Satellite Component of NGN: Integrated and Hybrid Networks

INTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS AND NETWORKING

Int. J. Satell. Commun. Network. 2011; 29:191–208Published online 13 May 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/sat.972

Satellite component of NGN: Integrated and hybrid networks

Sastri Kota1, Giovanni Giambene2,�,y and Sooyoung Kim3

1University of Oulu, Rajakyla, 90570 Oulu, Finland2Universita degli Studi di Siena, Via Roma, 56, 53100 Siena, Italy3Chonbuk National University, 664-14 Duckjin-dong, Jeonju, Korea

SUMMARY

Future mobile networks are expected to involve systems that are based on different technologies, such asWiFi, WiMAX, 2G/3G/3G1, LTE, and satellite. To address this scenario, ITU has defined integrated andhybrid networks in the framework of Next-Generation Networks. The interest is to exploit the cooperationof different wireless communication systems (segments) to provide service to mobile users in the mostefficient way, taking into account signal quality (coverage), traffic congestion conditions, and cost issues.Integrated and hybrid networks have the potential to be an efficient and cost-effective solution to employsatellite communications for mobile users. In the view of this, our paper focuses on the design ofintegrated/hybrid systems taking into account physical, MAC, and network layers issues. System examplesand standards are described as well. Then, cooperative diversity techniques and traffic engineering issuesfor overflow traffic are discussed. This paper concludes identifying some possible future trends. Copyrightr 2010 John Wiley & Sons, Ltd.

KEY WORDS: integrated networks; hybrid networks; cooperative communications; next generationnetworks

1. INTRODUCTION

Satellites are successful for their wide area coverage and speed to provide new services. Nicheareas such as coverage for planes and ships will persist, but land masses convergence of fixed,mobile, and broadcasting systems will dictate that the only way forward for satellite networks isin an integrated format with terrestrial systems [1]. Today the interest for global cellularnetworks is to define new systems, integrating segments offering differentiated coverage such asindoor, outdoor, and global [2].

The satellite network is mostly immune to terrestrial events, but mainly relies on Line-Of-Sight (LOS) communications and this could be a significant limit. This is the reason why anintegrated/hybrid network has to be considered with terrestrial wireless systems cooperatingwith the satellite one. This telecommunication infrastructure needs to be quickly deployable,scalable, reconfigurable, and must manage efficient interworking and user mobility.

The convergence of mobile and Internet technologies is now paving the way to thedevelopment of systems capable of supporting multimedia and interactive services previouslyavailable only to fixed users. International Telecommunication Union (ITU) is currentlydeveloping a vision of Next-Generation Networks (NGN) with the clear objective of providing ameans for true network integration. The purpose of this integration is effective utilization of therespective strengths of each network, within the context of their traditional roles and mandates.These integrated and hybrid systems enable an NGN by seamlessly interworking and

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

Copyright r 2010 John Wiley & Sons, Ltd.

cooperatively combining the most powerful aspects of satellite and terrestrial networks,according to the Always Best Connected (ABC) paradigm. In particular, the satellite networkcan provide the best and most comprehensive coverage for low-density populations, while theterrestrial network or the ground component can provide the highest bandwidth and lowest costcoverage for high-density populations in urban environments [3].

In this paper, we provide detailed definitions and examples concerning integrated systemsand hybrid systems that are both supposed to manage mobile users (Mobile Satellite Services,MSS). The main difference between integrated and hybrid systems is on whether both space andterrestrial parts use a common network and spectrum. The terrestrial part of an integratedsystem is a complementary part of the satellite system, and thus it uses the same frequency bandallocated to the satellite system and also it is operated by the same network. Such systems arereferred to as MSS-ATC (MSS-Ancillary Terrestrial Component) in the United States andCanada, and MSS-CGC (MSS-Complementary Ground Component) in Europe and areimplemented in the 1–3GHz bands. On the other hand, a hybrid system may combine a satellitesystem with a terrestrial one with different frequency bands, networks, and even air interfaces.

Primitive versions of these systems can be found in mobile satellite broadcasting services tohandheld terminals, although they are Broadcasting Satellite Services (BSS) instead of MSS.More recently, in Korea and Japan, Satellite-Digital Multimedia Broadcasting (S-DMB) serviceto hand-held user terminals was successfully deployed via a geostationary (GEO) satellite [4].In Europe, the Mobile Digital broadcast Satellite (MoDiS) project implemented an S-DMBexperimental platform where transparent terrestrial repeaters were adopted [5]. Moreover,‘Unlimited Mobile TV’ concept was introduced [6], in which a hand-held mobile terminal issupposed to receive broadcast signals from both satellite and terrestrial repeaters. 3GPP hasintroduced the Multimedia Broadcast and Multicast Service (MBMS) concept into 3G/beyond-3G networks: we can expect an integrated scenario where the satellite cooperates with theUMTS terrestrial segment to provide mobile users with MBMS services [7].

The multi-segment nature of the broadcast service entails the adoption of suitably newmulticast routing schemes [8]. Media companies require prompt multicast of extremely largefiles from a single source to a collection of geographically dispersed destinations. In addition tothis, there is the emerging application of digital cinema, a digital technology to distributegeographically a movie via satellite and to project it [9].

One of the main characteristics of the ETSI Broadband Satellite Multimedia (BSM) standardarchitecture is allowing services and networks to be realized and offered separately and withoutany dependence. This approach is very suitable to support integrated and hybrid networks [10].The key element of this standard is the SI-SAP (Satellite-Independent Service Access Point)interface between satellite technology-dependent layers (i.e. OSI layers 1 and 2) and the higherlayers that are satellite technology-independent.

The remaining part of this paper is organized as follows. Section 2 provides ITU definitionsand examples of integrated and hybrid satellite systems, including basic architectures. Section 3introduces the main technical issues that are typical of integrated/hybrid networks. Section 4first introduces a cooperative diversity technique that can be used to enhance transmissionQuality of Service (QoS), then provides an analytical approach to deal with the management ofrerouted traffic to the satellite segment. Section 5 focuses on future trends and drawsconclusions.

2. SYSTEM DEFINITIONS AND EXAMPLES

2.1. Integrated systems

2.1.1. Definition and architecture. ITU defined ‘an integrated Mobile Satellite Service (MSS)system’ as a system employing MSS and a ground component where the ground component iscomplementary to and operates as part of the MSS system and, together with the satellitecomponent, provides an integrated service offering [11]. In such systems, the ground segmentis controlled by the satellite resource and network management system. Further, the ground

S. KOTA, G. GIAMBENE AND S. KIM192

Copyright r 2010 John Wiley & Sons, Ltd. Int. J. Satell. Commun. Network. 2011; 29:191–208

DOI: 10.1002/sat

component uses the same designated portions of the frequency band as the associatedoperational MSS system.

An integrated system provides a combined (integrated) single network that uses both atraditional MSS link and terrestrial transmission paths to serve mobile end-users. With propernetwork planning and control of both the space and terrestrial segments of the system, theoperators can use the assigned spectrum extensively and efficiently to provide indoor andoutdoor coverage in urban, suburban, rural, and remote areas, including direct satellite serviceto small handsets. A typical integrated system comprises one or more multi-spot beam satellitesand a nation-wide or regional ensemble of terrestrial cell sites, where both terrestrial and spacecomponents communicate with mobile terminals using a common set of MSS frequencies. Theglobal ‘umbrella’ coverage is supplied by the satellite systems, mainly based on GEO satellites.

MSS systems can provide ubiquitous connectivity through their wide-area coveragecharacteristics and offer instant and reliable communications within their coverage area,while terrestrial-based networks (ATC) have their strength and traditional role in providinghigh capacity communication networks in suburban and urban areas, including inside buildings.In 2003, FCC first allowed MSS providers to modify their licenses to incorporate ATC services.MSS operators must provide an integrated MSS-ATC service with the MSS component alwaysbeing the primary part [12].

Integrated systems will likely have various service components, including traditional MSSservices; a rather large component of the portfolio of their services will be devoted to theprovision of broadband services, including multimedia broadband services, to handheld orportable terminals of form and cost factors very similar to terrestrial cellular terminals oftoday [13]. There are various approaches that system operators may choose to implement theirbaseline service provisioning, coverage goals, and end-user demand. Figure 1 shows an exampleof an integrated system designed by SkyTerra Communications [14]. As shown in Figure 1, theground component of an integrated system is not an independent stand-alone network and usesthe same frequencies assigned to the satellite component, although the two components do notnecessarily use the same frequencies simultaneously in the same geographic area. In thisexample, the ground component of the integrated system reuses frequencies multiple timeswithin each satellite antenna spot-beam. The satellite component also reuses frequenciesmultiple times among its spot-beams over the coverage area.

2.1.2. System examples. Initial examples of integrated systems are represented by satellitebroadcasting to mobile receivers in vehicles in the early 2000s. Broadcasting of high-qualitydigital radio channels to mobile users was achieved by means of S-band frequencies around2GHz. This was called Satellite-Digital Audio Radio Service (S-DARS). Two companies in theUSA, XM-Radio and Sirius, provided these services [15, 16]. The remarkable success achieved

Figure 1. Integrated satellite-terrestrial network.

SATELLITE COMPONENT OF NGN 193


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by XM-Radio and Sirius was based on traditional counter-measures such as a high link margin,time, frequency and satellite diversity, and usage of terrestrial repeaters within urban areas.

More recently, in Korea and Japan, S-DMB service to hand-held user terminals wassuccessfully deployed via a GEO satellite [4]. The S-DMB system was based on Code DivisionMultiplexing (CDM) technology, described in Recommendation ITU-R BO.1130-4 [17]. InEurope, the ‘Unlimited Mobile TV’ concept was introduced [6], which is based on the ETSIstandard of DVB-Satellite services to Handheld (SH) devices published in March 2008 [18]: ahand-held mobile terminal is supposed to receive broadcast signals from both the satellite andterrestrial repeaters (i.e. CGC component). Video services from ICO G1 (a GEO satellite thatcovers the entire United States, Puerto Rico and the US Virgin Islands) are based on the DVB-SH standard as well. The DVB-SH system was designed for frequencies below 3GHz (S-band).It complements the existing DVB-H (handheld) terrestrial coverage and analogously uses theDVB IP Datacast (IPDC) set for content delivery, electronic service guide, and service purchaseand protection standards. DVB-SH specifies two operational modes: (i) SH-A adopts CodedOrthogonal Frequency-Division Multiplexing (COFDM) on both satellite and terrestrial linkswith the possibility of running both links in Single-Frequency Network (SFN) mode; (ii) SH-Buses Time Division Multiplexing (TDM) on satellite with COFDM on the terrestrial link [19].

2.2. Hybrid systems

2.2.1. Definition and architecture. ITU defined ‘a hybrid satellite and terrestrial system’ asemploying satellite and terrestrial components where the satellite and terrestrial systems areinterconnected, but operate independently of each other [11]. In such systems, satellite andterrestrial components have separate network management systems and do not necessarilyoperate in the same frequency band. The hybrid system utilizes a satellite component that is partof either an FSS or an MSS network and the terrestrial component operates in a fixed, mobile ornomadic mode. Figure 2 shows a hybrid architecture with a satellite network (DVB-S2/-RCS)and a terrestrial wireless network (WiMAX, IEEE 802.16e) [20].

2.2.2. System examples. Recently, INMARSAT first started a Broadband Global AreaNetwork (BGAN) service based on 3GPP waveforms [21]. BGAN is providing a new serviceon commercial Airbus and Boeing flights by allowing passengers to use their mobile phones onboard through a base station and related GEO satellite link. BGAN provides circuit-switchedand packet data services via a standard Ericsson 3G Release-4 Core Network (CN) coupled to aspecialized Satellite Radio Access Network (S-RAN). The first generation of BGAN mobileterminals included a range of portable devices supporting packet data rates between 200 and

Figure 2. DVB-S2/-RCS star topology with WiMAX networks connected to satellite terminals.



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500 kbit/s, and delivered by GEO satellites to spot-beams, which are similar in concept toterrestrial mobile network ‘cells’.

The European Integral Satcom Initiative (ISI) Technology Platform has developed theIntegrated Space Infrastructure for global Communications (ISICOM) system concept, basedon an heterogeneous satellite/terrestrial communication infrastructure for ubiquitous access toservices, such as next-generation broadband access for high-speed Internet services (in Ku, Kabands), next-generation two-way mobile communications evolving towards 4G, next-generationDirect-To-Home television (DTH), DMB systems, Global Monitoring for Environment andSecurity (GMES), and downstream services [22].

One of the most powerful applications of hybrid systems can be found in emergencytelecommunications. Disasters are able to disrupt terrestrial services while generally not affectingsatellite services [23]. Telecommunication services are critically needed not only during the early phaseright after the disaster by victims but also during the response phase [24]. It has demonstrated theadded-value of hybrid networks composed of a satellite backhaul link and a local wireless coverage(GSM, WiFi, and WiMAX). For instance, the ESA Telecom Start-up Project STICK aims atproviding broadband communications capabilities to the Public Protection and Disaster Relief(PPDR) sector. STICK will deliver scalable on-demand communication capabilities in the form ofbroadband wireless access to deployed and roaming subscribers, operating over unlicensed frequencybands (2.4 and 5GHz). STICK network nodes connect to each other over wireless point-to-point orpoint-to-multipoint LOS bridges, over one or more hops as shown in Figure 3. All nodes will haveIP-based satellite access to some degree (via BGAN and Digital Video Broadcasting-Return Channelvia Satellite, DVB-RCS, respectively), through different terminals [26].

Figure 4 shows the detailed architecture and protocol stack of the hybrid network introducedin Figure 2 with DVB-S2/-RCS satellite component and WiMAX terrestrial coverage. Thissimple hybrid architecture may reduce the complexity of service deployment and provisioning forfixed and mobile networks in non-urban environments. The DVB-S2/-RCS terminal (i.e. RCST)is fixed while the user device is either fixed or nomadic. The satellite is GEO with MultibeamBroadband Satellite (MBS) links based on DVB-S2/-RCS. DVB-S2 allows Adaptive Coding andModulation (ACM) on a time slot basis depending on the Signal-to-Noise-plus-InterferenceRatio (SNIR) at the destination terminal. WiMAX has a well-defined QoS support mechanismwith several resource management options designed for different traffic classes, such asUnsolicited Grant Service (UGS), real-time Polling Service (rtPS), extended-rtPS (ertPS), andBest Effort (BE). Other network elements in this hybrid system include the Access ServiceNetwork (ASN) gateway, and the DVB-RCS Radio Resource Management (RRM). The RRMof the satellite terminal checks if enough resources are available to enable admission of new usersrequesting services from the gateway to the satellite link. The RCST communicates with the Hub,which is associated to a Network Control Center (NCC). The NCC controls the interactivenetwork and manages the satellite capacity allocation depending on satellite terminals requests.The IEEE 802.16e/DVB-RCS hybrid architecture shown in Figure 4 has been adopted in theEuropean BRASIL (Broadband to Rural America over Satellite Integrated Links) project, aimedat developing DVB-RCS in cooperation with Brazilian initiatives over Latin America [20, 27, 28].

Finally, the European MONET (Mechanisms for Optimization of hybrid ad-hoc and satelliteNETworks) project is currently involved in studying a hybrid network architecture consideringa wireless ad hoc network for field communications and a satellite segment for long rangeconnectivity to the Internet [29]. The envisaged scenario is that of emergency rescue,environmental observation, home and enterprise networking, educational applications, etc.Research issues are here dealing with routing adaptation for dynamic network architecture andoptimized satellite resource utilization.

3. INTEGRATED/HYBRID NETWORKS DESIGN ISSUES

This Section provides a survey of the technical issues that are relevant to design integrated andhybrid networks considering physical layer, radio resource management, and network



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standpoints. A multi-layer coverage scenario is typical of an integrated system, where terrestrialand satellite coverages contribute to provide service with different ranges and capacity whenneeded; a joint management of resources is necessary to allocate new sessions or handed-oversessions. Moreover, in integrated networks, a cooperative approach could be adopted at thephysical layer to improve the quality of the signal (diversity). In hybrid networks, the bestwireless network that is available should be selected to support a session, according to the ABCapproach. Finally, IP protocols have to account for the possibility of a user to roam from onenetwork to the other, thus involving MAC layer inter-segment handover and IP layer reroutingtechniques. Inter-segment handovers have to be executed with a criterion based on link quality,congestion, costs, etc. Basically, there are two main reasons to perform inter-segment handovers:

� Improvement in received signal quality,� Distribution of traffic load to avoid congestion and to improve performance.

3.1. Physical layer aspects

It is expected that integrated/hybrid systems are able to provide MBMS services over a widegeographic area with a uniform quality. The unidirectional nature of MBMS prohibits the useof control commands for power control and ACM. Therefore, downlink strategies should be

Figure 3. Network configuration of STICK [25].



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focused on improving the performance; a possible approach is to adopt a transmit diversitytechnique. This approach using multiple antennas enjoys several advantages that make it veryattractive for high bit-rate wireless applications in the terrestrial system, where it is possible toassume that the path components from antennas are independent. On the other hand, it isdifficult to expect the same gain to be achieved in a satellite system, because the distancebetween the satellite and the user terminal is much higher than that among antennas, so thateach path seems to be similar. In the integrated/hybrid satellite, however, we can still obtaindiversity gains by using the signal paths from the satellite and/or those from the terrestrial parts.At a user terminal, the signals from the satellite and those from the terrestrial parts will beindependent of each other. In this situation, the antennas are no longer co-located at thetransmitter or receiver.

Recently, transmit diversity techniques have been proposed for integrated networks, usingrepeaters as the ground components with appropriate signal processing capabilities.For example, an efficient diversity technique using Space-Time Coding (STC) for anintegrated MSS system was proposed in [3]. This transmit diversity technique is well suitedfor the MBMS service, since it does not require any Channel Quality Information (CQI) fromthe return link. In this scenario, ground components are terrestrial repeaters that cooperate withthe satellite to transmit space–time coded signals.

The cooperative diversity concept was originally proposed to achieve diversity gain in theuplink of terrestrial systems, where user terminals cannot support multiple antennas due to sizeor other constraints. In this cooperative scheme, user terminals share their antennas to achieveuplink transmit diversity by repeating the received bits from the partner [30]. Especially, due torecent advances in turbo coding techniques, various coded cooperation techniques using turbocodes have been proposed [31].

Motivated by this, a cooperative diversity technique has been envisaged for integrated/hybridsatellite systems [32]. Because we need a downlink diversity scheme in the satellite system, weneed a modification in the cooperative manner, i.e. cooperation is performed in the downlinkinstead of uplink cooperation between user terminals. Therefore, the satellite and terrestrialrepeaters cooperate so that the diversity gain is achieved at the user terminal. For this, satellite

Figure 4. Reference architecture and protocol stack for the hybrid IEEE 802.16e/DVB-RCS network.



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and terrestrial repeaters first select different parity symbols to be transmitted, by incorporatingrate-compatible punctured turbo codes, and then apply their own space–time encoding format.The user terminal with multi-path signals from the satellite and terrestrial repeaters first achievesspatial and time diversity gain from the STC encoded signals, and then achieves additionaldiversity gain from the channel coding gain by combining different parity symbols.

Moreover, Reference [33] presented cooperative diversity techniques for integrated satellitesystems using various STC schemes for two to four transmit antennas, and evaluated the BitError Rate (BER) performance for these diversity schemes. Although receiver diversity mayachieve high performance gain, it is impractical to implement multiple receive antennas at asmall user terminal. As a future research area, Multiple Input Multiple Output (MIMO)schemes can be a good candidate to achieve spatial diversity for high-speed transmissions.

To enjoy the performance gain by the above-mentioned diversity techniques, the groundcomponents should have additional encoding capability rather than simply being used as afrequency converter and amplifier, and the user terminal should have decoding capability.Because of this requirement, backward compatibility may not be guaranteed if the user terminalcannot obtain a signal from the satellite. We also note that this performance gain should beconditioned on the time synchronization of two different signals. In the system using CDMAschemes, we can resolve multi-paths by means of a rake receiver. However, time synchronizationcan be a very difficult problem to achieve this performance gain for a system using multi-carrierschemes such as Orthogonal Frequency Division Multiplexing (OFDM) [3].

3.2. Resource management aspects

We distinguish below the RRM issues in the two cases of integrated and hybrid networks thatentail some different implications.

3.2.1. Integrated networks. The resource allocation has to be coordinated between the terrestrialand the satellite segment, where the terrestrial part is based on 2G/3G/3G1/LTE cellularsystems and where satellite and terrestrial coverages are in overlap or complementary. Radioresources of the satellite segment are precious and costly. In addition to this, satellite coverage isbroad and well beyond the coverage area of a terrestrial cellular system; hence, the satellitenetwork is expected to service the traffic coming from wide areas on the earth. These are thereasons why satellite radio resources have to be used only when needed and released as soon aspossible. Inter-segment (vertical) handover from the terrestrial segment to the satellite one hasto be performed only when strictly necessary and a corresponding inter-segment handover backfrom the satellite to the terrestrial segment has to be executed as soon as the terrestrial networkbecomes available again. Hence, if a new terrestrial cell is reached due to user mobility, ahorizontal handover is performed; if no resources are available in the destination cell an inter-segment handover is executed. Inter-segment handover initiation may also depend on loadbalancing decisions. Another possible use of inter-segment handovers is when the user is movingfar away the terrestrial system coverage and the satellite segment is the only service option. Thesatellite segment has to accommodate resources in the following cases: new (redirected) sessions,intra-segment handover (horizontal handover), and inter-segment handover (vertical handover).The allocation priority in the satellite segment should be: inter-segment handover, intra-segmenthandover, and new (redirected) sessions. This prioritization is needed to maintain the quality ofalready-admitted traffic sessions.

Many European projects address inter-segment handover and demonstrations. Among those,we can consider: SINUS (Satellite Integration into Networks for UMTS Services) [34],INSURED (Integrated Satellite UMTS: Real Environment Demonstrator) [35], and VIRTU-OUS (Virtual Home UMTS on Satellite) [36]. In particular, INSURED adopted a networkarchitecture with InterWorking Functions (IWF) between terrestrial (2G) and satellite segmentswith the purpose of translating the different protocols on each side; the INSURED mobileterminal is permanently registered in both GSM and Iridium Home Location Register (HLR)



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using a dual mode approach. While in the VIRTOUS project a Terminal Inter-Working Unit(T-IWU) performing adaptation functions has been adopted.

In Asia, Korea is developing radio interfaces for the satellite component of the IMT-Advanced system. In particular, the satellite component will be an integral part offuture IMT infrastructure with optimized service delivery [37]. Japan is developing a systemcalled STICS (Satellite/Terrestrial Integrated mobile Communication System). This system isintended to improve the bandwidth utilization efficiency by integrating the frequency of theground and the satellite with about 100 high-gain multi-beams to cover Japan and the ExclusiveEconomic Zone (EEZ) with a 30m deployable antenna on the satellite. A handheld terminal isconsidered [38].

3.2.2. Hybrid networks. Hybrid networking calls for different segments to be involved togetherin service delivery, where the terrestrial component is based on a Wireless Local Area Network(WLAN), for instance, WiFi or WiMAX, having complementary coverage with respect to thesatellite system. The inter-working of QoS mechanisms is a critical aspect in hybrid systems:each segment should have QoS support mechanisms with end-to-end consistent choices. Thisentails a mapping function for traffic classes and coherent scheduling decisions and prioritylevels in the different segments. As far as the traffic classification is concerned, we may refer tothe categorization in ITU-T Y.1541, which defines 6 QoS traffic classes that not only refer toapplication layer characteristics but also to connectivity requirements (queuing mechanisms atnodes and routing types) and loss and jitter (delay variation) tolerance.

As presented in [39], the interest is in identifying a SI-SAP extension, named TI-SAP(Technology-Independent Service Access Point) to permit the seamless interconnection ofdifferent radio technologies. TI-SAP operates above different (terrestrial and satellite) MAClayers and uniforms them from the standpoint of upper layers, irrespective of the fact that lowerlayers belong to a terrestrial or to a satellite air interface.

The European WISECOM (Wireless Infrastructure over Satellite for EmergencyCommunications) project studied and demonstrated hybrid networks to be used to supportcommunications in emergency scenarios considering an architecture using local 2G, 3G,WiMAX, WiFi base stations interconnected to the network through a satellite back-haulinglink [40].

Another application scenario could be the interconnection of a satellite DVB-RCS networkwith WiFi based on the IEEE 802.11e standard. The WiFi network is used to provide a localcoverage on a plane, a train, or a ship. In QoS provisioning, over DVB-RCS, SatLabs, a not-for-profit organization initiated by the European Space Agency (ESA), has defined a QoSarchitecture and mapping mechanisms of DVB-RCS allocation methods [41] and IP layer whereQoS is supported by DiffServ [42]. DVB-RCS MAC layer and IP layer need to make consistentchoices in managing the related queues for QoS support. Hence, a cross-layer air interfacedesign could be adopted to coordinate the management of resources at IP and MAC layers (e.g.queue congestion signaling). In particular, TI-SAP has to be configured to support suitabletraffic mapping with the adoption of cross-layer signaling primitives (control plane) tocoordinate layer 2 and layer 3.

In the hybrid scenario, inter-segment handovers are needed when the mobile terminal leavesthe coverage of a given WLAN in urban area in order to handle this traffic via the satellitenetwork; the opposite inter-segment handover occurs when the mobile terminal enters a urbanarea. The inter-segment handover should be based on the Media Independent Handover (MIH)standard developed by IEEE 802.21 to enable the handover of IP sessions from one layer 2access technology to another [43]. Note that the TI-SAP approach is consistent with the NGNarchitecture and the separation of transport from control functions; moreover, TI-SAP shouldsupport the MIH standard.

Finally, IP layer Forward Error Correction (FEC) coding could be used to protect the trafficflow traversing different network domains with different characteristics in terms of signalimpairments and, then, packet loss.



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3.3. Networking aspects

In the change of wireless network (inter-segment handover) an IP traffic rerouting process has tobe supported. Efficient schemes have to be implemented for a fast rerouting; avoiding latency isimportant to reduce the risk of huge amount of packets that need to be recovered during thepath change process. IP mobility protocols have to be employed for this task.

4. PERFORMANCE ENHANCEMENT TECHNIQUES

4.1. Cooperative diversity techniques for integrated/hybrid systems

4.1.1. System concept and model. To improve the performance of integrated/hybrid systems forproviding MBMS, recently various cooperative diversity techniques have been proposed [3, 32, 33].Figure 5 shows the system model of the integrated/hybrid satellite-terrestrial network that canutilize cooperative diversity techniques using STC and/or FEC coding schemes. In this network, asatellite in the GEO orbit and an ensemble of terrestrial repeaters (CGC or ATC) are deployed.The satellite transmits an encoded signal format, C1, to all repeaters in the ground. Then, each ofthese repeaters transforms the received signal from the satellite to a different encoded signal andretransmits it to the user terminal. Contemporarily, the same signal is directly transmitted to theuser terminal from the satellite.

The repeaters have the ability to encode signals rather than being simple amplifiers. A userterminal has the ability to receive the STC encoded signals. If the user terminal receives multiplesignals from repeaters and also from the satellite, then it can achieve diversity gains. For thesake of simplicity and by considering a practical situation, we assume that the user terminal canreceive multiple signals with at maximum three different encoded formats, i.e. from C1 to C3.

In addition, a delay compensation algorithm is required, which can make the signals from thesatellite and the repeaters arriving at the same time to the user terminal. Since we can estimateprocessing delay to transform to a given STC encoded format at the repeaters as well as timedifference of propagation delay between the links from the satellite and terrestrial parts, thedelay compensation for the signal paths from the terrestrial parts can achieve successfulsynchronization at the user terminal [33].

The performance of these cooperative techniques can be varied depending on the encodingschemes, i.e. format of C1 to C3, and how we combine all available paths. Table I shows variouscooperative diversity schemes, where multiple signals with different codes of C1 to C3 can beeffectively combined at a user terminal to produce diversity gains. In each scheme, xi denotes

Figure 5. Integrated/hybrid satellite-terrestrial network incorporating cooperative diversity schemes.



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information symbol at the i-th time period, and � denotes complex conjugate operation. Weconsider a rate compatible FEC code with its mother code rate of 1/3. The code is composed ofa systematic part, x5 [x1x2yxN], a parity part, p1 ¼ ½p1

1p12 :::p

1N �, and another parity part,

p2 ¼ ½p21p2

2 :::p2N �. In a codeword, the lengths of x, p1, and p2 are the same, and thus a code can be

consisted of x and p1 with a code rate of 1/2, and another code can be consisted of x and p2 withthe same code rate of 1/2.

All the schemes in Table I adopt rate 1/2 FEC schemes by utilizing space–time encodedversion of x and p1 or p2. Although scheme DA and DB look similar to each other, scheme DA

allocates the same encoded signal format to C1 and C2, while scheme DB allocates two differentrate-compatible FEC codes to C1 and C2. In the user terminal, we can combine two differentparities, p1 and p2, thus utilizing the code of rate 1/3, when both signal paths from the satelliteand repeaters are available. Because each transmitter sends signal sets punctured from the sameoriginal mother code, we can use a unique decoder with any kinds of signal combinations, as ina conventional Rate-Compatible Punctured Coding (RCPC) scheme.

4.1.2. Simulation results. This section presents the simulation results of the BER performance ofthe various cooperative diversity schemes presented in Table I. We assume that the signal ismodulated by using QPSK, that the amplitude of fading from the satellite to the receiver isRician-distributed, with factor K of 10 dB, and those from the repeaters to the receiver areuncorrelated Rayleigh-distributed. In each case, we assume that fading is constant across two orfour consecutive symbols depending on the STC schemes, and the average signal powers at thereceiver, from each transmitter, are the same. The receiver has perfect knowledge of the channel.For the channel coding scheme, we use a duo-binary turbo code that is specified in manywireless systems including the DVB-RCS standard [41]. The encoder generates 3 N symbols withthe code rate of the mother code, 1/3 for the information with symbol length of N. In oursimulation, we use the information block size N of 212 symbols. The Max-log-MAP algorithmwas used as an iterative decoding algorithm, and the maximum iteration number was set to 8.

Table I. Encoded signal formats from C1 to C3 in various cooperative diversity schemes.

Diversity scheme Encoding format (information vector, x)

DA C1 ¼ ½xp1�

C2 ¼ ½xp1�

C3 ¼ ½�x�2x�1:::� x�N x�N�1 � ðp12Þ�ðp1

1Þ�:::� ðp1

N Þ�ðp1

N�1Þ��

(C3 is Alamouti encoding of C2[43])

DB C1 ¼ ½x1 � x�2x3 � x�4:::ðp1N�3Þ � ðp

1N�2Þ

�p1N�1 � ðp

1N Þ��

C2 ¼ ½x2 � x�1x4x�3:::ðp1N�2Þ � ðp

1N�3Þ

�p1N � ðp

1N�1Þ

��

C3 ¼ ½x3 � x�4x1 � x�2:::ðp1N�1Þ � ðp

1N Þ�p1

N�3 � ðp1N�2Þ

��

(C1 to C3 are QO-STBC encoded signals [33])DC C1 ¼ ½x11x3 � x�2 � x�4x3 � x1x�2 � x�4::::

p1N�31p1

N�3 � ðp1N�2Þ

� � ðp1N Þ�p1

N�1 � p1N�3ðp

1N�2Þ

� � ðp1N Þ��

C2 ¼ ½x21x4x�11x�3x4 � x2x�3 � x�1::::

p1N�21p1

N ðp1N�3Þ

�1ðp1N�1Þ

�p1N � p1

N�2ðp1N�1Þ

� � ðp1N�3Þ

��

C3 ¼ ½x3 � x1x�2 � x�4x11x3 � x�2 � x�4::::

p1N�1 � p1

N�3ðp1N�2Þ

� � ðp1N Þ�p1

N�11p1N�3 � ðp

1N�2Þ

� � ðp1N Þ��

(C1 to C3 are QO-STBC encoded signalswith linear decoding capability [33])

DD C1 ¼ ½xp1�

C1 ¼ ½xp2�

C3 ¼ ½�x�2x�1:::� x�N x�N�1 � ðp22Þ�ðp2

1Þ�:::� ðp2

N Þ�ðp2

N�1Þ��

(C3 to Alamouti encoding of C2[44])



DOI: 10.1002/sat

Figure 6 compares BER performance of various diversity schemes in Table I. As shown inTable I, all the schemes including the no-diversity scheme, which is denoted as NA in Figure 6,use rate 1/2 turbo codes that are punctured from the rate 1/3 mother code. As shown inFigure 6, cooperative diversity scheme can provide power gains with respect to the no-diversitycase. Although scheme DD can provide much better performance than the other schemes, thisscheme basically requires a means to identify p1 and p2. This is because these two parity parts aretransmitted at the same time via different transmitters. In our application, we used two differentreceiving antennas to identify these by assuming these two parities are transmitted by usingdifferent frequency bands or polarization types. If we use two antennas at the receiver we canachieve additional combining gain. For this reason, we also plotted the performance of schemeDD with two receiving antennas showing 3 dB performance gain with respect to scheme DD witha single receiving antenna. Another aspect to be noted in Figure 6 is for scheme DB that is acooperative diversity scheme using QO-STBC with three transmit antennas; adopting a softdecision scheme we should pay special attention to get precise soft detection information that isrequired for iterative decoding of turbo codes, because simple zero-forcing detection could causeserious performance degradation. For this reason, we applied a simple but computationallyefficient soft detection scheme to get reasonable performance [45], denoted as D�B in Figure 6.Similarly to the Alamouti code in schemes DA and DD, the QO-STBC code in scheme DC canutilize a simple linear maximum likelihood detection scheme.

Although we may not be able to see great performance gain by the various proposed diversitygains in Table I, an alternative interpretation of the proposed diversity schemes is as follows.The repeater for the integrated satellite and terrestrial system is used as a complementarymethod to fill the gap in a heavy urban area where we can hardly receive the direct signal fromthe satellite. At the edge of the terrestrial repeater coverage where the signal strength from therepeater becomes weak and thus none of the signal powers is strong enough, then cooperativecombining of the signal from the repeaters as well as from the satellite can be effectively used to

Figure 6. BER performance comparison of various cooperative diversity schemes for integrated/hybridsatellite systems.



DOI: 10.1002/sat

improve the performance, thus achieving constant service quality. In other words, near the edgeof the terrestrial repeater coverage, the signal from the satellite can be used as the compensationof power loss from the terrestrial repeaters. Accordingly, this will lead to the extension of therepeater coverage, thus reducing the number of repeaters to be installed. This will definitelycontribute to reduce the system installation costs.

4.2. Traffic engineering issues for integrated networks

Integrated systems are typically characterized by complementary coverage between satellite andterrestrial component or overlapped coverage where the satellite acts as an ‘umbrella system’, aguard capacity to be used when needed. Rerouting of traffic sessions from the terrestrialsegment to the satellite one could be adopted to exploit the guard capacity, thus maintaining theQoS. In particular, RRM could divert sessions to the satellite when there is resource congestionin the terrestrial network. If the satellite network can only accommodate a limited number ofparallel traffic flows (sessions) there is the risk of session blocking. In addition to this, therecould be sessions with different weights (e.g. with different requested bandwidth) for which thecapacity allocation becomes a classical knapsack problem. If the session generation in theterrestrial (preferential) segment is according to a Poisson process, the overflow arrival processto the satellite segment is peaked (i.e. variance bigger than mean) [46].

The Call Admission Control (CAC) model at the session level for the satellite system is of theG/G/S/S type with session arrival traffic with mean intensity A and variance V and denotingwith S the maximum number of simultaneous sessions that the satellite segment canmanage. Figure 7 describes a typical situation with session overflow traffic reaching a queuemodeling a cell (spot-beam) of a satellite network. To study the blocking probability in thiscase we could generalize the Erlang-B formula by means of the Wilkinson approach describedbelow [46].

The blocking probability PB experienced by the generic overflow traffic (traffic overflowaddressed to the satellite) with mean intensity A and variance V (V>A) for S servers isapproximated by means of the following generalization of the Erlang-B formula to the case ofnoninteger number of servers:

PB ¼A1

APBðS1S1;A1Þ ¼

1

AR110 e�A1yð11yÞS1S1dy

; ð1Þ

where A1 and S1 are parameters of a fictitious queuing system (see Figure 7) of theM/G/S1/S1 type with Poisson arrival process with mean intensity A1 and non-integer

Figure 7. Traffic overflow from terrestrial to satellite segment (umbrella coverage scenario) producingtraffic with mean intensity A and variance V in the satellite component.



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number of servers S1.z In particular, A1 and S1 can be derived by numerically solving the

following non-linear system with given A and V values characterizing the rejected trafficfrom the fictitious queue that is used to generate the input traffic to our queue with parametersA and V:

A ¼1R11

0 e�A1yð11yÞS1dy; ð2Þ

V ¼ A 1� A1A1

S1 � A11A11

� �: ð3Þ

Note that this method can be extended to the case where the input traffic is the sum ofindependent traffic contributions coming from different terrestrial areas of coverage with meanAi and variance Vi; in fact, we employ the above formulas where

A ¼X

i

Ai ; V ¼X

i

Vi : ð4Þ

The utilization f of satellite resources can be computed as A(1�PB)/S. An approximatedsolution of (3) entails A1 5V13z(z�1) with z5V/A. Hence, we can numerically invert (2) toobtain S1 to be used in (1) to compute PB.

The graph in Figure 8 shows the behavior of the traffic rejected from the satellite network,PB, in a case where A5 5 Erlang, S5 8 satellite resources as a function of the traffic variance V(equivalently, the peakedness factor z5V/A with V4A for the peaked traffic of the overflowtraffic case). We can note that the blocking probability PB significantly increases with the trafficpeakedness with respect to the classical Erlang-B case with Poisson arrivals (i.e. V5A). Finally,it is important to note that both intra-segment and inter-segment handovers entail a smoothed

Figure 8. Impact of traffic peakedness on the CAC blocking performance for an integrated satellite network.

zEven if Figure 7 is intended for analyzing the blocking due to CAC for the satellite segment where the M/G/S1/S1

system is a fictitious queue used to generate the equivalent traffic with the same parameters A and V, this queue can alsorepresent the actual situation of traffic overflow from a cell of the terrestrial system.



DOI: 10.1002/sat

traffic contribution (i.e. variance lower than mean) for the satellite to be considered in (4)depending on user mobility.

5. FUTURE TRENDS AND CONCLUSIONS

Broadband communications are a key element to open Information Society doors to less-developed regions; in such a context, satellites can represent an interesting approach to solve thedigital divide problem. Since we are moving towards an information-driven society, thedevelopment of a converged communication and service infrastructure that gradually replacescurrent Internet, mobile, and fixed networks is essential. According to the NGN approach, theinterest is for a converged service capability across heterogeneous accesses for ubiquitousbroadband services, integrating wired and wireless, fixed and mobile technologies in integrated/hybrid access systems. It is therefore important to achieve service portability and continuityacross composite networks through the service–network interface with ubiquitous access,involving any network, any technology, any domain, and any administrative domain. Thefuture architecture will be service-oriented, the network will be pervasive, and importantapplications will include peer-to-peer applications, video on demand, and multicast.

For the long-term IP-based network architecture, intelligent On-Board Processing (OBP)capabilities will enable adequate distribution of control for an integrated satellite and terrestrialnetwork [47, 48]. Moreover, routing at network layer and allocation of resources at link layerhave to be performed in a joint way for the different segments to satisfy traffic requests andminimizing at the same time the cost of the allocated bandwidth among the heterogeneousnetwork components [49]. More advanced form of cooperative diversity and relay techniqueswill be developed not only to enhance the performance but also to increase spectral efficiency.Compatibility of radio interfaces between terrestrial and satellite components is one of the keyissues to be investigated to achieve true integration.

Current interest is in identifying the potentialities and showing advantages in adoptingintegrated/hybrid network solutions involving the satellite component. The contribution of thispaper has been to provide a wide survey on integrated/hybrid network issues including relatedinitiatives and projects. Then, we have provided analytical details to describe the performance ofcooperative communications at the physical layer showing that we can effectively achieve powergains, by using cooperative diversity techniques with appropriately encoded signals from thesatellite and terrestrial repeaters. We also discussed that the cooperative combining of thesignals can be used to maintain the QoS. Finally, traffic engineering methods have beenexplained to analyze the service capability of the satellite segment in managing overflow trafficcoming from the terrestrial segment.

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AUTHORS’ BIOGRAPHIES

Sastri Kota ([email protected]): Dr. Kota received his BS in Physics fromAndhra University, BSEE from BITS, Pilani, MSEE from Indian Institute ofTechnology (IIT), India. He received the Electrical Engineer’s Degree fromNortheastern University, Boston, U.S.A. and PhD in Electrical and InformationEngineering from University of Oulu, Finland. From 2003 to 2009, he has been aSenior Scientist in Harris Corporation involved with Corporate Technologies andStandards with special emphasis on Wireless and Mobile Ad Hoc Networks, satellitecommunication networks and Standardization. He is an Adjunct Professor in theTelecommunications Laboratory of the University of Oulu. His research interestsinclude wireless and mobile Information networks, satellite IP networks, QoS andtraffic management, broadband satellite access, and ATM networks. Over the years,he has held technical and management positions and has contributed to military and

commercial communication systems at Loral Skynet, Lockheed Martin, SRI International, The MITRECorp and Xerox Corp. He has been very active in telecommunications and networking standardsdevelopment. Currently he is the Head of the US delegation and the US chair for ITU-R, Working Party4B and International Rapporteur for Ka-Band Fixed Satellite Systems. He was the chair for Wireless ATMWorking Group and has been an ATM Forum Ambassador. He was the recipient of the ATM ForumSpotlight award, publication awards from Lockheed Martin, and the Golden Quill award from HarrisCorporation for his contributions to Broadband Satellite Communications and Assured Communications.

Dr. Kota is the principal author of the book Broadband Satellite Communications for Internet Accesspublished by Kluwer Academic Publishers, and is co-editor of the book ‘Emerging Location AwareBroadband Wireless Ad Hoc Networks’ by Springer, and has contributed book chapters to Encyclopedia ofTelecommunications, John Wiley & Sons, High Performance TCP/IP Networking, Prentice Hall, andModeling and Simulation Environment for Terrestrial and Satellite Networks, Kluwer Academic Publishers.He has published and presented over 120 technical papers in book chapters, journals, and conferenceproceedings. He has served as a guest editor for IEEE Communications Magazine, Special Issues on Cross—Layer Protocol Engineering for Wireless Mobile Networks; Satellite ATM architectures, Broadband SatelliteNetwork Performance, and International Journal of Satellite Communications and Networking, Special Issueon Satellite IP QoS and Cross Layer Design for Satellite Networks; and special issues on satellite networkprotocols and Satellite Networks for Mobile Services in Space Communications—international journal. Hecurrently serves on the editorial boards of the International Journal of Satellite Communications andNetworking (Wiley Interscience), and the International Journal of Space Communications (IOS Press).

Dr. Kota has been a keynote speaker, invited speaker, and panelist at various InternationalConferences. He is the Unclassified Technical program Chair for MILCOM 2007. He also served as theTutorial chair and Asst. Technical chair of MILCOM2004, 1997, 1990; symposium co-chair of satellitecommunications symposium of GLOBECOM 2000, 2002 and invited session chair of PIMRC 2004, 2005and 2006. He was the co-chair of Wireless Communications and networking symposium ofGLOBECOM2006 and Technical chair of ISWPC2007. He was the co-chair of Workshop on Co-operative Communications held in conjunction with ICC 2008 and panel co-chair of WCNC 08. He is theco-chair of Wireless Networks Symposium of GLOBECOM 2009 and Mobile Satellite and PositioningTrack Chair of VTC 2009 spring. He has been a member of the technical program committees of severalIEEE, AIAA, SPIE and ACM conferences and workshops. He is a senior member of the IEEE, AssociateFellow of AIAA, and a member of ACM.



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Giovanni Giambene ([email protected]) Giovanni Giambene was born in Florence,Italy, in 1966. He received the Dr. Ing. degree in Electronics in 1993 and the PhDdegree in Telecommunications and Informatics in 1997, both from the University ofFlorence, Italy. From 1994 to 1997, he was with the Electronic EngineeringDepartment of the University of Florence, Italy. He was the Technical ExternalSecretary of the European Community COST 227 Action (‘Integrated Space/Terrestrial Mobile Networks’). He also contributed to the SAINT Project (‘SatelliteIntegration in the Future Mobile Network’, RACE 2117). From 1997 to 1998, hewas with OTE of the Marconi Group, Florence, Italy, where he was involved in aGSM development program. During the same period he also contributed to theCOST 252 Action (‘Evolution of Satellite Personal Communications from Second toFuture Generation Systems’) research activities by studying PRMA protocols for

voice and data transmissions in low earth orbit mobile satellite systems. In 1999, he joined the InformationEngineering Department of the University of Siena, Italy, first as research associate and then as assistantprofessor and aggregate professor. He teaches the advanced course of Telecommunication Networks at theUniversity of Siena. From 1999 to 2003 he participated in the project ‘Multimedialita’, financed by theItalian National Research Council (CNR). From 2000 to 2003, he contributed to the ‘Personalised Accessto Local Information and services for tourists’ (PALIO) IST Project within the EU FP5 programme. Hewas vice-Chair of the COST 290 Action (www.cost290.org) for its whole duration 2004–2008, entitled‘Traffic and QoS Management in Wireless Multimedia Networks’ (Wi-QoST). He participated in theSatNEx I & II network of excellence (FP6 programme, 2004–2009) as work package leader of two groupson radio access techniques and cross-layer air interface design for satellite communication systems(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/) and in the COST Action IC0906 ‘‘Wireless Networking for Moving Objects’’(WiNeMO). Giambene is an IEEE senior member. He has 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.

Sooyoung Kim ([email protected]) received the BS degree in Electrical andElectronics Engineering from KAIST, Korea, in 1990. After having worked at theSatellite Communication Technology Division, ETRI, Korea from February 1990 toSeptember 1991, she received the MSc and the PhD degrees in Electrical andElectronics Engineering from the University of Surrey, U.K in 1992 and 1995respectively. From November 1994 to June 1996 she was employed as a researchfellow at the Centre for Satellite Engineering Research, University of Surrey, U.K.In 1996 she re-joined the Satellite Communication Technology Division, ETRI,Korea, and worked as a team leader until February 2004 to develop efficienttransmission techniques for digital satellite communication systems. She is now anassociate professor in Chonbuk National University. Her research interests includemodulation and coding schemes for digital satellite communication systems. She has

been working on ITU-R since 2000, and has contributed to making the radio interface standard of satellitecomponents in the IMT-2000 system. Now, she is actively working on Working Party 4B of ITU-R, andshe was appointed as an international standardization expert in Korea. She has been a Technical ProgramCommittee (TPC) member at various conferences, and has co-chaired satellite systems track at VTC 2008spring.



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Satellite Component of NGN: Integrated and Hybrid Networks

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