Physical Layer Technologies And Challenges In Mobile Satellite Communications


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The central features of the future fourth-generation
mobile communication systems are the provisioning of highspeed
data transmissions (up to 1 Gb/s) and interactive
multimedia services. For effective delivery of these services,
the network must satisfy some stringent quality-of-service
(QoS) metrics, defined typically in terms of maximum delay
and/or minimum throughput performances. Mobile satellite
systems will be fully integrated with the future terrestrial
cellular systems, playing important roles as back-bones or
access satellites, to provide ubiquitous global coverage to
diverse users. The challenges for future broadband satellite
systems, therefore, lie in the proper deployments of state-ofthe-
art satellite technologies to ensure seamless integration of
the satellite networks into the cellular systems and its QoS
frameworks, while achieving, to the extent possible, efficient
use of the precious satellite link resources. This paper presents
an overview of the future high-speed satellite mobile
communication systems, the technologies deployed or planned
for deployments, and the challenges.

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Physical Layer Technologies And Challenges In Mobile Satellite Communications

  1. 1. ACEEE Int. J. on Communication, Vol. 01, No. 03, Dec 2010 Physical Layer Technologies And Challenges In Mobile Satellite Communications. Sonika Singh1, and R.C.Ramola2 1 Depatment of Electronics & Communication Engineering, Uttarakhand Technical University, Dehradun (U.K), India. 2 Depatment of Electronics & Communication Engineering, ICFAI University, Dehradun (U.K),India. rramola@gmail.comAbstract :The central features of the future fourth-generation the presence of obstacles or return link budget restrictionsmobile communication systems are the provisioning of high- caused by the low power and small antenna size availablespeed data transmissions (up to 1 Gb/s) and interactive on portable terminals. In order to address these problems,multimedia services. For effective delivery of these services, two similar, but distinct, innovative design approaches canthe network must satisfy some stringent quality-of-service be adopted: (i) hybrid networks and (ii) integrated(QoS) metrics, defined typically in terms of maximum delayand/or minimum throughput performances. Mobile satellite networks. In the first case, terrestrial gap fillers (repeaters)systems will be fully integrated with the future terrestrial can be employed to retransmit locally the satellite signal incellular systems, playing important roles as back-bones or non-LoS conditions. Moreover, the return link can beaccess satellites, to provide ubiquitous global coverage to supplied by a terrestrial cellular system to simplify thediverse users. The challenges for future broadband satellite power management of mobile terminals. Finally, thesystems, therefore, lie in the proper deployments of state-of- satellite coverage can be extended (e.g. indoor or urbanthe-art satellite technologies to ensure seamless integration of cases) by means of a local wireless system where the basethe satellite networks into the cellular systems and its QoS station ‘converts’ the satellite signal to the wireless one andframeworks, while achieving, to the extent possible, efficient vice versa. For what concerns the integrated networks, ause of the precious satellite link resources. This paper presentsan overview of the future high-speed satellite mobile terrestrial cellular network can be used as an alternativecommunication systems, the technologies deployed or planned system to connect the mobile user (both forward and returnfor deployments, and the challenges. links) with respect to the satellite one. Some examples of integrated networks are analyzed in [2], referring to theIndex Terms: Mobile satellite Systems, Design issues for Mobile Applications & sErvices based on Satellite &MSSs,PHY layer technologies, QoS. Terrestrial inteRwOrking (MAESTRO) project. In order to define the terrestrial segment, the European Commission I. INTRODUCTION has introduced the concept of Complementary Ground Component (CGC); while, FCC in U.S. has used the term Satellite networks are an attractive approach for Ancillary Terrestrial Component (ATC). These conceptscommunication services in areas of the world not well are quite interchangeable, even if CGC is more related toserved by existing terrestrial infrastructures. There is a vast hybrid networks and ATC to integrated networks. In anyrange of sectors (e.g. land-mobile, aeronautical, maritime, case, terrestrial systems could be based on3rd generationtransports, rescue and disaster relief, military, etc.) needing (3G), Wireless Fidelity (WiFi, IEEE 802.11mobile communication services and where the satellite is a/b/g), or Worldwide Interoperability for Wirelessthe only viable option [1]. This is the reason why at present Microwave Access (WiMAX) technologies.there is a renewed interest and market opportunities forMobile Satellite Systems (MSSs). Technologies for multi- Current Mobile Satellite Systems:spot-beam antennas, low-noise receivers, and on boardprocessing have permitted to achieve the direct access to The following MSS projects deal with the challengesthe satellite for small, portable or even handheld terminals and the efforts for providing broadband multimediaby using S, L, and recently Ku and Ka bands. Satellites can services to users in land vehicular, aeronautical, andalso be equipped with a regenerating payload and inter- maritime environments:satellite links, thus respectively permitting to switch traffic MObile Wideband Global Link sYstemflows from different beams of a satellite and traffic (MOWGLY) [3];forwarding/routing in the sky through satellites. Satellites Mobile Broadband Interactive Satelliteare on suitable orbits around the earth; on the basis of their multimedia Access Technology(MoBISAT) byaltitude, they can be categorized into Geosynchronous ETRI [4]; andEarth Orbit (GEO) and non-GEO. MSSs may suffer from Broadband Global Area Network—eXtensionnon-Line-of-Sight (non-LoS) propagation conditions due to (BGAN-X) by the European Space Agency, ESA [5].Moreover, the Satellite-based communication 28© 2010 ACEEEDOI: 01.IJCOM.01.03.2
  2. 2. ACEEE Int. J. on Communication, Vol. 01, No. 03, Dec 2010 systems within IPv6 (SATSIX) project aims, low latency delivery, high capacity, high throughput, and among others, to incorporate the IPv6 protocol high data rate capabilities. Table 2 shows several mobile inside broadband MSSs [6]. satellite systems currently in use which also support high- Standards for Mobile Satellite systems: speed data, Internet, and multimedia applications [7],[8]. The five standards that are directly related to MSSs are: Table 1. Satellite Mobile Communication Systems [7],[8]. 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). Broadband Satellite Architectures and Constellations:Broadband satellite architectures may be based on ATMwith sophisticated onboard processing (OBP), onboardswitching (OBS), and intersatellite links (ISLs), whileothers employ simple bent-pipe transponder relays. Thesystem design choices depend on factors includingcoverage, cost, user service, and traffic demands. II. DESIGN ISSUES FOR MSSS NETWORKS.Constellations may be LEO, MEO, geostationary earthorbit (GEO), or combinations thereof, dependent on the A. Frequency bands and regulations.required coverage and the supported services. Future Frequency bands are assigned at the World Radio-broadband satellite mobile systems will deploy high communication Conferences (WRCs), periodicallynumbers of satellites in the nongeostationary constellations, organized by the International Telecommunication Unionsuch as MEO and LEO. Though the coverage of GEO radio-communication sector (ITU-R). While fixedsatellites is a primary advantage over the LEO systems, the services use high C and K frequency bands, mobilelonger delay of GEO however makes them typically less services are better suited for lower L and S frequencysuitable for mainstream 4G applications such as interactive bands that were assigned at the World Administrativemultimedia than the LEO systems. Radio Conference (WARC) 92. MSSs have exploited L/S- For satellites in LEO, propagation delay is on the band technology for a long time: L/S-band systems permitorder of 10 ms. In MEO the delay is on the order of 80 ms, small on-board antennas due to lower signal attenuationand in GEO orbits it is 250–270 ms. Other delays due to and reduced impact of atmospheric effects. However, theprocessing and transmissions are on the order of 80–100 need of broadband services and the limited amount ofms for regional traffics and 140–180 ms for international available L/S-band resources (2-30 MHz) have pushedones. When all delays are considered GEO satellite-based toward the use of Ku and Ka bands for MSSs. ITU-R hascommunications may be marginal for quality due to time assigned Ka band frequency portions to MSSs and Fixeddelays. However, LEO and MEO orbits have their peculiar Satellite Systems (FSSs) on a primary basis in all regionsproblems. Due to the low altitude, LEO and MEO satellites (29.9–30 GHz for earth-to-space link and 20.1–21.3 GHzmove at rapid speeds, causing frequent handovers between for space-to-earth link) and Ku band frequency portions tothe ground terminal and the satellites, which are in view for MSS on a secondary basis in all regions (14–14.5 GHz fora relatively short period. The high mobility causes regular- earth-to-space link and 10–12 GHz for space-to-earth link).changing network topology and the transmission is At present, Ku-based MSSs are available to providesubjected to Doppler shifts and small-scale multipath broadband services in many mobile environments, such asfading. Additionally, LEO and MEO satellites rely on ISLs trains, boats, planes, and cars. However, Ku-band satellites,between neighboring satellites to increase coverage. The as opposed to L/S-band satellites, do not provide a goodmain challenge here lies in the proper handling of ISLs so coverage over seas, because antenna spot-beams footprintsthat they do not lead to problems with delay jitter, which are focused on landmasses [9]. In fact, Ku-band satellitescan degrade voice and video QoS performances over the are mainly intended for fixed users, so that there are notsatellite systems. Buffering is a good solution known to enough Ku/Ka band satellites providing coverage overwork well for jitter problems and must be employed. oceans. Hence, a trade-off has to be achieved between theSeveral satellite mobile systems have been deployed, or are need of increased bandwidth and coverage the process of being deployed, employing specificconstellations or mixture of constellations carefullyselected to achieve as much as possible, combinations of 29© 2010 ACEEEDOI: 01.IJCOM.01.03.2
  3. 3. ACEEE Int. J. on Communication, Vol. 01, No. 03, Dec 2010B. Mobile terminal antenna. satellite antennas; dozens of beams for LEO and MEO The antenna design is a crucial issue for mobile satellite antennas. The allocated frequency band is dividedterminals. An important aspect is the antenna size, the cost, into some carriers that are distributed among beams inand the adopted technology. Moreover, the antenna system order to avoid interferences among adjacent beams; carriersshould be reliable and efficient in terms of sensitivity, gain, can be reused in sufficiently far beams. A cluster is a set ofand interference. It is important to highlight some beams where all the system carriers are used. Somedifferences between fixed and mobile services: fixed examples of (average) cluster sizes (i.e. number of beamsterminals use directional antennas, while mobile terminals per cluster) for MSSs are]:12 beams/cluster for Iridium,27can also use omni-directional antennas (where ,phased- beams/cluster for BGAN, and 21 beams/cluster forarray directional antennas with fast tracking algorithms Thuraya [14]. GEO systems, such as BGAN and Thuraya,could be adopted instead of omni-directional antennas in are characterized by higher values of the cluster size: inorder to improve the link budget). Typically, mobile GEO systems ‘narrower’ (i.e. higher directivity) beamsterminals can transmit in all the directions and receive than in non-GEO ones are needed to irradiate the same areasignals from all the directions as well. For this reason, on the earth. Hence, beams are much ‘closer’ each other inmobile terminals could interfere with other satellite antennas on GEO satellites, thus entailing higher levels ofnetworks. In [17],the study analyzes non-GEO fixed and mutual interference and the need for a larger frequencymobile satellite service constellations, providing some reuse cluster. Here, ‘closer beams’ means a greater densitysuggestions for regulations (in terms of maximum of satellite antenna beams per unit of solid angle related totransmitted power and elevation angles) to avoid the satellite; such density is much higher in GEO cases thaninterference among them. Further considerations on in non-GEO ones. On the contrary, the spot-beamterminal antenna design can be done by taking into account footprints (i.e. cells) irradiated on the earth by non-GEOthe different application environments: for example, the satellites are smaller and closer each other than in the caserailway scenario is well served by Ku-band satellites of GEO satellites.(coverage over landmasses), but the antenna on trains D. Elevation angle.should be small (low-directivity gain), thus generating Another important issue for a good quality of thehigher interference levels for adjacent satellites. In communication is the minimum elevation angle accordingaeronautical and maritime scenarios, planes and boats to which a mobile terminal can see the satellite in an MSS.could be at the edge of spot-beam coverage, thus requiring While the requirements on this angle are not so stringenta suitable antenna design. However, big antennas could be for FSSs due to the fact that the location and orientation ofused in the case of big boats that have lower design the user antenna can be optimized (e.g. LoS conditions canconstraints. The antenna size on the mobile terminal be achieved for GEO satellites by selecting appropriatedetermines the characteristics of interference for both earth station locations), in the MSS scenario (in particularuplink and downlink transmissions. Moreover, there are for land-mobile users) a low value of the minimumoff-axis power flow limitations for uplink transmissions in elevation angle should be avoided , otherwise frequentKu band (there are only secondary allocations for MSSs). shadowing and blockage events due to trees, buildings andThis entails constraints on the Effective Isotropic Radiated hills may occur. Increasing the elevation angle, the signalPower (EIRP) for the mobile user. In order to mitigate quality improves (reduction of shadowing/blockageinterference, spread-spectrum schemes can be used. Several effects), but also system costs increase (higher number ofspread-spectrum techniques can be adopted (e.g. Direct satellites in the constellation). The minimum elevationSequence, DS, Frequency Hopping, FH, and burst angle requirement entails suitable design constraints for therepetition). The standardization for the mobile extension of number of satellites in a constellation and also entails thatDigital Video Broadcasting—Satellite version 2/ Digital GEO satellites cannot service Polar Regions.Video Broadcasting—Return Channel via Satellite (DVB-S2/DVB-RCS) has considered DS spreading for the E. Channel models.forward link and burst repetition for the return link For the purpose of satellite system analysis, design,(maximum spreading factor of 16 with Single Channel Per and simulation, mathematical models for the land mobileCarrier, SCPC) [11]. satellite channel are needed. Extensive research worksC. Satellite antenna and frequency reuse. have, therefore, been carried out to develop measurements- based statistical models [13],[14], that are particularly One of the key aspects in realizing MSSs is the use of suitable for Ka- band and higher frequencies. For example,a high-directivity multi-spot-beam satellite antenna, the authors in [15] gave a comprehensive survey of theconsisting of a large deployable reflector and a feeder most accepted statistical models proposed in the scientificsystem. At present, typical big-antennas on GEO satellites literature, considering large-scale and small-fading, singlecan reach a diameter up to 25 m, a diameter around 2 m and multiple-state structures, narrowband and widebandcan be expected for LEO systems. Spot-beams are needed channels, and first and second-order statistics. Buildingin order to focus the covered area on the earth with a high upon a thorough characterization of propagation effects, theantenna gain. Current MSSs exploit satellite antennas with authors focus on performance analysis of coded anda high number of beams: hundreds of beams for GEO 30© 2010 ACEEEDOI: 01.IJCOM.01.03.2
  4. 4. ACEEE Int. J. on Communication, Vol. 01, No. 03, Dec 2010uncoded systems based on closed-form expressions, upper receiving resources in the destination cell, otherwise thebounds, and numerical simulations. related session could be terminated by higher layers.Channel Modeling Challenges: H. Network layer issuesOne fundamental characteristic of future satellite mobile With reference to satellites with on-board IP routingcommunication systems is the necessity to be fully capabilities,Mobile IP(MIP) developed by IETF could beintegrated into the other terrestrial networks in order to used to support handover procedures. Unfortunately, MIPenable global, seamless, and ubiquitous communications. has the problem of high handover latency. NASA andWith emerging non-geostationary LEO and MEO satellite CISCO have carried out many projects to improve MIP forsystems and high data rate applications, accurate and handover procedures in IP-based satellite networks [18]. Asflexible channel models are needed in order to allow an alternative to the above MIP approach, Connexion byrealistic QoS predictions and perform system comparisons Boeing (an in-flight GEO-based Internet connectivityunder different multiple-access, modulation, coding and service, not anymore active since 2006) allowed global IPdiversity schemes. For future satellite mobile systems, a mobility using the Border Gateway Protocol (BGP) [19]. Insuitable channel model should satisfy the following particular, a Class C IP address block is assigned to acharacteristics: the model should be based on accurate mobile platform (i.e. a plane or a ship, having on-board a dataestimation and modeling of propagation statistics, the transceiver/router box and some 802.11 a/b/g wireless access points). These addresses are ‘selectively announced’ by themodel should combine very well the effects of weather nearest terrestrial gateway, for the period the plane/ship passesattenuation process and the multipath fading and through the region where the gateway is located (four gatewaysshadowing process, and the model should consider the have been used to cover North America, Europe,different channel state changes, for example, from a and Asian regions). When the plane/ship leaves the region, theshadowing to a non-shadowing state or vice versa. The gateway stops advertising the IP address block that ischoice of channel modeling and estimation should take into advertised by the neighbor gateway. Finally, the IEEE 802.21account the computational complexity and implementation Media Independent Handover (MIH) standard could be adopted toissues for real time processing. manage handovers between IP-based satellite networks and other mobile networks in an integrated system. F. Physical (PHY) layer issues An important aspect for MSSs is to use an adaptive air III. ISSUES INVOLVED IN QOS SUPPORT:interface with the possible choice among several The main issues involved in QoS provisioning is themodulation and coding techniques to adapt to channelvariations due to user movement;where adaptation to fact that different traffic types have different QoS requirements, which results in different service levels. Pocketsizechannel variations implies the use of a feedback channel to voice traffic is characterized as relatively low bandwidthinform the transmitter about the most suitable physical (typically 8 Kb/s), but requires very low latency delivery tolayer transmission parameters to guarantee a certain quality ensure high-quality audio at the destination. Such traffic forat the receiver. Such adoption is viable only for land- example, is tagged high priority to protect its service (low speed) users and becomes critical for higher Video traffic, on the other hand, generally has higher bandwidthfrequency bandsThe signal blockage effects can cause a (128 to 384 Kb/s or more), but still similarly require low latencydemodulator synchronization loss with a period of for high-quality video images at the destination. Data traffics likeunavailability during the resynchronization process. file transfer, e-mail messages, etc., can generally be allowed to suffer latency through the network without appreciable QoSDifferent solutions may be used to face this non-LoS deterioration. While an e-mail message is typically lowproblem: for example, gap fillers (in the bandwidth, file transfer takes significantly high bandwidth. Thepresence of extended or permanent obstacles), space goal of resource management for QoS is to share properly anddiversity (e.g. using two receiving antennas that are distant efficiently access to the available resources among these differentmore than the length of obstacles), and time diversity (e.g. traffic types with the aim of keeping their required quality. Largeusing a time interleaver for spreading the errors occurring packets delivered from a high-bandwidth delay tolerant dataduring a persistent fading event). service like file transfer, for example, may cause quality- degrading delay to latency intolerant application such as voice. IfG. Medium Access Control (MAC) layer issues a 1500-B packet delivered as part of a file transfer over a 64 Kb/sAccording to [18], many handover scenarios can be link will take 187 ms to be transmitted, voice and video packets inconsidered: in a non-GEO case, user mobility is dominated queue behind this data packet must keep waiting for this time interval. As a result, voice cuts will be heard for the voice traffic,by the satellite constellation mobility; while in a GEO case, while jitter may be observed in the video images. Effectivemobility is present only for users accessing the service resource sharing mechanisms, therefore, play important roles infrom planes, trains, and ships. The resource assignment at QoS provisioning.the MAC layer (layer 2) has to provide adequate prioritiesfor handover management: handed-over traffic typicallysuffers from extra switching delays (and, in some cases, re-routing delays when gateway changes are involved) and,hence, it needs an adequate layer 2 prioritization in 31© 2010 ACEEEDOI: 01.IJCOM.01.03.2
  5. 5. ACEEE Int. J. on Communication, Vol. 01, No. 03, Dec 2010 IV. CONCLUSIONS: Communications: New Services and Systems, Co-located with Globecom ’05, St. Louis, MO, December 2,2005. Currently, there is a renewed R&D interest for [6]. Martinez RM, de Domingo MC, Guerra Expo´sito JA. SATSIX PROJECT: a first approach to IPv6 over SatelliteMSSs due their capabilities to provide services anytime and Networks. Proceedings of the 16th IST Mobile and Wirelessanywhere. This paper surveyed the current mobile satellite Communications Summit, 2007, July 1–5, 2007; 1–4.networks and services with respect to design issues, [7] J. Farserotu and R. Prasad, “A survey of future broadbandphysical layer technologies and challenges, recent multimedia satellite systems, issues and trends,” IEEE Commun.standardization advances (eg. Mobile extension for DVB- Mag.,vol. 38, pp. 128–133, June 2000.S2/RCS,DVB-SH) and some operational systems (eg [8] A. Jamalipour, “Broad-band satellite networks—The global ITGlobalstar, Inmarsat BGAN, Iridium and Thuraya.). The bridge,” Proc. IEEE, vol. 89, pp. 88–104, Jan. 2001.paper discusses the available frequency bands for the [9]. Arcidiacono A, Finocchiaro D, Grazzini S. Broadbandcurrent MSSs, the design characteristics of mobile terminal mobile satellite services: the Ku-band volution.Proceedings of the 2006 Tyrrhenian International Workshop on Digitalantenna and satellite antenna. The paper discussed the Communications(TIWDC’06), Island of Ponza, Italy, Septemberminimum elevation angle requirement for land mobile 5–8, 2006.users and channel models to characterize a mobile satellite [10]. Henri Y. Non-GSO MSS/FSS constellations and thesystem. The last part of the paper discusses the important international regulations, Regional Radio communicationdesign issues for physical layer, medium access control Seminar, Mexico City, Mexico, September 24–28, 2001.layer , network layer and QoS requirements. Satellite [11].DVB, Interaction channel for satellite distribution systems,services in previous-generation systems were limited to low bit- BlueBook A054r4.1, January 2009, available on-line at the URL:rate applications. In the 4G system, the trend is toward information networks offering flexible multimedia [12]. ASMS-Task Force Technical Group, Overview of existing standards and architectures, Internal Report, May 2001, availableinformation services to users on demand, anywhere, on-line at the URL:anytime. Satellite-based mobile systems will be used in this in a complementary mode to the terrestrial system to [13] C. Loo and J. Butterworth, “Land mobile satellite channelmeet user demands better. 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