3 to layer 2. We assume to have anencapsulation from layer 3 to layer 2, where we use always the efficiency value that istypical of the Multi Protocol Encapsulation (MPE); on the basis of [52], Zenc 5 0.96.

� Z5 total efficiency of lower layers: Z5 Zphy� Zguard� ZTDD� ZMAC� Zenc.� H5 satellite orbit altitude.� S5 number of subscribers in the MSS.� D5 spot-beam footprint diameter.� A5 average area irradiated on the earth by a beam.

Let us consider a reference ‘equivalent’ user requiring an average capacity of C kbit/s. Then,the number of equivalent users Ueq that can be supported per beam is given by:

Ueq ¼Z� B

K� C� G

max equivalent users

beam

� �ð1Þ

Let us assume that each real user is active only for h hours per day, thus contributing a ‘load’equal to h/24 Erlangs. Hence, we multiply Ueq by 24/h to express the capacity in terms of realequivalent users, U, as follows:

U ¼Z� 24� B

h� K� C� G

maxusers

beam

h ið2Þ

The U value in (2) represents the maximum capacity of users per beam that can be supportedby the system, depending on the different parameters of our modelization. Thus, the maximumcapacity of the system is U�N. We can therefore compare such a value with the currentnumber of subscribers in the MSS, S, to evaluate the system utilization parameter, G, definedbelow:

G ¼S

U�N¼

h� K� C� G� S

Z�N� 24� B¼

h� C� G� S

Z� F� 24� Bð3Þ

G parameter is useful to understand the possible expansion in terms of subscribers that isallowed for a given MSS. The lower G, the bigger is the potential expansion of an MSS. Viceversa, a high G value denotes an MSS well exploiting its traffic capabilities.



DOI: 10.1002/sat

Alternatively, we can consider the maximum user density a that can be supported by an MSSby using (2) divided by A, as follows:

s ¼Z� 24� B

h� K� C� G� A

max users

km2

� �ð4Þ

It is also possible to define another MSS efficiency parameter ZSI-SAP that is related to theefficiency at the Satellite Independent-Service Access Point (SI-SAP) level [53]. In particular, thisparameter is computed as the bit-rate capacity of a beam multiplied by the number of activebeams in the system and divided by the total one-way bandwidth. This formula has beenproposed by some mobile satellite service providers as a response to a public consultation madeby the European Commission in 2007 [13].

ZSI�SAP ¼ðbeambit� rate capacityÞ � ð beams=systemÞ

ðone�way total bandwidthÞ¼

Z�N

K� G¼

Z� F

G

bit=s

Hz

� �ð5Þ

Then, we have considered the system cost of an MSS in US$ (Sysc) normalized to the MSStraffic capacity (i.e. B� ZSI�SAP); this parameter, denoted as b, represents a measure of thesystem cost per unit of traffic rate. The most convenient’ MSS is that with the lowest b value.

b ¼Sysc

B� ZSI�SAP

¼Sysc� G

B� Z� F

$

bit=s

� �ð6Þ

Finally, we are interested to compare the mobility conditions (with consequent signaling load)in the different MSS scenarios. In order to characterize the user mobility, we introduce themobility index t that represents a measure of the cell (5 spot-beam) crossing time by a user:

t ¼D

V½s� ð7Þ

where V5Vuser, the mean user terminal speed in the GEO case (for instance the speed of a train,a plane, or a ship), while V5Vtrk, the satellite ground-track speed in the case of a LEO or MEOsystem where the user mobility is dominated by the satellite constellation mobility.

As for the Vtrk derivation we can consider the following proportionality of the satellite orbitalspeed, Vorb, with respect to the satellite orbit radius (5RT1H, where RT is the mean earthradius and H is the satellite orbital altitude) as follows:

Vorb

Vtrk¼

RT þH

RT) Vtrk ¼

RT

RT þHVorb

m

s

h ið8Þ

where Vorb is obtained by equaling the earth gravitational attraction to the centrifuge force(denoting ms the satellite mass, mT the earth mass, and g the gravitational constant):

gmTms

RT þHð Þ2¼ ms

V2orb

RT þH) Vorb ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffigmT

RT þH

rm

s

h ið9Þ

We consider mT 5 5.9742� 1024 kg, g5 6.67� 10�11m3/(kg� s2), and RT 5 6378 km.The lower the satellite orbit, the faster it moves, and the smaller the covered area on the

earth.Parameter t, evaluated from Equation (7), can be used to derive the mean number of beam

handovers, nh, occurring during the lifetime of a session with mean duration Tsession from [54] as:

nh /Tsession

tbeamhandovers

session

� �ð10Þ



DOI: 10.1002/sat

Therefore, we can consider that the ratio Tsession/t is a measure of the signaling load that thesystem has to support to manage a session of a given duration by switching it from one beam toanother during its lifetime. The higher this ratio is, the larger the system capacity lost to supportthe signaling for handovers and the lower the capacity available for data traffic.

5.2.2. Results comparison. On the basis of parameters G, a, ZSI-SAP, b, and nh we have been ableto compare in Figures 5–9 our reference MSSs by using the data summarized in Table III. It hasbeen very difficult to find these values since they are on disparate documents in the literature aswell as on the Web.

Figure 5 shows the utilization comparison in terms of G among BGAN, Globalstar, Iridium,and Thuraya, by using (3). We can note that the Globalstar system achieves the highest G value,considering that it has acquired in the years a high number of subscribers. Moreover, we can

GEO BGAN GEO Thuraya LEO Globalstar LEO Iridium0

0.2

0.4

0.6

0.8

1

, Util

izat

ion

Figure 5. Utilization comparison in terms of G for the considered MSSs.


0.02

0.04

0.06

0.08

0.1

σ [u

sers

/km

2 ]

Figure 6. Maximum user density comparison for the considered MSSs.


20

40

60

ηS

I-S

AP

[bit/

s/H

z]

Figure 7. Efficiency ZSI�SAP comparison for the considered MSSs.



DOI: 10.1002/sat

expect that the BGAN system, with very low G value, has the potentiality to increase the numberof subscribers (BGAN started its services more recently in 2005).

The comparison among the MSSs in terms of maximum supported user density s (a user ishere characterized by the C and h values in Table III) is shown in Figure 6, according to (4). Wecan note that GEO-based systems allow s values greater than those of LEO-based ones.

In Figure 7, the ZSI-SAP efficiency values are shown according to (5). These results show thatIridium achieves a higher efficiency than GEO-based systems. This is due to the fact that in theIridium case we have a smaller cluster size and a larger number of beams than in GEO-basedsystems. Moreover, the better ZSI-SAP value of Iridium with respect to Globalstar is due to thehigh spreading factor value G used by Globalstar that reduces the available information bit-rate.Finally, it is important to note that the antenna technology on GEO satellites plays a crucial rolein order to improve the MSS efficiency. In particular, it is important that a high insulation beachieved among adjacent beams to reduce the interference and to allow the use of lower-sizeclusters. It is interesting to note that if we evaluate the ZSI-SAP value for an advanced GEO-basedFSS, like the HotBird 6 satellite referring to its payload of 4 Ka-band Skyplex DVB-RCStransponders (each of them with a bandwidth of 33MHz), we obtain an efficiency value of2.8 bit/s/Hz. This is a much lower efficiency than that of MSSs and shows a significant designdifference between MSSs and FSSs.

Figure 8 shows MSSs comparison in terms of parameter b (system cost per unit of trafficrate) according to (6). We can note that, in this case, BGAN achieves the best b value meaningthat this system could be able to support a given traffic with lower costs than the other envisagedMSSs. Thuraya achieves a performance quite close to BGAN, while LEO-based systems appearto be less convenient.


10

20

30

40

β [$

/bit/

s]

Figure 8. Comparison in terms of b parameter for the considered MSSs.


2

4

6

8

Tse

ssio

n/τ

Figure 9. Comparison of Tsession/t parameter for the considered MSSs.



DOI: 10.1002/sat

On the basis of Tsession/t and for a given Tsession 5 600 s value, we compare MSSs in terms ofuser mobility as shown in Figure 9 by assuming the worst-case scenario for GEO mobilitycondition (i.e. users on a plane with V5 1000 km/h). These results clearly show that the GEOcases are characterized by a much lower mobility and lower related signaling load than LEOones.

As a conclusion, we can state that, among the examined MSSs, GEO systems (and, inparticular, BGAN) result very interesting since they may support a higher user density with lowsystem cost per unit of traffic rate so that they have a concrete possibility to increase the numberof subscribers.

6. FUTURE TRENDS FOR MSSS

It is possible to identify some challenges that future MSSs have to address in order to providenew and improved services, reducing system costs and increasing the service penetration. Inparticular, some interesting R&D aspects are: all-IP mobile satellite networks with end-to-endQoS support; hybrid satellite/terrestrial networks able to provide uninterrupted services tomobile users also when non-LoS conditions with the satellite are present; adoption of high-directivity multi-spot-beam satellite antennas to increase the resource reuse factor in the satelliteconstellation; user mobility management with suitable handover protocols; provision of newbroadband services; achievement of small-size smart terminals of reasonable costs with power-saving techniques to improve the battery autonomy. On the basis of the above, we can identifysome important trends, new implementation issues and related technology needs, as outlinedbelow.

Currently, many MSSs operators have announced the intention to launch IP-based MSSs,considering both second-generation systems (e.g. Iridium and Globalstar) and new ones (e.g.MSV and TerreStar). Moreover, the DVB Project is now interested to the Next Generation ofDVB-RCS (DVB-RCS NG), supporting enriched mobility features and commonly-used IP-based protocols [55].

Future plans of WARC 2011 are to consider new frequency bands for MSSs, reaching acompromise with terrestrial services in C band.

Future MSSs will need to be integrated with a terrestrial broadband segment (e.g. WiMAXand WiFi) with a suitable degree of commonality of interfaces and services [56]. Moreover, tosupport small-size mobile terminals, optimized antenna technologies (on both the satellite andthe mobile terminal) are needed to close the link budget. This technology improvement shouldalso be addressed toward the interference reduction (thus allowing a higher degree of resourcereuse) and the adoption of high-efficiency transmission techniques (e.g. OFDMA air interface tobe consistent with the prospected use of UMTS LTE for terrestrial 4G cellular systems). Inaddition to this, the novel approach based on cross-layer air interface design could permit anoptimization of the whole protocol stack, thus improving capacity and QoS [2]. Theachievement of low-cost mobile terminals should be the important outcome of suchoptimization process, thus paving the way for a larger diffusion of mobile satellite services.

A critical issue for MSSs is the seamless support of user mobility with handover proceduresinvolving satellite antenna beams of the same or of different satellites or even between a satellitebeam and a terrestrial cell, when more segments are involved. According to these examples,intra-system handovers and inter-system handovers have to be supported for IP-based traffic in



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MSSs. Let us assume future satellites having regenerative payload and operating at the IP layer[57]. Then, intra-satellite handover should adopt IP micro-mobility protocols [58], while inter-satellite and inter-system handovers should involve the MIP scheme. An interesting study onmobility support for IP-based traffic has been made by the SATSIX project and can be found in[8–10]. In any case, efficient mobility management protocols need to be employed at layer 3 toprevent excessive delays incurred in re-routing the IP data flows during handover phases.

Finally, an interesting future-proof service for MSSs is represented by mobile broadcasting(encompassing mobile TV and multimedia download) that is already widely used in Japan,Korea, and U.S., but experiences some delays in Europe due to regulatory issues. As describedin Section 4.5, Eutelsat and SES-ASTRA have launched the W2A satellite with S-band payloadin order to support mobile TV according to DVB-SH [47]. With the diffusion of DVB-SH-basedservices, it will be possible to reach a new market slice for satellite communications.

7. CONCLUSIONS

Currently, there is a renewed R&D interest for MSSs due to their capabilities to provide servicesanytime and anywhere. In this paper, we have studied MSSs focusing on their specificcharacteristics that are very unique with respect to other satellite systems. Important standardsfor mobile satellite communications and services have been presented with a special attention tohybrid and integrated networks with satellite and terrestrial segments.

The second part of this paper has surveyed the main characteristics of some MSSs(operational or planned), thus allowing their comparisons in terms of a proposed analyticalframework based on a wide range of efficiency parameters, including user mobility degree.Globally, GEO systems appeared more interesting than non-GEO ones, because they supporthigher user density, entail lower costs, and have lower mobility degree. Moreover, it has beenhighlighted that the design of high-directivity multi-spot beam antenna for GEO satellites willrepresent a particularly crucial aspect in order to increase system efficiency, and, then, capacity.We believe that the framework presented in this paper may represent a good tool for the designand the market study of MSSs.

According to the envisioned future challenges and trends for MSSs, we can conclude thateven if mobile satellite services represent a niche part of the whole satellite communicationmarket, there are now new opportunities (in terms of technologies and cross-layer protocoldesign) that can be exploited to increase the diffusion of these services.

NOMENCLATURE

3G 3rd generation4G 4th generationACeS Asia Cellular SystemAPSK Amplitude Phase Shift KeyingATC Ancillary Terrestrial ComponentBGAN Broadband Global Area NetworkBGAN-X BGAN eXtensionCGC Complementary Ground Component



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DS Direct SequenceDVB-H Digital Video Broadcasting—HandheldDVB-IPDC Digital Video Broadcasting—IP DataCastingDVB-RCS Digital Video Broadcasting—Return Channel via SatelliteDVB-RCS NG Next Generation of DVB-RCSDVB-S2 Digital Video Broadcasting—Satellite version 2DVB-SH Digital Video Broadcasting—Satellite to HandheldEIRP Effective Isotropic Radiated PowerESA European Space AgencyETRI Electronics and Telecommunications Research Institute (South Korean

organization)ETSI European Telecommunications Standards InstituteFBB FleetBroadBandFDD Frequency Division DuplexingFDMA/TDMA Frequency Division Multiple Access/Time Division Multiple AccessFSS Fixed Satellite SystemGEO Geosynchronous Earth OrbitGMR GEO Mobile RadioGPRS General Packet Radio ServiceGSM Global System for Mobile CommunicationsIAI2 Inmarsat Air Interface-2ICO Intermediate Circular OrbitIETF Internet Engineering Task ForceIMR Intermediate Module RepeaterISL Inter Satellite LinkITU-R International Telecommunication Union–Radiocommunication sectorLEO Low Earth OrbitLoS Line-of-SightLTE Long Term EvolutionMAC Medium Access ControlMAESTRO Mobile Applications & sErvices based on Satellite & Terrestrial inteR-

wOrkingMBMS Multimedia Broadcast Multicast ServiceMEO Medium Earth OrbitMIP Mobile IPModCod Modulation and CodingMoBISAT Mobile Broadband Interactive Satellite multimedia Access TechnologyMoDiS Mobile Digital broadcast SatelliteMOWGLY MObile Wideband Global Link sYstemMPE Multi Protocol EncapsulationMPEG2-TS Motion Picture Experts Group 2—Transport StreamMSS Mobile Satellite SystemMSV Mobile Satellite Venturesnon-LoS non-Line-of-SightOBP On Board ProcessingOFDM Orthogonal Frequency Division Multiplexing



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OFDMA Orthogonal Frequency Division Multiple AccessQoS Quality of ServiceR&D Research and DevelopmentSATSIX Satellite-based communication systems within IPv6SBB SwiftBroadBandSCPC Single Channel Per CarrierSI-SAP Satellite Independent-Service Access PointS-DMB Satellite—Digital Multimedia BroadcastingS-UMTS Satellite—Universal Mobile Telecommunications SystemTDD Time Division DuplexingWARC World Administrative Radio ConferenceWCDMA Wideband—Code Division Multiple AccessWiFi Wireless FidelityWiMAX Worldwide Interoperability for Wireless Microwave AccessWRC World Radiocommunication Conference

ACKNOWLEDGEMENTS

This paper has been carried out within the framework of the European SatNEx II (contract No. IST-027393), network of excellence, www.satnex.org. The authors thank the anonymous Referees for theirimportant comments and suggestions that have permitted to improve the quality and the clarity of thispaper.

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AUTHORS’ BIOGRAPHIES

Paolo Chini was born in Siena, Italy, in 1972. He received the Dr Ing. Degree inTelecommunications Engineering in 2005 from the University of Siena, Italy, with athesis entitled ‘Cross-layer management of resources in an interactive DVB-RCS-based satellite network’ in the framework of the SatNEx ‘Satellite CommunicationsNetwork of Excellence’, in FP6 network of excellence, in cooperation with theUniversity of Rome ‘Tor Vergata’. From 2005 he has been performing researchactivities at the University of Siena; his fields of interests include satellitecommunications, DVB-RCS standards, TCP and MAC layer protocols, mobileand wireless networks, HAPs platforms. In 2008 he participated in the FP7 EURADICAL project, whose objective is to create a roadmap for enhancing Securityand Privacy, in order to protect medical and genetic data.

Giovanni Giambene was born in Florence, Italy, in 1966. He received the Dr Ing.degree in Electronics in 1993 and the PhD degree in Telecommunications andInformatics in 1997, both from the University of Florence, Italy. From 1994 to 1997,he was with the Electronic Engineering Department of the University of Florence,Italy. He was Technical External Secretary of the European Community COST 227Action (‘Integrated Space/Terrestrial Mobile Networks’). He also contributed to theSAINT Project (‘Satellite Integration in the Future Mobile Network’, RACE 2117).From 1997 to 1998, he was with OTE of the Marconi Group, Florence, Italy, wherehe was involved in a GSM development program. In the same period he alsocontributed to the COST 252 Action (‘Evolution of Satellite Personal



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Communications from Second to Future Generation Systems’) research activities by studying PRMAprotocols for voice and data transmissions in low earth orbit mobile satellite systems. In 1999, he joined theInformation Engineering Department of the University of Siena, Italy, first as research associate and thenas assistant professor. He teaches the advanced course of Telecommunication Networks at the Universityof Siena. From 1999 to 2003 he participated in the project ‘Multimedialita’, financed by the ItalianNational Research Council (CNR). From 2000 to 2003, he contributed to the ‘Personalised Access to LocalInformation and services for tourists’ (PALIO) IST Project within the EU FP5 programme. He was alsovice-Chair of the COST 290 Action (http://www.cost290.org) for the whole of its durations 2004–2008,entitled ‘Traffic and QoS Management in Wireless Multimedia Networks’ (Wi-QoST). At present, he isinvolved in the SatNEx network of excellence of the FP6 programme in the satellite field, as work packageleader of two groups on radio access techniques and cross-layer air interface design (http://www.satnex.org). He also participated in the FP7 Coordination Action ‘Road mapping technology forenhancing security to protect medical & genetic data’ (RADICAL) as work package leader (http://www.radicalhealth.eu/). Giambene is IEEE senior member. He has recently published the following books:G. Giambene, ‘Queuing Theory and Telecommunications: Networks and Telecommunications’, Springer,May 2005; G. Giambene (Ed.), ‘Resource Management in Satellite Networks: Optimization and Cross-Layer Design’, Springer, April 2007.

Dr Kota is a Senior Scientist in Harris Corporation and Adjunct Professor in theUniversity of Oulu, Finland. He has held technical and management positions at Loral,Lockheed Martin, SRI International, the MITRE Corp, and Xerox Corporation, andcontributed to both military and commercial satellite communication systems andbroadband (IP, ATM) network design and analysis. He made significant contributionsto various programs e.g. MILSTAR, AEHF, AFSATCOM, DSCS, GBS and MSE-programs and early phase of TSAT and WIN-T programs. He was the chief networkarchitect of broadband multimedia services (BMS), a two way IP satellite network andhe developed the first phase of Astrolink -ka-band satellite network architecture.Currently he is head of the U.S. delegation and the U.S. chair of the ITU-R WorkingParty 4B involved with Fixed and Mobile satellite system performance. He led theefforts of development of Recommendations of TCP over Satellite Networks and

Satellite ATM performance. He is the principal author of a book Broadband Satellite Communications forInternet Access, and co-edited a book Emerging Location Aware Broadband Wireless Ad Hoc Networks, andwrote book chapters on Satellite TCP/IP in High Performance Networking, Trends in Broadband Networkingin Wiley Encyclopedia of Telecommunications. He has published more than 130 papers in conferenceproceedings, and journals. He served as a guest editor of special issues for IEEE Communications Magazine,International Journal of Satellite Communications and Networking, Space Communications- an InternationalJournal and Int’l Journal of Wireless Information Networks. He is member of the editorial boards on theInternational Journal of Satellite Communications and Networking, and the Space Communications Journal.Dr Kota received his PhD from the University of Oulu, Finland; Electrical Engineer’s Degree fromNortheastern University, Boston, USA; MSEE from IIT; BSEE from BITS, India. Dr Kota has been akeynote speaker, invited speaker and a panelist at various international conferences. He served as theUnclassified Technical Program Chair of MILCOM 2007, Technical Committee member of MILCOM2004,1997, and Asst Technical Chair of MILCOM 1990; Satellite Communications symposium chair of IEEEGLOBECOM 2002, 2000, co-chair of Wireless Communications and networking symposium ofGLOBECOM2006, and Technical chair of ISWPC2007, WCNC 2008 panel chair, and invited sessionchair for PIMRC 2006, 2005, 2004. He is the co-chair of Wireless Networking Symposium for GLOBECOM2009. He is the recipient of Golden Quill awards from Harris Corporation, publication awards from LockheedMartin and ATM Forum Spotlight Award. In addition, he is a Senior Member of IEEE, Associate Fellow ofAIAA, and Member of ACM.



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Mobile Satellite Systems