WORLDSPACE The first DAB satellite service for the world by pengtt

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									ASMS-TF
Technical Group                                       ASMS_04_T16_0

Date:             August 2001
Source:           Alcatel Space

Title:            Contribution to the Internal Report of the technical Group of
                  the Advanced Satellite Mobile System Task Force
                  Section : 4.3 DIGITAL AUDIO BROADCAST-DERIVED
                  TECHNOLOGY
Agenda item:
Document for:



                  Decision
                  Discussion        X
                  Information




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4.3 Digital Audio Broadcast-derived technology

4.3.1 Introduction
Satellite Digital Radio Broadcasting is currently in full expansion worldwide.
WorldSpace, the first worldwide Digital Radio Broadcasting satellite system, has
started satellite broadcasting in developing countries of Africa and Asia. XM™
Satellite Radio and Sirius Satellite Radio, the two Digital Audio Radio System (DARS)
for the United States, are implementing their systems, which will operate by year
2001, and at least two satellites programs are being started in Japan.
These systems offer at the same time wide service areas, high digital sound quality,
user friendliness, and portable and mobile reception. In addition to sound and
thanks to the digital format, complementary services such as broadcasting of
multimedia and internet data are part of the delivery to enrich the content
received.
The expansion of these systems, demonstrates that satellite is now optimized for radio
broadcasting, particularly to portable or mobile receivers, compared to terrestrial
broadcasting that requires far more investment.
Europe has been involved for several years in the development of terrestrial DAB, but
this system has a lot of difficulties to emerge commercially, partly due to its lack of
coverage outside large cities.


4.3.2 The WorldSpace system
4.3.2.1 Overview
WorldSpace is the first satellite Digital Radio Broadcasting system providing portable
reception [1].
The WorldSpace system offers a worldwide coverage, over developing countries,
using geostationnary satellites, and broadcasting in L-band. The service targets are
mainly underserved radio markets, where low cost radio and radio portability are
key. Alcatel Space was contracted in 1995 by WorldSpace for the turn-key delivery
of the entire system.
The WorldSpace system is composed of 3 medium size geostationary satellites:
AfriStar™, launched in late 1998, covering Africa and Middle East, AsiaStar™,
launched in early 2000, covering Asia, and AmeriStar™, to be launched in 2001,
covering Latin America. Each satellite provides three beams of coverage (see figure
1).
In addition to the satellites, the system comprises a comprehensive ground
infrastructure deployed on the five continents comprising various control centers
(satellite, mission and broadcast) and service providers feeder link stations.
Obviously such service needed the development of a new generation of radio
receivers, available today on the market. These receivers are based on chipsets
developed by ST Microelectronics and Micronas. First generation receivers are
manufactured by Hitachi, JVC, Panasonic, Sanyo.




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                           Figure 1 : WorldSpace Coverage Area

There are currently about fifty radio programs per beam, a large part of them
provided by well-known broadcasters, such as BBC, RFI, CNN, Bloomberg.
In addition to audio content, WorldSpace is also providing multimedia contents,
such as Direct Media Service (DMS). DMS provides huge amounts of the best web
content, selected by the subscriber, directly to the hard drive of the user's PC,
without the need for a telephone line.
The early experience with the system has confirmed the sound technical choices
and demonstrated excellent performance in the various reception conditions,
portable and mobile, as well as in interference environments.
4.3.2.2 Key parameters of the system
The WorldSpace system is referenced as System D within ITU recommended systems
for Broadcasting Satellite Service (Rec. ITU-R BO.1130) [2].
The signal audio sources are digitally coded using the ISO/Audio MPEG 2 layer III
standard, worldwide known as MP3. The digitally coded source bit rates range from
16 kbps for monoral near AM quality to 64 kbps for stereo, FM quality.
Each satellite has the capacity to transmit a capacity of 50 to 200 programs per
beam.
The WorldSpace system uses a TDM QPSK transmission on downlink, including high
efficiency concatenated FEC (Convolutional and Reed Solomon codes), well suited
for satellite broadcasting.
Each downlink beam offers a link margin which helps combat typical signal losses in
the path between the satellite and the receiver, providing full quality reception.
Radio receivers in disadvantaged locations can be connected to high gain
antennas, or to antennas located in a unobstructed positions. For example,
reception in large buildings may need a common roof antenna for the entire
building or an individual reception antenna near a window.




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4.3.2.3 Uplink access to the satellite
To cope with the various broadcaster requirements, the WorldSpace system uses two
communication missions:
 A processed communication mission offers the capability for all potential
  broadcasters to have a direct access from theirs own facilities to the satellite using
  a Frequency Division Multiplex Access (FDMA) mode. Use of FDMA for the uplinks
  offers the highest possible flexibility between multiple independent Feeder Link
  stations. These Feeder Link stations for the processed mission are VSAT-type
  terminals, using small antennas, of about 2 to 3 meters diameter, and low power
  amplifiers, less than 100 W RF power.
 A transparent communication mission deals with time division transmission of
  bouquets of programs and with broadcasters that have no direct access to the
  satellite, for which a hub station is preferred.
Whatever the mission, the complete WorldSpace transmission path is transparent to
the overall channel capacity allocated to a broadcaster, so that the broadcaster
has full dynamic control, and may configure it at his discretion.
4.3.2.4 The WorldSpace satellites
To meet the system requirements, a specific payload design has been implemented
by Alcatel Space on a standard Eurostar 2000+ platform from Matra Marconi Space
(now Astrium) to form the WorldSpace satellites.
These satellites are 3 axis stabilized geostationary satellites, have a 2.75 tons launch
mass, with a 15 years maneuver lifetime. Solar arrays generate more than 6 kW of
power.
The WorldSpace satellite includes two communication payloads to cope with the
two possible uplink accesses:
   The WorldSpace processed mission is based on the baseband processing
    concept of the satellite payload, already used on advanced technology
    satellites, but only recently emerging on commercial programs.
   The WorldSpace transparent mission is based on the bent pipe concept of the
    satellite payload, widely used on commercial programs, except that it
    specifically uses a small frequency band with narrow channel bandwidth leading
    to a unique payload design for filtering and demultiplexing.
Figure 2 shows the block diagram of the satellite payload. The key elements of the
payload are:
 Baseband processing:
  It offers higher performance than a "classical" transparent payload, with an
  improved link budget, and provides flexibility to broadcasters on the uplink.
  WorldSpace is the first commercial satellite to use this technology.
 High power amplification:
  RF power is 300 W for each downlink signal, performed with paralleling of two 150
  W TWTA.
 L band antenna :
  2 antennas offer a total of 3 spot beam coverages.




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                                           3 Way
                                                                                                          Switch
                                           Hybrid
                                                                                                          Switch




                                                                  IF Demultiplexer          L Band
                  X band / IF   2 Way                                                                              L band Transmit
                                                                        &
                   Receiver     Hybrid                                                                             A type Antenna
                                                                L band Up Converter       300 W TWTA
                                                                                                                      (2 beams)
                                                                                          Amplification
        X band
                                                                      96
        Receive
        Antenna                           Multi Carrier            PRC
                                                                                                                   L band Transmit
                                         Demultiplexer    288         96
                                                                                                                   B type Antenna
                                                &         PRC      PRC                                                (1 beam)
                                         Demodulator                  96

                                                                   PRC
                                                                        TDM Generator
                                                            Routing
                                                                              &
                                                            Switch
                                           Baseband                    L Band Modulator
                                           Processor


                       Figure 2: WorldSpace satellite payload block diagram
4.3.2.5 WorldSpace mobile applications
The WorldSpace system was primarily tailored for portable and fixed reception. But
the selected waveform (TDM QPSK), together with the high satellite EIRP also allows
for straightforward mobile reception. It is common use to listen to the WorldSpace
programs in a car with the receive antenna behind the windscreen or on the roof!
Achieved availability is very high in rural and sub urban areas, even in Southern
Europe.
To improve availability in dense urban areas suffering blockages, WorldSpace has
extended its system to an hybrid satellite/terrestrial system, identified as System D h
within ITU [2]. A complementary terrestrial component retransmits the satellite TDM
signal using Multi-Carrier Modulation (MCM) in L band. MCM is a Orthogonal
Frequency Division Multiplex technique, especially designed for multipath
environment, allowing for Single Frequency Network, and used in digital terrestrial
broadcasting systems, such as EU 147 DAB (ITU system A) or DVB-T. WorldSpace MCM
has been optimized for L band mobile reception in high multipath environment.
MCM also benefits from the same high efficiency FEC as the satellite component,
compatible with any audio compression scheme and data transmission. Dedicated
radios receive signals from both satellite and terrestrial components, with seamless
hand over between the two.
To improve the availability in conditions where only a satellite signal is present,
WorldSpace has also incorporated a time diversity technique, based on the
broadcasting of the same content twice but with a time interval of several seconds.
The time interval makes the two contents uncorrelated with respect to blockages for
mobile reception. A dedicated receiver is able to combine the two contents for a
seamless reception.
This hybrid system has been implemented and successfully tested in Germany
(Erlangen) and in South Africa (Pretoria), using the AfriStar satellite.
4.3.3 XM™ Satellite Radio
4.3.3.1 Overview
The XM™ Satellite Radio service is a Satellite Digital Audio Radio Service (DARS)
licensed by the FCC for the United States. It is being developed for commercial
operation in 2001, and will provide over the United States high-quality compressed
audio, as well as text and other digital data to car, home, and portable personal


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receivers via a pair of geostationary satellites and a network of terrestrial repeaters.
Distinctive features of this new radio service include: a coast-to-coast coverage,
creating a truly national listening audience in the United States; a superior digital
quality reception; and a superior choice of programming, catering to wide range of
listener’s preference.
4.3.3.2 Key parameters of the system
The XM™ Satellite Radio system [3] uses the 2332.5 to 2345 MHz frequency band. It
has been optimized for this S band spectrum in order to ensure reliable performance
in both urban and rural environments throughout CONUS.
The system’s flexible time-division multiplexing (TDM) scheme allows broadcast of up
to one hundred of channels of music and voice, each of which can be
supplemented by service components (sub-channels) for carrying text and other
data.
Each satellite transmits the same content, using QPSK modulation, so that a receiver
can construct the service signal from either satellite signal. The two satellites
configuration provides space and time diversity, which improve the availability of
the service.
All content is up-linked to the satellites from the XM™ Satellite Radio programming
center.
To maximize the signal availability to mobile receivers everywhere within the
Continental United States (CONUS), the system employs two high-powered
geostationnary satellites and a network of urban repeaters for re-broadcasting. To
cope with terrestrial transmission environment, broadcasting from repeaters uses
COFDM modulation. Repeaters are fed by the signal broadcast by satellites in S
band. The system provides seamless reception between the satellite and repeater
components.
4.3.3.3 The XM™ satellites
The XM™ Radio S-band satellites are constructed by Boeing Space Systems, with its
payload being supplied by Alcatel Space. The satellites are located at 115° W and
85° W longitude on the geostationnary orbit. Satellites have a 15 years lifetime, and
are based on HS-702 platform, using high-efficiency solar arrays, and Xenon-Ion
thrusters.
These satellites, with a 4.45 tons launch mass, are launched by Sea Launch. Upon
launch, these satellites are the most powerful commercial satellites in operation,
producing over 15.5 kW of power at end of life.
The XM™ Satellite Radio payload is composed of 2 bent pipe transponders.
Each transponder includes an antenna composed of a shaped 5-meter aperture
reflector with a single feed. The beam coverage is tailored to the shape of CONUS,
with high EIRP biased towards the areas with lower elevation angles to the satellites
and also to areas with higher population density.
Each transponder also includes High Power Amplification composed of sixteen 216-
Watt TWTA’ s operating in parallel at saturation. Combination of both antenna and
High Power Amplification produces a peak EIRP in excess of 68 dBW.
4.3.4 Sirius Satellite Radio
[Refer to Alenia contribution]


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4.3.5 Satellite Digital Radio Broadcasting in Europe
Satellite Digital Radio Broadcasting will surely become a large success in coming
years, as experienced through the WorldSpace and XM™ Satellite Radio systems.
This success is largely based on the involvement of European companies that have
participated to these programs. But surprisingly, no satellite Digital Radio
Broadcasting system for Europe has ever succeeded up to now, although Digital
Radio Broadcasting studies started more than ten years ago in Europe.
More than 10 years ago, Europe decided to promote satellite Digital Radio
Broadcasting. Through the project EU 147, DAB (Digital Audio Broadcasting) was
born. The system was based on the use of OFDM modulation, and MPEG 2 layer 2
(MUSICAM) audio coding. OFDM allows the use of the Single Frequency Network
(SFN) concept (reuse of the same frequency on all transmitters), which was
expected to be used for the satellites and the terrestrial repeaters of the system. In
the frame of DAB, the Archimedes satellite project was envisioned, using the L band
frequency resource allocated at WARC 92 only with OFDM. Unfortunately, satellite
technology was not able to provide satisfactory performance at that time, and the
thought in Europe that satellite could not be used for Digital Radio Broadcasting
emerged.
Yet, DAB activities continued, but only limited to terrestrial broadcasting (T-DAB).
Today, even after 10 years, the EU 147 based T- DAB has enjoyed only limited
development. Main reasons are:
   A limited number of attractive programs, compared to the current analog radio
    offerings, especially in FM
   A limited coverage, due to the lack of a huge terrestrial infrastructure investment
   A limited capacity of programs, due to the use of rather old, inefficient audio
    coding scheme (older than MP3)
   A limited FEC performance, only optimized for the specified audio coding
    scheme, but not to higher performance audio coding scheme, nor to data
    transmission which needs high error protection.
4.3.6 Satellite Digital Radio Broadcasting technology
Significant improvements have been achieved in these last years, allowing for
satellite based Digital Radio Broadcasting to become a reality. The reasons are
described below.
4.3.6.1 Digital broadcasting
Radio, as most transmission systems, is going to digital:
 Digital audio signal compression has improved to a point where the digital audio
  signal occupies less bandwidth than an analog signal for the same quality, much
  lower bandwidth than the CD standard. Current ISO MPEG 2 Audio layer 3 (MP3)
  provides quasi-stereo CD quality at 64 kbps, that is reduced by more than 15 fold
  compared to CD. New ISO MPEG-2 AAC/AAC+ audio coding provides CD quality
  at 48 kbps.
 Digital signals are more robust with respect to external interference thanks to
  dedicated error correcting digital signal processing.


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 Digital audio signals may be multiplexed with any other signal, such as image, test
  or files, and offers an enriched multimedia content. Also interfaces with other
  applications are eased.
 Digital signal processing is cheaper than analog processing thanks to silicon large
  scale integration.
4.3.6.2 Digital Radio Broadcasting
Analog radio is available from terrestrial transmission in well-established frequency
bands (FM, MW, SW). But the analog signal susceptibility to digital signals and the
lack of available frequency spectrum make it difficult to share these frequency
bands.
To avoid these compatibility problems, other frequencies have been allocated.
WARC 92 allocated the 1452 -1492 MHz band (L band) for Digital Radio Broadcasting
over the major part of the world, including the WorldSpace service area, and
Europe. In the US, the FCC has allocated the 2320 to 2345 MHz band for Digital Radio
Broadcasting.
But use of these new frequency bands for terrestrial only broadcasting is expensive,
because it requires the implementation of a huge terrestrial infrastructure, denser
than FM as frequencies are higher, involving larger propagation attenuation.
The best solution to limit the amount of ground infrastructure is to use satellites, which
are able to offer large coverage from their vantage point in the sky. An analogy
may be performed with digital TV broadcasting, where satellite reception is already
largely used, while there is still no digital terrestrial reception available yet.
4.3.6.3 Satellite Digital Radio Broadcasting
The largest possible availability in portable and/or mobile receive conditions for the
end user is based on the following key parameters:
   The visibility of the satellite
   The modulation scheme for satellite transmission
   The satellite EIRP
   The complementary terrestrial retransmission

Visibility of the satellite
Reception from satellite is sensitive to blockage, occurring not only when a satellite is
seen with low elevation angle. Even with high elevation angle, trees and houses may
shadow the satellite. So whatever the satellite configuration, HEO or GEO, high
availability performance requires the use of specific techniques, such as space and
time diversity.
Space diversity is achieved with two satellites having the same earth coverage,
providing the same content, but from different orbital locations. In that case, a
location on earth is in visibility of the two satellites, and the probability of blockage
occurrence is accordingly reduced.
An HEO satellite configuration, providing high elevation angle on the coverage
area, requires generally at least three satellites.
A GEO satellite configuration, requires two satellites if space diversity is used.
Modulation scheme for satellite transmission


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Unlike terrestrial transmission, modulation used for satellites does not require provision
for multipaths environments. Thus power efficient modulation, such as TDM QPSK
may be used for satellite broadcasting.
MCM/COFDM, an efficient type of modulation adapted to multipath environment, is
largely used in digital terrestrial broadcasting systems, such as the WorldSpace
hybrid system, viz. ITU System Dh or ITU System A.
Using the same satellite parameters, the same bit rates and the same propagation
environment, an overall link budget difference of at least 7 dB in favor of the TDM
QPSK modulation is obtained between COFDM and TDM QPSK link budgets, due to:
 The need for COFDM modulation to operate in linear mode as it uses multicarrier
  signals, leading to output back-off and non linearity losses (about 3 dB difference)
 The use of non coherent demodulation for COFDM (about 3 dB difference)
 The specific losses of the COFDM modulation, that is guard time and overhead
  allowance (about 1 dB difference)
Satellite EIRP
Availability is function of the link budget margin, defined from the service to be
provided. For mobile applications, high power satellites are necessary, with EIRP in
excess of 60 dBW. Such levels of EIRP in L band have been possible only in the last
few years.
Complementary terrestrial retransmission
Even if high satellite link margins may be achieved now, there are still some receive
areas where obstacles are numerous (dense urban areas). There, terrestrial repeaters
may complement satellite reception. Concepts developed and tested by
WorldSpace and DARS systems are applicable.
4.3.7 References
[1] : O. Courseille, P. Fournié, J.F. Gambart    "On-air with the WorldSpace Satellite
System"       48th IAF Congress, 1997
[2] : “Systems for digital sound broadcasting to vehicular, portable and fixed
receivers for broadcasting satellite service (sound) bands in the frequency range
1400-2700                                                                    MHz”
Rec. ITU-R BO.1130-3
[3] : D.H. Layer, “Digital Radio Takes to the Road”, IEEE Spectrum, July 2001




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