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.

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.
gsonika@gmail.com
2
Depatment of Electronics & Communication Engineering,
ICFAI University, Dehradun (U.K),India.
rramola@gmail.com
Abstract :The central features of the future fourth-generation the presence of obstacles or return link budget restrictions
mobile communication systems are the provisioning of high- caused by the low power and small antenna size available
speed 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 can
the network must satisfy some stringent quality-of-service be adopted: (i) hybrid networks and (ii) integrated
(QoS) metrics, defined typically in terms of maximum delay
and/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 in
cellular systems, playing important roles as back-bones or non-LoS conditions. Moreover, the return link can be
access satellites, to provide ubiquitous global coverage to supplied by a terrestrial cellular system to simplify the
diverse users. The challenges for future broadband satellite power management of mobile terminals. Finally, the
systems, therefore, lie in the proper deployments of state-of- satellite coverage can be extended (e.g. indoor or urban
the-art satellite technologies to ensure seamless integration of cases) by means of a local wireless system where the base
the satellite networks into the cellular systems and its QoS station ‘converts’ the satellite signal to the wireless one and
frameworks, while achieving, to the extent possible, efficient vice versa. For what concerns the integrated networks, a
use of the precious satellite link resources. This paper presents
an overview of the future high-speed satellite mobile
terrestrial cellular network can be used as an alternative
communication systems, the technologies deployed or planned system to connect the mobile user (both forward and return
for deployments, and the challenges. links) with respect to the satellite one. Some examples of
integrated networks are analyzed in [2], referring to the
Index 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 concepts
communication services in areas of the world not well
are quite interchangeable, even if CGC is more related to
served by existing terrestrial infrastructures. There is a vast
hybrid networks and ATC to integrated networks. In any
range of sectors (e.g. land-mobile, aeronautical, maritime,
case, terrestrial systems could be based on3rd generation
transports, rescue and disaster relief, military, etc.) needing
(3G), Wireless Fidelity (WiFi, IEEE 802.11
mobile communication services and where the satellite is
a/b/g), or Worldwide Interoperability for Wireless
the only viable option [1]. This is the reason why at present
Microwave Access (WiMAX) technologies.
there is a renewed interest and market opportunities for
Mobile Satellite Systems (MSSs). Technologies for multi- Current Mobile Satellite Systems:
spot-beam antennas, low-noise receivers, and on board
processing have permitted to achieve the direct access to The following MSS projects deal with the challenges
the satellite for small, portable or even handheld terminals and the efforts for providing broadband multimedia
by using S, L, and recently Ku and Ka bands. Satellites can services to users in land vehicular, aeronautical, and
also be equipped with a regenerating payload and inter- maritime environments:
satellite links, thus respectively permitting to switch traffic MObile Wideband Global Link sYstem
flows from different beams of a satellite and traffic (MOWGLY) [3];
forwarding/routing in the sky through satellites. Satellites Mobile Broadband Interactive Satellite
are on suitable orbits around the earth; on the basis of their multimedia Access Technology(MoBISAT) by
altitude, they can be categorized into Geosynchronous ETRI [4]; and
Earth Orbit (GEO) and non-GEO. MSSs may suffer from Broadband Global Area Network—eXtension
non-Line-of-Sight (non-LoS) propagation conditions due to (BGAN-X) by the European Space Agency, ESA
[5].Moreover, the Satellite-based communication
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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 ATM
with sophisticated onboard processing (OBP), onboard
switching (OBS), and intersatellite links (ISLs), while
others employ simple bent-pipe transponder relays. The
system design choices depend on factors including
coverage, cost, user service, and traffic demands. II. DESIGN ISSUES FOR MSSS NETWORKS.
Constellations may be LEO, MEO, geostationary earth
orbit (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), periodically
numbers of satellites in the nongeostationary constellations, organized by the International Telecommunication Union
such as MEO and LEO. Though the coverage of GEO radio-communication sector (ITU-R). While fixed
satellites is a primary advantage over the LEO systems, the services use high C and K frequency bands, mobile
longer delay of GEO however makes them typically less services are better suited for lower L and S frequency
suitable for mainstream 4G applications such as interactive bands that were assigned at the World Administrative
multimedia 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 permit
order of 10 ms. In MEO the delay is on the order of 80 ms, small on-board antennas due to lower signal attenuation
and in GEO orbits it is 250–270 ms. Other delays due to and reduced impact of atmospheric effects. However, the
processing and transmissions are on the order of 80–100 need of broadband services and the limited amount of
ms for regional traffics and 140–180 ms for international available L/S-band resources (2-30 MHz) have pushed
ones. When all delays are considered GEO satellite-based toward the use of Ku and Ka bands for MSSs. ITU-R has
communications may be marginal for quality due to time assigned Ka band frequency portions to MSSs and Fixed
delays. However, LEO and MEO orbits have their peculiar Satellite Systems (FSSs) on a primary basis in all regions
problems. Due to the low altitude, LEO and MEO satellites (29.9–30 GHz for earth-to-space link and 20.1–21.3 GHz
move at rapid speeds, causing frequent handovers between for space-to-earth link) and Ku band frequency portions to
the ground terminal and the satellites, which are in view for MSS on a secondary basis in all regions (14–14.5 GHz for
a 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 provide
subjected to Doppler shifts and small-scale multipath broadband services in many mobile environments, such as
fading. 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 good
main challenge here lies in the proper handling of ISLs so coverage over seas, because antenna spot-beams footprints
that they do not lead to problems with delay jitter, which are focused on landmasses [9]. In fact, Ku-band satellites
can degrade voice and video QoS performances over the are mainly intended for fixed users, so that there are not
satellite systems. Buffering is a good solution known to enough Ku/Ka band satellites providing coverage over
work well for jitter problems and must be employed. oceans. Hence, a trade-off has to be achieved between the
Several satellite mobile systems have been deployed, or are need of increased bandwidth and coverage issues.
in the process of being deployed, employing specific
constellations or mixture of constellations carefully
selected to achieve as much as possible, combinations of
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3. ACEEE Int. J. on Communication, Vol. 01, No. 03, Dec 2010
B. 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 divided
terminals. An important aspect is the antenna size, the cost, into some carriers that are distributed among beams in
and the adopted technology. Moreover, the antenna system order to avoid interferences among adjacent beams; carriers
should be reliable and efficient in terms of sensitivity, gain, can be reused in sufficiently far beams. A cluster is a set of
and interference. It is important to highlight some beams where all the system carriers are used. Some
differences between fixed and mobile services: fixed examples of (average) cluster sizes (i.e. number of beams
terminals use directional antennas, while mobile terminals per cluster) for MSSs are]:12 beams/cluster for Iridium,27
can also use omni-directional antennas (where ,phased- beams/cluster for BGAN, and 21 beams/cluster for
array 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: in
order to improve the link budget). Typically, mobile GEO systems ‘narrower’ (i.e. higher directivity) beams
terminals can transmit in all the directions and receive than in non-GEO ones are needed to irradiate the same area
signals from all the directions as well. For this reason, on the earth. Hence, beams are much ‘closer’ each other in
mobile terminals could interfere with other satellite antennas on GEO satellites, thus entailing higher levels of
networks. In [17],the study analyzes non-GEO fixed and mutual interference and the need for a larger frequency
mobile satellite service constellations, providing some reuse cluster. Here, ‘closer beams’ means a greater density
suggestions for regulations (in terms of maximum of satellite antenna beams per unit of solid angle related to
transmitted power and elevation angles) to avoid the satellite; such density is much higher in GEO cases than
interference among them. Further considerations on in non-GEO ones. On the contrary, the spot-beam
terminal antenna design can be done by taking into account footprints (i.e. cells) irradiated on the earth by non-GEO
the different application environments: for example, the satellites are smaller and closer each other than in the case
railway 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 the
higher interference levels for adjacent satellites. In communication is the minimum elevation angle according
aeronautical 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 stringent
a suitable antenna design. However, big antennas could be for FSSs due to the fact that the location and orientation of
used in the case of big boats that have lower design the user antenna can be optimized (e.g. LoS conditions can
constraints. The antenna size on the mobile terminal be achieved for GEO satellites by selecting appropriate
determines the characteristics of interference for both earth station locations), in the MSS scenario (in particular
uplink and downlink transmissions. Moreover, there are for land-mobile users) a low value of the minimum
off-axis power flow limitations for uplink transmissions in elevation angle should be avoided , otherwise frequent
Ku band (there are only secondary allocations for MSSs). shadowing and blockage events due to trees, buildings and
This entails constraints on the Effective Isotropic Radiated hills may occur. Increasing the elevation angle, the signal
Power (EIRP) for the mobile user. In order to mitigate quality improves (reduction of shadowing/blockage
interference, spread-spectrum schemes can be used. Several effects), but also system costs increase (higher number of
spread-spectrum techniques can be adopted (e.g. Direct satellites in the constellation). The minimum elevation
Sequence, DS, Frequency Hopping, FH, and burst angle requirement entails suitable design constraints for the
repetition). The standardization for the mobile extension of number of satellites in a constellation and also entails that
Digital 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 mobile
Carrier, SCPC) [11]. satellite channel are needed. Extensive research works
C. 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 the
consisting of a large deployable reflector and a feeder
most accepted statistical models proposed in the scientific
system. At present, typical big-antennas on GEO satellites
literature, considering large-scale and small-fading, single
can reach a diameter up to 25 m, a diameter around 2 m
and multiple-state structures, narrowband and wideband
can be expected for LEO systems. Spot-beams are needed
channels, and first and second-order statistics. Building
in order to focus the covered area on the earth with a high
upon a thorough characterization of propagation effects, the
antenna gain. Current MSSs exploit satellite antennas with
authors focus on performance analysis of coded and
a high number of beams: hundreds of beams for GEO
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uncoded systems based on closed-form expressions, upper receiving resources in the destination cell, otherwise the
bounds, and numerical simulations. related session could be terminated by higher layers.
Channel Modeling Challenges: H. Network layer issues
One fundamental characteristic of future satellite mobile With reference to satellites with on-board IP routing
communication systems is the necessity to be fully capabilities,Mobile IP(MIP) developed by IETF could be
integrated into the other terrestrial networks in order to used to support handover procedures. Unfortunately, MIP
enable global, seamless, and ubiquitous communications. has the problem of high handover latency. NASA and
With emerging non-geostationary LEO and MEO satellite CISCO have carried out many projects to improve MIP for
systems and high data rate applications, accurate and handover procedures in IP-based satellite networks [18]. As
flexible channel models are needed in order to allow an alternative to the above MIP approach, Connexion by
realistic QoS predictions and perform system comparisons Boeing (an in-flight GEO-based Internet connectivity
under different multiple-access, modulation, coding and service, not anymore active since 2006) allowed global IP
diversity schemes. For future satellite mobile systems, a mobility using the Border Gateway Protocol (BGP) [19]. In
suitable channel model should satisfy the following particular, a Class C IP address block is assigned to a
characteristics: the model should be based on accurate mobile platform (i.e. a plane or a ship, having on-board a data
estimation 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 the
model should combine very well the effects of weather
nearest terrestrial gateway, for the period the plane/ship passes
attenuation process and the multipath fading and through the region where the gateway is located (four gateways
shadowing 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, the
shadowing to a non-shadowing state or vice versa. The gateway stops advertising the IP address block that is
choice of channel modeling and estimation should take into advertised by the neighbor gateway. Finally, the IEEE 802.21
account the computational complexity and implementation Media Independent Handover (MIH) standard could be adopted to
issues 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 the
modulation and coding techniques to adapt to channel
variations due to user movement;where adaptation to fact that different traffic types have different QoS
requirements, which results in different service levels. Pocketsize
channel variations implies the use of a feedback channel to voice traffic is characterized as relatively low bandwidth
inform the transmitter about the most suitable physical (typically 8 Kb/s), but requires very low latency delivery to
layer transmission parameters to guarantee a certain quality ensure high-quality audio at the destination. Such traffic for
at the receiver. Such adoption is viable only for land- example, is tagged high priority to protect its service quality.
mobile (low speed) users and becomes critical for higher Video traffic, on the other hand, generally has higher bandwidth
frequency bandsThe signal blockage effects can cause a (128 to 384 Kb/s or more), but still similarly require low latency
demodulator synchronization loss with a period of for high-quality video images at the destination. Data traffics like
unavailability during the resynchronization process. file transfer, e-mail messages, etc., can generally be allowed to
suffer latency through the network without appreciable QoS
Different solutions may be used to face this non-LoS
deterioration. While an e-mail message is typically low
problem: for example, gap fillers (in the bandwidth, file transfer takes significantly high bandwidth. The
presence of extended or permanent obstacles), space goal of resource management for QoS is to share properly and
diversity (e.g. using two receiving antennas that are distant efficiently access to the available resources among these different
more than the length of obstacles), and time diversity (e.g. traffic types with the aim of keeping their required quality. Large
using a time interleaver for spreading the errors occurring packets delivered from a high-bandwidth delay tolerant data
during a persistent fading event). service like file transfer, for example, may cause quality-
degrading delay to latency intolerant application such as voice. If
G. Medium Access Control (MAC) layer issues a 1500-B packet delivered as part of a file transfer over a 64 Kb/s
According to [18], many handover scenarios can be link will take 187 ms to be transmitted, voice and video packets in
considered: 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. Effective
mobility is present only for users accessing the service resource sharing mechanisms, therefore, play important roles in
from planes, trains, and ships. The resource assignment at QoS provisioning.
the MAC layer (layer 2) has to provide adequate priorities
for handover management: handed-over traffic typically
suffers 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
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5. ACEEE Int. J. on Communication, Vol. 01, No. 03, Dec 2010
IV. CONCLUSIONS: Communications: New Services and Systems, Co-located with
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