A Comparison Framework for MSSs - Semantic Scholar

A Comparison Framework for MSSs Paolo Chini, Giovanni Giambene

Sastri Kota

Universit`a degli Studi di Siena Via Roma, 56, 53100 Siena, Italy Email: [email protected]

Harris Corporation-GCSD 1134 East Arques Avenue, Sunnyvale, CA, 94086 Email: [email protected]

Abstract—Satellite networks can provide the possibility to access communication services on wide areas of the earth; this characteristic coupled with the support of user mobility allows satellite networks having unique features well suited for different scenarios, like: land mobile, aeronautical, maritime, transports, rescue and disaster relief. Mobile satellite networks, however, entail crucial technological challenges, such as: link budget, antenna technologies for both the satellite and the mobile terminal, and efficient utilization of the radio spectrum. This paper investigates these aspects and provides an analytical framework that can be used to evaluate the characteristics of satellite networks in the design phase as well as for their operational phase. In particular, quantitative comparisons are presented here for different systems in terms of supported user density, bandwidth efficiency, and mobility aspects.

I. I NTRODUCTION Satellite networks are attractive to provide communication services in areas of the world not 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.) needing mobile communication services via satellite [1]. In such a context, Mobile Satellite Systems (MSSs) have an increasing role since they allow the provision of personal communication services anytime and anywhere. Recent technology advances in terms of multi-spotbeam antenna, low-noise receivers, and processing capabilities have permitted to achieve the direct access to the satellite for small, portable or even handheld terminals by using S, L, and, recently, Ku and Ka bands. Satellites can be also equipped with regenerating payload, processing capabilities, and intersatellite links, thus respectively permitting to switch traffic flows from different beams and traffic routing in the sky. Satellites are on suitable orbits around the earth; on the basis of their altitude, they can be categorized as GEO (Geosynchronous Earth Orbit) and non-GEO. A GEO satellite is on the earth’s equatorial plane at a height of about 35,800 km, a significant distance that entails huge signal propagation delay and attenuation. Typical GEO satellite communications use high frequencies (e.g., S, L and even Ku and Ka bands), thus exacerbating the path loss experienced by the signal. Non-GEO satellites are on orbits of two types: Low Earth Orbit (LEO), at a height between 500 and 2,000 km and Medium Earth Orbit (MEO), at a height between 8,000 and 12,000 km. MEO and LEO systems need several satellites (i.e., a constellation) to cover a region or the whole earth: due to the non-stationarity of these satellites, many satellites

alternate to cover the same region on the earth, so that frequent handover procedures are needed to switch a connection from one satellite beam to another or even from a satellite to another. At present, there is a renewed R&D interest on MSSs with the aim to define new systems and related standards. There are several research projects studying the integration between terrestrial and satellite networks for user mobility support. One interesting example is represented by the Mobile Applications & sErvices based on Satellite & Terrestrial inteRwOrking (MAESTRO) project that investigated MSSs integrated with 3rd generation (3G) and beyond-3G mobile terrestrial networks [2]. The focus of this paper is on MSSs and their capability to provide multimedia services to users. Nowadays, we can consider at least the following 5 standards that are directly related to the development of MSSs: Global System for Mobile Communications (GSM) via satellite; Satellite - Universal Mobile Telecommunications System (S-UMTS); Digital Video Broadcasting - Satellite Version 2 (DVB-S2); DVB - Satellite to Handheld (DVB-SH); Satellite - Digital Multimedia Broadcasting (S-DMB). The current interest is for future IP-based satellite networks that will adopt the Broadband Satellite Multimedia (BSM) standard and reference protocol architecture defined in the ETSI TR 101 985 specification. According to the BSM concept, the protocol stack is divided into two blocks connected by the Satellite Independent-Service Access Point (SI-SAP). Primitives are used to exchange signaling across SI-SAP between these two blocks. Future MSSs are expected to be compliant with the BSM standard and provide accordingly Quality of Service (QoS) support for multimedia traffic. The aim of this paper is to provide an analytical framework for the comparison of MSSs, taking into account different characteristics, supported user density, bandwidth efficiency, and mobility issues. II. MSS- RELATED D ESIGN I SSUES This Section addresses important improvements, special solutions and modifications that are needed in order to design satellite communication systems supporting user mobility. Our study is carried out below by focusing on specific aspects of the physical layer, including regulatory frameworks and technologies. Bandwidth frequencies are assigned at the World Radiocommunication Conferences (WRC) that are periodically organized by ITU-R. While fixed services use high C and K

978-1-4244-2324-8/08/$25.00 © 2008 IEEE. This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings.

frequency bands, mobile services are better suited for lower L and S frequency bands that were assigned at WARC-92 [3]. MSSs have exploited a technology based on L-band infrastructures for a long time: L-band systems permit small on-board antennas and lower signal attenuation due to atmospheric effects. However, the need of broadband services and the limited amount of available L-band resources (2×30 MHz) have pushed towards the use of Ku and Ka bands for MSSs. ITU-R has assigned the 14-14.5 GHz frequency portion (Ku band) for mobile satellite services in the earth-to-space link. Moreover, also a portion of the Ka band can be used for mobile services; in particular, the 29.9-30 GHz band in the earthto-space link and the 20.1-21.3 GHz band in the space-toearth link. At present, Ku-based satellite systems are available to provide broadband services in many mobile environments, such as trains, boats, planes and cars. However, differently from L-band satellite, Ku-band satellites do not provide a good coverage over seas. Hence, a trade-off has to be found between the need of increased bandwidth and coverage issues [4]. The European regulatory framework for the use of L band by MSS is becoming obsolete. For this reason, on February 2007 the European Commission decided a public consultation among the MSS manufacturers and operators for the use of the L band for MSSs (i.e., the band [1980 - 2010] MHz in uplink and band [2170 - 2200] MHz in downlink). An interesting question that such consultation had to address was the bandwidth to allocate to each MSS once auctions will be done. The replies to the public consultation can be seen at link in Reference [5]. The main outcome was the confirmation of the interest to these frequency bands by many MSS operators and the indication of possible number of operators to be selected with auctions for the use of L band. There are some examples in the literature that address interference issues between fixed and mobile satellite services. In [6], the authors describe interference characteristics between a non-GEO MSS and a GEO fixed satellite service system. In [7], the author analyzes non-GEO fixed and mobile satellite service constellations, providing some suggestions for regulations (in terms of maximum transmitted power and elevation angles) to avoid interference between services. The antenna design is a crucial issue for mobile terminals. An important aspect is the antenna cost and the related adopted technology. Moreover, the antenna system should be reliable and efficient in terms of sensitivity, gain and interference. It is important to highlight some differences between fixed and mobile services: mobile terminals use omni-directional antennas, while fixed terminals use directional antennas. Hence, mobile terminals can transmit in all the directions and receive signals from all the directions as well. For this reason, mobile terminals could interfere with other satellite network frequencies (not assigned to mobile services, but only to fixed services). Further considerations on the antenna system design can be done by taking into account the different application environments: for example, the railway scenario is well served by Ku-band satellites (coverage over land masses), but the antenna on trains should be small (low directivity gain), thus

generating higher interference levels for adjacent satellites. In the aeronautical and maritime scenarios, planes and boats could be at the edge of a spot-beam and this requires a suitable link optimization. Finally, big antennas could be used in the case of big boats that therefore can have less system constraints. One of the key technologies in realizing MSSs is the use of a high-performance multi-spot-beam satellite antenna consisting of a large deployable reflector and a feeder system. Typical big-antennas on GEO satellites can reach a diameter up to 25 m, while we can expect a diameter around 2 m for LEO systems. Spot-beams are need in order to focalize the covered area on the earth and to guarantee there a high antenna gain. The adoption of a multi-spot-beam antenna allows increasing the system capacity by re-using the same frequency bands in sufficiently far beams so the mutual interference is negligible. Current MSSs exploit satellite antennas with a high number of beams. Some examples of frequency reuse cluster sizes for MSSs are: 12 beams/cluster for Iridium, 27 beams/cluster for BGAN, and 21 beams/cluster for Thuraya. We can note that some GEO systems, like BGAN and Thuraya, are characterized by a high value of the cluster size due to the complexity in the cell structure illuminated by each antenna: in these GEO systems narrower beams than in non-GEO systems are used to irradiate a given area on the earth; hence, beams are much ‘closer’ each other, thus entailing higher levels of mutual interference and the need for a larger frequency reuse cluster. The adoption of low-directional antennas can cause a higher interference with adjacent satellites in the forward link and gives problems as well for the Effective Isotropic Radiated Power (EIRP) in the return link. In particular, the strict regulations for mobile user return link operating in Ku band may require the adoption of spread spectrum techniques, as in the proposed DVB-S2/-RCS mobile extension, especially for those users with low-profile small antennas (e.g., a minimum diameter of 30 cm for aeronautical and maritime environment). The EIRP value can be reduced by means of spread spectrum techniques, thus preserving the requested Signalto-Noise Ratio (SNR) even if a lower spectral efficiency is achieved. The standardization for the mobile extension of DVB-S2/-RCS has considered Direct Sequence (DS) spreading for the forward link and burst repetition for the return link (maximum spreading factor of 16 with Single Channel Per Carrier, SCPC). Note that for the return link the new DVBRCS+M specification is ready for the DVB-RCS extension to support mobile users. Another important issue for a good quality of the communication is the minimum 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 fixed satellite systems due to the fact that the location and orientation of the user antenna can be optimized (e.g., lineof-sight conditions can be achieved for GEO satellites on the basis of a suitable design of the earth stations locations), in the MSS scenario (land mobile users) we need to avoid a low value of the minimum elevation angle, otherwise we could have


frequent shadowing and blockage events for the signal due to trees, buildings, hills, etc. Such problem is less important for aeronautic and maritime users, where we could not expect blockage events. Recommended minimum elevation angles are around 30o in the case of mobile users on landmasses. The European Space Agency has carried out a measurement campaign at Ku and Ka bands that has permitted to define a channel model for MSSs [8]. In particular, in the land mobile case, the MSS channel at both Ku and Ka bands can be characterized by a 3-state Markov chain model, featuring line-ofsight, shadowing and blockage conditions; for what concerns the Ku (Ka) band, a Rice (Loo) distribution characterizes each state. Multipath, shadowing and blockage effects are also present at lower L frequency bands. However, a typical choice for the L-band channel is to consider a two-state (good-bad) channel model [9]. The parameters of these models depend on the environment (city, suburban area, rural area) as well as the minimum elevation angle. Finally, another important aspect is the choice among several possibilities in terms of modulation and coding techniques to adapt to fast channel variations due to user movement (note that adaptation to channel variations implies the use of a feedback channel to inform the transmitter about the most suitable physical layer transmission parameters to be used to guarantee a certain quality at the receiver). It is important to highlight that signal blockage effects can cause a demodulator synchronization loss with a period of effective unavailability during the resynchronization process. Different potential solutions may be used to face this problem: for example, gap fillers (in the presence of extended or permanent obstacles), spatial diversity (e.g., using two receiving antennas that are distant more than the length of obstacles), and time diversity (e.g., using a time interleaver for spreading the errors during a persistent fading event). III. A NALYTICAL F RAMEWORK FOR MSS C OMPARISONS In this Section, we provide a quantitative comparison among some MSSs in terms of different important efficiency and mobility parameters. All the considered systems use air interfaces of the Frequency Division Duplexing (FDD) type. We introduce below an analytical approach that is based on the following parameters: • B = bandwidth available for the whole MSS in downlink. • N = number of beams simultaneously active in the whole MSS (counting all the satellites and the transponders in them). • K = size of the frequency reuse cluster of the MSS, depending on the antenna technology, air interface type and tolerance to interference of the multiple access system. Hence, the bandwidth available in each beam is equal to B/K. If a 2-D hexagonal-like cellular layout is used, possible values of the reuse cluster K belong to the set {i2 +j 2 +ij}, where i and j are natural numbers, ij = 0. • F = reuse factor of the MSS (i.e., the number of times that a frequency is reused among active beams); K = N/F .

•

•

•

• • • • •

ηphy = efficiency of the modulation and coding scheme 2 (M ) adopted, ηphy = r×log , where r is the code rate, 1+α M is the number of symbols in the modulation and α is the roll-off factor that is equal to 0 for a perfect raisedcosine impulse and 1 for a rectangular impulse (null-tonull band). We have used r = 3/4 and α = 1 for all the compared MSSs. ηenc = efficiency of the encapsulation from layer 3 to layer 2. We have assumed for all the compared MSSs ηenc = 0.96, an efficiency value that is typical of the Multi Protocol Encapsulation (MPE) [10]. ηM AC = MAC layer efficiency. We have assumed for all the compared MSSs, ηM AC = 184/188 ≈ 0.97, considering the ratio between the payload and the packet length according to the MPEG2-TS (Motion Picture Experts Group 2 - Transport Stream) format, typical of the DVBS/DVB-RCS standard. η = total efficiency of lower layers: η = ηphy × ηM AC × ηenc . H = satellite orbit altitude. S = number of subscribers in the whole MSS. D = spot-beam footprint diameter. A = average area irradiated on the earth by a beam.

Let us consider a reference high-profile ‘equivalent’ data user requiring an average capacity of C kbit/s. Then, the number of equivalent users Ueq supported per beam is max equivalent users η×B . (1) Ueq = K ×C beam Let us assume that each real user is active for h hours per day, thus contributing a ‘load’ equal to h/24 Erlangs. Hence, each equivalent user can be mapped to 24/h real users, U , thus modifying the above formula as: η × 24 × B max users . (2) U= h×K ×C beam U in (2) represents the maximum capacity of users per beam that can be supported by the system depending on the different parameters of our modelization. Dividing (2) by A, we obtain the user density σ that can be supported by the MSS: max users η × 24 × B . (3) σ= h×K ×C ×A km2 It is also possible to define another MSS efficiency parameter ηSI−SAP that is related to the efficiency at the SI-SAP level [11]. In particular, this parameter is computed as the bitrate capacity of a beam (= η×B/K) multiplied by the number of active beams in the systems (= N ) and divided by the total one-way bandwidth (= B). This formula has been proposed by some mobile satellite service providers (e.g., EUTELSAT, SES ASTRA, etc.) as a response to a public consultation made by the European Commission in 2007 [5]. Through simple manipulations, ηSI−SAP can be expressed as: bit/s η×N =η×F . (4) ηSI−SAP = K Hz


Then, we have evaluated the system cost of the MSS in US$ (Sysc) normalized to the MSS traffic capacity (i.e., B × ηSI−SAP ); this parameter, denoted as β, represents a measure of the system cost per unit of traffic rate. The lower β is, the more convenient the system is from this standpoint. $ Sysc Sysc . (5) = β= B × ηSI−SAP B × η × F bit/s 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 parameter τ that represents a measure of the cell (= spot-beam) crossing time by a user [12]: D [s] , (6) V where D is the spot-beam footprint diameter and V = Vuser , the mean user terminal speed in the GEO case (for instance the speed of a train, a plane or a ship) or V = Vtrk , the satellite ground-track speed in the case of a LEO or MEO system 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 orbital speed, Vorb , with respect to the satellite orbit radius (= RT + H) as follows: m RT Vtrk = Vorb , (7) RT + H s where Vorb is obtained by equaling the earth gravitational attraction to the centrifuge force (denoting with mT the earth mass, and with γ the gravitational constant): m γmT . (8) Vorb = RT + H s τ=

We consider mT = 5.9742 × 1024 kg and γ = 6.67 × 10−11 m3 /(kg × s2 ). The lower the satellite orbit, the faster it moves, and the smaller the covered area on the earth. Parameter τ evaluated according to the above formula can be used to derive the mean number of beam handovers, nh , occurring during the lifetime of a session with mean duration Tsession . On the basis of [12], we can write: Tsession beam handovers . (9) nh ∝ τ session Therefore, we can consider that the ratio Tsession /τ is a measure of the signaling load that the system has to support to manage a session of a given duration by switching it from one beam to another in order to maintain it uninterrupted along its life. The higher this ratio is, the higher is the system capacity lost to support the signaling for handovers and the lower the capacity available for data traffic. Note that when inter-satellite handover occurs (especially in the LEO case), a rerouting of the communication has to be quickly performed, otherwise the higher layer protocols could experience bad performance or, even, the connection may be dropped.

IV. R ESULTS AND C OMPARISONS On the basis of parameters σ, ηSI−SAP , β, and nh we have been able to compare some reference MSSs by using the data provided in Table I (note that ηM AC , ηenc , C, and h/24 have been considered as common values for all the MSSs, for a fair comparison). In particular, we consider currently-operational GEO (i.e., BGAN and Thuraya) and LEO (i.e., Iridium) MSSs. The comparison among the MSSs in terms of user density σ is provided in Figure 1, according to equation (3). In this case, we can note that the GEO-based systems have a σ value greater than that of the LEO-based system. In Figure 2, the ηSI−SAP efficiency values are shown according to equation (4). These results show that LEO systems are characterized by a higher efficiency than GEO-based ones; this is due to the fact that in the Iridium case we have a smaller cluster size (with respect to GEO-based systems) and there is also a higher number of beams; therefore, Iridium achieves a higher degree of resource reuse. On the basis of these results, we can argue that the antenna technology on GEO satellites plays a crucial role in order to improve the MSS efficiency; in particular, it is important that a high insulation is achieved among adjacent beams to reduce the interference and to allow the use of lower size clusters. It is interesting to note that if we evaluate the ηSI−SAP value for an advanced Fixed Satellite System (FSS), like the HotBird 6 satellite referring to its payload of 4 Ka-band Skyplex DVB-RCS transponders (each of them with a bandwidth of 33 MHz), we obtain ηSI−SAP = 2.8 bit/s/Hz. This is a much lower efficiency than that of MSSs and proves the significant difference in the design between MSSs and FSSs; in particular, FSSs use very large transponder bandwidths in Ku or Ka bands, but have a very low number of beams corresponding to the number of transponders. Figure 3 compares our reference MSSs in terms of parameter β that, according to (5), represents the cost per unit of traffic rate. We can note that in this case, the Thuraya-like GEO system achieves the best β value meaning that this system could be able to support a given traffic with lower costs than the other envisaged MSSs. On the basis of Tsession /τ and for a given Tsession = 600 s we can compare our reference MSSs as shown in Figure 4 by assuming in the GEO case the scenario of users on a plane (V = 1000 km/h) that is the worst-case for GEO mobility conditions. These results clearly show that the GEO cases are characterized by a much lower mobility and lower related signaling load. As a conclusion, we can state that BGAN and Thuraya achieve interesting efficiency results; in addition to this, BGAN has the advantage of supporting broadband connections to users. V. C ONCLUSION In this paper we have presented MSSs and their specific characteristics that are very unique with respect to other satellite communication systems. Then, we have introduced an analytical framework that permits to compare MSSs in terms of a wide range of parameters, such as user density, efficiency, and user mobility. We believe that this comparison framework


TABLE I T RAFFIC ENGINEERING PARAMETERS AND OTHER DATA FOR OUR MSS S OF REFERENCE . Parameter B [MHz/downlink/system] N [# beams/system] K [# beams/cluster] ηP HY [bit/s/Hz] ηM AC ηenc η [bit/s/Hz] H [km] A [km2 ] D [km] S [# subscr./system] C [bit/s/user] h/24 [Erl/user] Number of beams/satellite System cost [US$ billions]

GEO BGAN − like 34 ∼456 ∼27 1.3125 0.97 0.96 1.22 35800 2.76 × 105 ∼592 19000 9.6 k 3/24 228 1.5

GEO Thuraya − like 34 ∼600 ∼21 0.75 0.97 0.96 0.70 35800 1.97 × 105 ∼500 250000 9.6 k 3/24 300 0.96

LEO Iridium − like 16.5 ∼2150 ∼12 0.75 0.97 0.96 0.70 780 3.16 × 105 ∼630 203000 9.6 k 3/24 48 7

ACKNOWLEDGMENT This work has been partly carried out within the framework of the SatNEx II network of excellence (contract No. IST027393), www.satnex.org. R EFERENCES Fig. 1.

Fig. 2.

Efficiency ηSI−SAP comparison for the considered MSSs.

Fig. 3.

Fig. 4.

User density comparison for the reference MSSs.

Comparison of β parameter for the considered MSSs.

Comparison of Tsession /τ parameter for the considered MSSs.

can provide a good tool to drive system designers towards optimized choices for a better diffusion of these services in the population.

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