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Cooperative satellite to land mobile gap-filler-less interactive system architecture

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  • Cohere Technologies

Abstract and Figures

Considerable efforts are being made in the area of interactive satellite broadcast services to mobiles, which range from the development of specific communication standards to the realization of practical proof-of-concept prototypes. The main reasons for such a big interest lie in the fact that interactive satellite services are a large potential market with many possible applications. Frequent and long-lasting shadowing caused by surrounding buildings in urban and suburban scenarios greatly limits system availability. The common approach to solve the problem consists in deploying fixed gap fillers, which imply a great consumption of economic and bandwidth resources. In the present paper we propose a novel hybrid satellite-terrestrial system architecture in which cooperative communication techniques are extensively adopted in order to reduce the number of gap-fillers needed to get a target system availability. Nodes are meant to have both networking and satellite communication capabilities so that each of them can possibly act as a Mobile Gap-Filler (MGF).
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Cooperative Satellite To Land Mobile
Gap-Filler-Less Interactive System Architecture
G.Coccoand C.Ibars
Centre Tecnol`
ogic de Telecomunicacions de Catalunya – CTTC
Parc Mediterrani de la Tecnologia
Av. Carl Friedrich Gauss 7 08860
Castelldefels – Spain
giuseppe.cocco@cttc.es christian.ibars@cttc.cat
O. del Rio Herrero
European Space Agency / ESTEC
Noordwijk – The Netherlands
Oscar.del.Rio.Herrero@esa.int
Abstract—Considerable efforts are being made in the area of
interactive satellite broadcast services to mobiles, which range
from the development of specific communication standards to
the realization of practical proof-of-concept prototypes. The main
reasons for such a big interest lie in the fact that interactive
satellite services are a large potential market with many possible
applications. Frequent and long-lasting shadowing caused by
surrounding buildings in urban and suburban scenarios greatly
limits system availability. The common approach to solve the
problem consists in deploying fixed gap fillers, which imply a
great consumption of economic and bandwidth resources. In the
present paper we propose a novel hybrid satellite-terrestrial sys-
tem architecture in which cooperative communication techniques
are extensively adopted in order to reduce the number of gap-
fillers needed to get a target system availability. Nodes are meant
to have both networking and satellite communication capabilities
so that each of them can possibly act as a Mobile Gap-Filler
(MGF).
I. INTRODUCTION
In the last decade specific standards have been developed
with the aim of enabling interactive satellite communications
to mobile users such as the ETSI DVB-SH [1] and the S-
UMTS [2]. Several private companies working in the satellite
sector have developed their own standards for satellite to land
mobile (SLM), maritime and aeronautical communications
which are now implemented in real systems [3] [4]. This
effort is justified by the fact that satellite broadcast and
relaying capabilities give rise to the possibility of creating
mobile interactive broadcast systems over wide geographi-
cal areas, which opens large market possibilities for both
handheld and vehicular user terminals. Mobile broadcasting
is of fundamental importance for services such as digital
TV or machine-to-machine communication related to road
safety and traffic congestion control. As a matter of fact the
support of a satellite in information diffusion about congestion
and road pavement conditions (e.g. in case of ice or rain
over vast geographical areas) largely overcomes the simple
vehicle-to-vehicle communication capabilities. Other potential
applications of mobile interactive satellite communications
G. Cocco is partially supported by the European Space Agency under the
Networking/Partnering Initiative.
range from disaster recovery to the updating of road maps
and tourist information.
Implementation of such services requires good coverage in
satellite broadcast transmission to mobiles, as well as a low
data speed return link for satellite access. However severe
availability problems are endemic in urban and suburban
environments, where most of the users are located. As a
matter of fact in SLM scenario, as described in DVB-SH
standard [1], only users with good enough channel condition
(i.e. Line Of Sight propagation from satellite to user) are
able to access interactive services broadcasted by the satellite
platform. Bad channel conditions frequently occur due to
the shadowing effect of surrounding environment especially
in case of low satellite elevation angles or when the user
is indoors. Whenever a line of sight between terminal and
satellite does not exist, terrestrial gap fillers are employed to
provide the missing coverage. The gap filler solution has two
main shortcomings: i)it is a fixed solution which is not able
to react quickly to changes in the propagation environment,
which may create new dead spots; ii)it is very costly in terms
of investment, management, and bandwidth usage.
A hybrid satellite-terrestrial networking approach has sev-
eral advantages with respect to the fixed gap filler solution.
Previous works showed how the intrinsic flexibility of hy-
brid satellite-terrestrial network architectures can be extremely
helpful when dealing with random environments in disaster
recovery [5].
In this paper we propose a hybrid solution based on a co-
operative ad-hoc networking approach, which is largely more
flexible than the fixed gap-fillers solution. In the proposed
system land mobile users have both satellite communication
and cooperative-networking capabilities. Cooperation among
nodes can significantly improve satellite coverage especially in
those areas which mostly are affected by availability problems
such as urban and suburban areas. Actually urban and subur-
ban scenarios are characterized by high node density, which
increases the efficiency of cooperative techniques. An ad-hoc
cooperative approach determines a reduction of the number of
fixed gap fillers and, if the target system outage probability
is not too restrictive, can lead to a terrestrial gap-filler-less
scheme, with a large reduction in the cost of satellite systems.
2010 5th Advanced Satellite Multimedia Systems Conference and the 11th Signal Processing for Space Communications Workshop
978-1-4244-6833-1/10/$26.00 ©2010 IEEE 309
We consider two types of users: handheld and vehicular. The
IEEE 802.11 family of standards is adopted for terrestrial com-
munications, while the ETSI DVB-SH and S-UMTS standards
are considered for forward (satellite broadcasting) and return
(user terminal to satellite) links respectively.
Cooperative communication techniques applied at different
layers of the open system interconnection (OSI) structure
have been chosen taking into account their compatibility
with existing commercial standards for terrestrial mobile
and satellite communication. Through the application of
cooperative techniques at non-PHY layers (layers 2,3) we
designed a system where there is almost no need for PHY
modifications to IEEE 802.11 standards. Compatibility
with the 802.11 air interface is a key feature for speeding
up an eventual practical implementation of the proposed
architecture. As a matter of facts modifications at non-PHY
level in general only imply firmware/software changes
without the need for hardware redesign of the communication
platform.
This paper is organized as follows: in Section II an overview
of the system architecture is given. In Section III we give a
description of the different possible communication scenarios
and channel models. In section IV cooperative techniques for
land mobile-to-land mobile communication are described, with
a particular emphasis to vehicle to vehicle communications. In
Section V we analyze uplink and downlink communications
and finally in Section VI we draw conclusion of the present
work and give indications on related future work.
II. SYSTEM ARCHITECTURE
The system architecture for the satellite to land mobile gap-
filler-less interactive system we propose is depicted in Fig.1.
Sband(DVB-SH)
MGF
Shadowed nodes
Direct-to-mobile
DVB-SH GEO satellite
Internet /
Broadcast
Distribution
Network
Sband (S-UMTS)
Terrestrial communications (802.11)
Handheld terminal
Gateway station
Vehicular terminal (MC)
Fig. 1. Architecture for a cooperative satellite interactive broadcasting system
without gap-fillers. Nodes with LOS help enhancing coverage in urban and
suburban areas taking the role of mobile gap-fillers (downlink) and mobile
collectors (uplink). End users can access the Internet or broadcast contents
(such as DTV) through the satellite.
It consists of:
A space segment which includes
A transparent multibeam GEO satellite. DVB-SH
standard is adopted for satellite broadcasting in the
2170 2200 MHz band.
A gateway station (GS) that provides access to
the Internet through the satellite and feeds satellite
broadcast transmission.
A ground user segment which includes terminals with
different capabilities [1]:
Handheld user terminals (HT). For simplicity we call
HT all non-vehicular terminals referred to in the S-
UMTS standard (i.e. Handheld, Palmtop-sized and
Laptop-sized terminals [2]). These are characterized
by:
Limited antenna diversity (order more than 2 is
very challenging) and low antenna gain (can be
less than -3 dBi). Linear antenna polarization not
optimized for satellite reception.
Small battery, which requires an efficient power
management.
Difficult to embed telecom modems like GSM or
3G inside terminal without reducing the satellite
receiver sensitivity. Moreover RF filtering, an-
tenna design rules and compactness constraints
have an impact on the achievable receiver sensi-
tivity and immunity to high level blockers coming
from the terminal.
Memory limitation may, in some architectures,
not allow the support of a large Physical Layer
interleaver.
Vehicular terminals (VT) characterized by:
Adequate power supply able to support more
complex receiver processing.
Better antenna diversity (order 2 or more) imple-
mentation (form factor and antenna spacing). One
antenna can be optimized for satellite reception
(directivity and matching polarization). Low Noise
Amplifiers (LNAs) can be integrated with the
antenna(s) to reduce sensitivity loss.
Larger memory can be embedded in the receiver
so that longer Physical Layer interleaving can be
supported. The higher speed of the terminal allows
a better exploitation of time diversity (either at
Physical Layer or at Upper Layers), at equal
memory resource.
All user terminals have both ad-hoc networking and
satellite communication capabilities and each of them can
take the role of Mobile Gap-Filler (MGF). We propose
to use IEEE 802.11 family of standards for inter user
communications, while users access to satellite using S-
UMTS standard in the 1980 2020 MHz band. Table
I summarizes user terminals RF front-end and antenna
characteristics for satellite communications.
310
TABLE I
RF FRONT-END AND ANTENNA CHARACTERISTICS FOR DIFFERENT KINDS OF S-UMTS END US ER T ERM INA LS [2].
Handheld Palmtop-sized Laptop-sized Vehicular terminal
terminal terminal terminal terminal
Average transmit power 0.6 W 1 W 1 W 2 W
Max burst power 2 W 4 W 4 W 4 W
El.: cardioidal 2 elements: 3 dB 40 degrees 3 dB 40 degrees El.: covering 5-85 degrees
Antenna diagram Az.: Omnidirectional 1 element: 3dB 80 degrees 3 dB 40 degrees Az.: Omnidirectional
Antenna gain Peak: 0.5 dBi 7 dBi 10 dBi 4 dBi
(including feeder loss) Average -1 dBi
Receiver G/T -25.5 db/K -18.5 dB/K -15.5 dB/K -19 dB/K
Let us consider a typical urban scenario where a great
percentage of users are likely to be. The need for target
availability and QoS must face the severe restrictions imposed
by the harsh communication environment. Both SLM and
mobile-to-mobile (M2M) communications suffer from such
condition. The typical approach adopted in order to overcome
shadowing in the satellite link consists in the deployment
of ground fixed repeaters called gap-fillers. Fixed gap fillers
represent a quite rigid and resource-consuming solution in
terms of both bandwidth and economical resources.
We propose a system in which nodes with LOS take the role
of mobile gap fillers. In the forward link they collect data from
satellite and distribute them to the other users. In the return
link they act as mobile collectors (MC) (the equivalent of gap
fillers for the uplink), gathering messages from other users
and relaying them to satellite. Due to cost and technological
limitations, mobile nodes (both vehicular or handheld) are
meant to have lower transmission capabilities than fixed col-
lectors. This translates in a lower uplink capacity with respect
to fixed collectors. In case the number of shadowed users is
high compared to those with LOS, a limited uplink capacity
(64 Kbit/s return link according to S-UMTS specifications
[2]) can determine large delays in the access at the satellite
transponder, thus determining a bottleneck in the system.
To increase uplink capacity, two or more nodes with LOS,
possibly close one to each another, cooperate together creating
a distributed antenna array. Nodes acting as mobile gap fillers
change dynamically as network topology and satellite visibility
frequently change, particularly in a vehicular scenario with
high mobility. Data distribution (forward link) and collection
(return link) by mobile gap fillers is duty of the ground user
segment. Ad-hoc networking among users is adopted for such
purpose because of the several advantages it provides with
respect to the fixed gap fillers solution:
It is a scalable solution that can quickly adapt to nodes
joining or leaving the network.
It saves a lot of complexity and resources (in terms of
association time and authentication) needed in case of
a centralized or multi-centralized structure (e.g. basic
service sets approach adopted in common WiFi systems).
High node density in urban and suburban settings makes
cooperative communications more effective.
New standards are currently being developed for vehic-
ular ad-hoc networking physical and lower MAC levels
(IEEE 802.11p [6]).
In the proposed setup the directives of IEEE 802.11 family
of standards are considered for terrestrial communications. In
particular the IEEE 802.11p standard is adopted for vehicle-
to-vehicle communications. As for handheld terminals, we
also assume that the IEEE 802.11g (already available for
handheld terminals [7]) standard is adopted. Vehicular users
are meant to have multiple radio interfaces supporting different
standards (IEEE 802.11p/g) in order to enable vehicle-to-
handheld communications (e.g. communication between a HT
inside a vehicle and the vehicle itself).
Information diffusion in the network relies on Network
Coding (NC), a high level cooperative technique that have
been extensively studied during last years. Several works have
shown how NC can allow to handle in a distributed way
problems which typically occur in ad-hoc scenarios such as
packet losses, routing, delay and load balancing [8]. A key
feature that makes network coding suitable for the proposed
system architecture is the possibility to implement it at the
MAC layer or higher layers. This makes it compatible with
PHY standards such as those of the IEEE 802.11 family which
have a high market penetration.
III. CHA NN EL MO DE LS
From a signal propagation point of view the communication
environments are usually classified in three scenarios: urban,
suburban and rural. In both LMS and M2M communica-
tions, urban and suburban scenarios are the most challenging
ones due to signal blocking/scattering caused by surrounding
buildings. In the following, propagation channel models for
terrestrial and satellite communications in urban and suburban
areas are described.
A. Terrestrial Channel
Urban and suburban environments are challenging and
aggressive scenarios from a communication point of view.
Propagation is even worse if communicating nodes have high
mobility, which is the case in Vehicular Ad-Hoc Networks
(VANETS), leading to high doppler spread. Terrestrial links
are deeply affected by fading, severe fading, shadowing and
frequency selectivity [9]. All these factors contribute to in-
crease the error probability of land mobile channel.
311
B. Satellite Channel
Satellite link suffers from sudden and relatively long shut
downs due to the shadowing effect of tall buildings or overhead
roads in urban and suburban scenarios. Link unavailability is
exacerbated by low satellite elevation angles and peculiar ur-
ban topologies. Satellite link availability is generally modeled
as a two (Lutz model [1]) or more (Fontan model [1]) states
Markovian process. In the two-state model one represents the
case in which LOS is present while the other state models the
case in which satellite link is not available at all. This means
that, at a given time, some nodes will experience direct LOS to
satellite, while others will suffer from satellite link shut down.
The satellite channel can be modeled as an erasure channel.
The erasure probability of the channel depends on the channel
state transition of the correspondent Markov model which, on
its turn, is determined by the satellite elevation angle and the
urban topology (building height and layout).
IV. TERRESTRIAL COMMUNICATIONS
If on one hand urban and suburban characteristics tremen-
dously limit system efficiency, on the other hand they implic-
itly suggest the way to mitigate communication problems. As
a matter of fact the broadcast nature of the wireless channel
in land mobile-to-mobile communication in dense scenarios
provides rich diversity that can be exploited to enhance system
reliability using Cooperative Communication techniques [10].
Information propagation across the network is based on
network coding. Network coding consists in storing overheard
packets, linearly combining them and transmitting the linear
combination together with the combination coefficients. As the
combination is linear and the coefficients are known, a node
can decode all packets iff it receives a sufficient number of
linearly independent combinations of the same packets [11].
Theoretical results have shown that a class of low complex-
ity NC called random linear network coding [12] [13] can
achieve the packet-level capacity for both single unicast and
single multicast connections in wireless networks. A practical
implementation of network coding was proposed in [11] and
testbed implementations have been realized such as CodeCast,
suited for multimedia applications with low-loss and low
latency constraint [14] and COPE [15]. The above mentioned
works show how NC can increase network throughput and
limit the transmission delay in lossy ad-hoc networks while
simultaneously overcoming routing problems in rapidly chang-
ing topologies. These are suitable characteristic for the high
mobility ad-hoc networking scheme we propose, where both
the nodes’ positions and the propagation conditions rapidly
change. Actually high speed (e.g. vehicles or HT traveling on
vehicles) greatly complicates the formation of communication
clusters, basic elements in the typical approach for ad-hoc
networks.
New standards are currently being developed for vehicular
ad-hoc networking PHY and lower MAC (IEEE 802.11p
[6]) as well as higher MAC and higher OSI layers (IEEE
1609.2/.3/.4), which all together are generally referred to as
IEEE 802.11p WAVE (Wireless Access in Vehicular Envi-
ronment) standard. In the proposed system architecture land
mobile nodes communicate through IEEE 802.11p standard.
The MAC process is handled in the so called ”WAVE mode”,
a key amendment introduced by the IEEE 802.11p which al-
lows immediate communication between two vehicles without
any additional overhead needed for typical BSS association.
Nodes’ radios are set in promiscuous mode, which allows
broadcast message transmission as well as the possibility to
overhear and store packets transmitted by neighbor nodes.
V. SATE LL IT E COMMUNICATIONS
In order to enhance satellite coverage, in the proposed
system nodes with LOS act as mobile gap fillers relaying data
to other nodes. Taking into account the lower transmission
capabilities of mobile nodes with respect to fixed gap fillers,
two different techniques are adopted for uplink and downlink.
Forward link (FL) and return link (RL) are allocated two
distinct spectrum portions in the unlicensed S band.
A. Forward Link
In order to better understand the proposed cooperative
terrestrial networking scheme, let us consider a terminal i
with LOS receiving broadcast satellite messages from time
t0to time t1. After t1node iis shadowed and can no longer
receive messages transmitted by satellite. Node ibroadcasts
packets to neighbor nodes acting as a MGF. Cooperative
downlink scheme is depicted in Fig.2. Network coding is
Sband (DVB-SH)
MGF
Shadowed nodes
GEO satellite
Fig. 2. Users with LOS act as Mobile Gap Fillers receiving data from
satellite. Shadowed users receive network-coded data data from user with
LOS
used to compensate for packet losses in the wireless channel.
This means that it broadcasts linear combinations of packets
received in (t0, t1). After time t1(i.e. after downlink connec-
tion is lost) node icontinues transmitting linear combinations
of previously received packets until time t2> t1. These
retransmissions are received by shadowed users and help
them decode the satellite signal. The amount of redundancy
312
TABLE II
TYPICAL AND MAXIMUM BITRATES (MBP S)FO R A MU LTIB EAM
SATE LLI TE D OWN LIN K WI THO UT G AP FIL LER S IN DVB-SH STA NDAR D
[1].
Waveform SH-A SH-B
Typ Max Typ Max
Multibeam satellite
system with 5 2,5 10 2,66 10,64
MHz allocated to
each satellite beam
in the retransmission is controlled on the fly according to
the channel erasure probability characteristic of the specific
urban environment. This kind of protection against erasure
is generally called rateless coding. It has been shown in
several works that such codes can notably enhance reliability
in erasure channels [16]. Figure 3 shows how rateless codes
together with dynamical MGF choice can provide continuous
satellite coverage to users with non-line of sight (NLOS).
0
t
0't
1
't
2
t
1
t
2
't
Fig. 3. User ihas LOS in (t0, t1). During this interval it receives data from
satellite. During the whole interval (t0, t2)it acts as a MGF, broadcasting
linear combinations of data received in (t0, t1)to shadowed nodes. Similarly
user jacts as MGF in (t
0, t
2), broadcasting linear combinations of data
received in (t
0, t
1). In the interval (t
0, t2)channel access of the two users
is handled by the 802.11x MAC layer. With this scheme it is possible to have
continuous satellite coverage for shadowed users using dynamically changing
MGF.
Technical specifications for ETSI Digital Video Broadcast-
ing - Satellite Handheld (DVB-SH) in S band (2-4 GHz) have
been chosen as a baseline PHY for satellite downlink com-
munications. DVB-SH considers handheld terminals (defined
as light-weight and battery-powered apparatus such as PDAs
and mobile phones), vehicle-mounted, nomadic (e.g. laptops,
palmtops) as well as stationary terminals [1]. Supported bit
rates for DVB-SH-A (OFDM terrestrial OFDM satellite) and
DVB-SH-B (OFDM terrestrial TDM satellite) for a multi-
frequency network with satellite-only coverage (i.e. without
gap fillers) are indicated in Table II.
B. Return Link
Several air interface standards have been developed for
satellite uplink in S band. Among these we consider the air
interface for the S-UMTS, an extension to the satellite context
of the popular terrestrial universal mobile telecommunications
system (UMTS) [2].
In order to increase uplink capacity and avoid uplink
congestion in situations with a high ratio of shadowed-to-LOS
users, distributed array techniques are used by MGF, which act
as a distributed mobile satellite gateway. Two possibilities are
envisaged for a distributed MIMO implementation: beamform-
ing and space-time codes (STC). Beamforming consists in two
or more users transmitting synchronously to the satellite. The
signals add up at the satellite antenna, thus increasing received
carrier-to-noise ratio [17]. Application of such technique is
limited to those situations in which uplink phase synchroniza-
tion among the transmitters can be attained (e.g. nodes with
low speed and close to each other).
Information
exchange
Ad-hoc networking
(network coding)
Data collection
(a) Users with LOS act as mobile collectors receiving data from shadowed
users
Distributed MIMO
Sband(S-UMTS)
(b) Users with LOS use distributed MIMO techniques to relay data to satellite
Fig. 4. Satellite uplink access for nodes without LOS.
Space time codes are more robust against synchronization
errors and thus are more suited to a mobile context. Upon S-
UMTS air interface, distributed space time coding is used by
two or more mobile users. This allows to increase the uplink
capacity by emulating a coded multi-antenna system [18].
Fig.4(a) and Fig.4(b) depict the two phases of uplink access
scheme for shadowed nodes. In the first phase, information
313
exchange takes place using network coding techniques similar
to those described for the forward link.
VI. CONCLUSION
In this paper a novel architecture for a gap-filler-less
broadcast interactive satellite network has been described. The
underlying idea is to extensively apply advanced cooperative
communication techniques in order to overcome frequent
shadowing typical of satellite-to-land mobile communication
channels in urban and suburban scenarios. An ad-hoc net-
working approach has been chosen for M2M communication.
Cooperative techniques at different OSI level such as Net-
work Coding and distributed MIMO have been included in
the system architecture while keeping the compatibility with
existing VtoV and SLM communication standards. Standards
ETSI DVB-SH, S-UMTS and IEEE 802.11x have been con-
sidered as a reference for satellite down and uplink and M2M
communications respectively.
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... A lot of work has been done on the use of cooperation [5] in multicast and broadcast communications in both terrestrial [6] [7] and satellite networks [8][9] [10]. Many of the proposed solutions [4][11] [12], particularly in terrestrial networks, are based on network coding (NC) [13], that can achieve the Maxflow Min-cut capacity in ad-hoc networks. ...
... In order to evaluate the loss in performance due to finite code lengths and real coding schemes implementation, we also show in Fig. 6 the curves obtained for the same setup but with a finite block-length Raptor code (NC Raptor). The Raptor encoder is the one used in DVB-SH and introduced in Section VI 10 . A block length of K = 150 source symbols was chosen. ...
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... Diversity techniques such as time, frequency, and satellite diversity techniques [5] can tackle this problem by providing ground receivers with different versions of a satellite signal. 1 In the context of hybrid satellite/terrestrial systems, cooperative communications can also enhance the service availability by allowing relay nodes to forward satellite messages to masked nodes [6] [7] [8]. ...
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