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1. INTRODUCTION
We all have our favorite radio stations that we preset into our car
radios, flipping between them as we drive to and from work, on errands and
around town. But when travel too far away from the source station, the
signal breaks up and fades into static. Most radio signals can only travel
about 30 or 40 miles from their source. On long trips that find you passing
through different cities, you might have to change radio stations every hour
or so as the signals fade in and out.
Now, imagine a radio station that can broadcast its signal from
more than 22,000 miles (35,000 kill) away and then come through on your
car radio with complete clarity without ever having to change the radio
station.
Satellite Radio or Digital Audio Radio Service (DARS) is a
subscriber based radio service that is broadcast directly from satellites.
Subscribers will be able to receive up to100 radio channels featuring
Compact Disk digital quality music, news, weather, sports. talk radio and
other entertainment channels.
Satellite radio is an idea nearly 10 years in the making. In 1992, theU.S. Federal Communications Commission (FCC) allocated a spectrum in
the "S" band (2.3 GHz) for nationwide broadcasting of satellite-based
Digital Audio Radio Service (DARS).. In 1997. the FCC awarded 8-year
radio broadcast licenses to two companies, Sirius Satellite Radio former
(CD Radio) and XM Satellite Radio (former American Mobile Radio). Both
companies have been working aggressively to be prepared to offer their
radio services to the public by the end of 2000. It is expected that
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automotive radios would be the largest application of Satellite Radio.
The satellite era began in September 2001 when XM launched in
selected markets. followed by full nationwide service in November. Sirius
lagged slightly, with a gradual rollout beginning _n February, including a
quiet launch in the Bay Area on June 15. The nationwide launch comes July
1.
To the average user, these systems will look very similar to
conventional AM/FM .radio systems, whether they are used in the home,office, or on the road. However. the real difference is in what the listener
won't see. Rather than receiving a signal from a tower antenna of a local
radio station, these new radios will receive signals from a set of satellites in
geosynchronous orbit. Programming will be up linked from ground stations
to the satellites and then broadcast back to large geographic areas.
The programming will be up linked to the three geostationary
orbit satellites and then rebroadcast directly to radios in the vehicles of CD
Radio subscribers. Ground based repeaters will be used in urban areas to
provide a clear and uninterrupted radio signal.
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Fig. 1 The satellite station
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2. BASIC COMPONENTS OF SATELLITE RADIO
Each company has a different plan for its broadcasting system, but
the systems do share similarities. Here are the key components of the three
satellite radio systems:
• SATELLITES
• GROUND REPEATERS
• RADIO RECEIVERS
At this time, there are three space-based radio broadcasters in various
stages of development:
XM Satellite Radio launched commercial service in limited areas of
the United States on September 25, 2001. (They were originally going to
launch service September 12. but postponed the event because of the
terrorist attacks on the United States.)
Sirius Satellite Radio is now operational in the United States, with
its official launch on July I, 2002.
WorldSpace is already broadcasting in Africa and Asia, and will begin
broadcasting in South America sometime soon.
XM Satellite radio and Sirius Satellite Radio have both launched
such a service. Satellite radio, also called digital radio, offers' uninterrupted,
near CD-quality music beamed to the radio from space.
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Taking a closer look, you will see slight variances in the three satellite
radio companies' systems. In the next three sections, we will profile each of
the companies offering satellite radio services.
2.1 SATELLITES
2.1.1 XM SATELLITE RADIO
XM Radio uses two Boeing HS 702 satellites, appropriately dubbed
"Rock" and "Roll," placed in parallel geostationary orbit, one at 85 degrees
west longitude and the other at 115 degrees west longitude. Geostationary
Earth orbit (GED) is about 22.223 miles (35,764 km) above Earth, and is
the type of orbit most commonly used for communications satellites. The
first XM satellite, "Rock," was launched on March 18.2001, with "Roll"
following on May 8. XM Radio has a third HS-702 satellite on the ground
ready to be launched in case one of the two orbiting satellites fails.
XM Radio's ground station transmits a signal to its two GED
satellites. Which bounce the signals back down to radio receiver son the
ground. and the downlink will be in the 2.33-2.34 GHz frequency range. A
spare satellite will be kept on the ground for emergencies. The radio
receivers are programmed to receive and unscramble the digital data signal,which contains up to 100 channels of digital audio. In addition to the
encoded sound, the signal contains additional information about the
broadcast. The song title, artist and genre of music are all displayed on the
radio. In urban areas, where buildings can block out the satellite signal,
ground transmitters supplement XM's broadcasting system.
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2.1.2 SIRIUS SATELLITE RADIO
Unlike XM, Sirius does not use OED satellites. Instead, its three
SS/L-1300 satellites form an inclined elliptical satellite constellation. Sirius
says the elliptical path of its satellite constellation ensures that each satellite
spends about 16 hours a day over the continental United States , with at
least one satellite over the country at all times. Sirius completed its three-
satellite constellation on November 30, 2000. A fourth satellite will remain
on the ground, ready to be launched if any of the three active satellitesencounter transmission problems.
The Sirius system is similar to that of XM. Programs are beamed to
one of the three Sirius satellites, which then transmit the signal to the
ground where the radio receiver picks up one of the channels within the
signal. Signals are also be beamed to ground repeaters for listeners in urban
areas where the satellite signal-can be interrupted.
While XM offers both car and portable radios, Sirius is concentrating
on the car radio market. The Sirius receiver includes two parts -- the
antenna module and the receiver module. The antenna module picks up
signals from the ground repeaters or the satellite. Amplifies the signal and
filters out any interference. The signal is then passed on to the receiver
module. Inside the receiver module is a chipset consisting of eight chips.
The chip set converts the signals from 2.3 gigahertz (GHz) to a lower
intermediate frequency. Sirius also offers an adapter that allows
conventional car radios to receive satellite signals.
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2.1.3 WORLDSPACE
So far, WorldSpace has been the leader in the satellite radio industry.
It put two or its three satellites, AfriStar and AsiaStar, in geostationary orbit
before either of the other two companies launched one. AfriStar and
AsiaStar were launched in October 1998 and March 2000, respectively.
AmeriStar, which will offer service to South America and parts of Mexico,
is not yet scheduled for launch. Each satellite transmits three signal beams
carrying more than 40 channels of programming, to three overlapping
coverage areas or about 5.4 million square miles (14 million square km)
each. Each of WorldSpace satellites' three beams can deliver over 50
channels of crystal clear audio and multimedia programming via the
1,467- to 1,492- megahertz (MHz) segment of the L-band spectrum.
which is allocated for digital audio broadcasting.
AfriStar is positioned in a 210 East geosynchronous orbit and is
controlled by the WorldSpace Operations Center located in Washington,
DC. The prime contractor for the satellite is Alcatel Space Industries, and
Matra Marconi Space built the EuroStar 2000+ satellite bus. The uplink
frequencies are 7.025-7.075 GHz, and the downlink frequencies are 1.452-
1.492 GHz. Each AfriStar downlink spot beam has capacity for ninety-six
16 kbit/s mono-AM-quality signals that can be combined for fewer channels
of higher audio quality. The downlink signals in each spot beam are
combined into two Time Division Multiple Access (TDMA) carriers.
Uplink signals can be. accepted as TDMA signals from control stations or,
individually, as Frequency Division Multiple Access (FDMA) signals from
originating program locations.
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WorldSpace also launched AsiaStar in March 2000, a DBS radio
satellite that currently covers Asia(1050 East orbit). In late 2000,
WorldSpace plans to launch AmeriStar (950 West orbit) to cover Latin
America.
The United States is not currently part of WorldSpace's
coverage area The company has invested in XM Radio and has
an agreement with XM to share any technological developments .
WorldSpace is going beyond one nation and eyeing worlddomination of the radio market. That might be overstating the company's
intent a bit. But WorldSpace does plan to reach the corners of
our world that most radio stations cannot . There are millions of
people living in WorldSpace's projected listening area who
cannot conventional radio station. WorldSpace says it has a potential
audience of about 4.6 billion listeners spanning five continents.
Fig 2 WoridSpace will be able to broadcast to the majority of the
world's population when its AmeriStar satellite is launched.
WorIdSpace broadcasters uplink their signal to one of the three
satellites through a centralized hub site or an individual feeder link station
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located within the global uplink beam. The satellite then transmits the signal
in one, two or all three beams on each satellite. Receivers on the ground
then pick up the signal and provide CD-quality sound through a detachable
antenna.
Fig. 3 World space integrated solution
2.2 GROUND REPEATERS
Satellite radio reception, poses threats from weather, tall building_
and mountains that can potentially interfere with broadcasts.
To avoid the interference caused by tall structures, both Sirius and
XM Radio are supplementing their satellite coverage with terrestrial
transmitters, called ground repeaters. If the satellite radio antenna is blocked
by a skyscraper, it should pick up signals from one of the ground repeaters.
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Getting signals from a satellite to receivers in cars or in the home is a
tall order. Although the microwaves the satellites rely on are able to
penetrate the atmosphere from space, they need a "direct line of sight" and
can only reach their target if unimpeded by obstacles such as trees, houses,
or thunderstorms. Therefore, ground-based repeaters are needed to prevent
service interruption in cities where tall buildings otherwise would block the
line of sight between radio receivers and the satellites. XM has employed
more than 1,000 of these terrestrial repeaters, which have been strategically
placed throughout the continental United States to receive the XM signal
directly from the satellites, and then retransmit it to XM radios in cars and
homes. These repeaters have been installed in densely populated cities, on
the roofs of buildings, and in mountainous areas where line of sight can be
difficult to maintain.
2.3 THE SATELLITE RADIO RECEIVER
Existing AM/FM car radio will not be able to receive satellite radio
broadcasts. Two options are available. Replacement of the radio with a 3-
band capable receiver (AM, FM, Sirius or XM Satellite). Radios can be
purchased as a dealer option or can be directly purchased at consumer retail
stores, mail order and Internet stores. All major manufactures are prepared
to provide radios capable of satellite radio reception.
A second option is the purchase an adaptor for existing AM/FM
radios. The adaptor will contain the satellite receiver, along with display
and control functions. Sirius and XM have developed slightly different
technologies which mean that you can purchase a radio capable of receiving
satellite broadcasts from one company or the other. but not both.. You need
a receiver, about the size of squashed shoe box, which goes under a car in
the trunk, along with a fist-sized antenna that sits on the roof or trunk lid.
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The receiving end is virtually the same for both companies, but the
satellite configurations are different: XM Radio will use two satellites, and
Sirius will use a combination three. These receivers, somewhat akin to
AM/FM tuners, are made up of two parts: an "active" antenna and a
receiving module.
XM and Sirius Radio will work similarly. Each will beam a
combination of original and syndicated programming to orbiting
communications and terrestrial satellites which will send out signals to thesatellite radio receivers. These receivers, somewhat akin to AM/FM tuners,
are made up of two parts: an "active" antenna and a receiving module.
The antenna is active because it basically looks for available signals
to pick up from. Satellites it recognizes. When it finds them, it amplifies
them, filters out any accompanying noise and interference, and then sends
them to the receiver, where most of the real work is done. En route to the
receiver, the signals are converted from analog to digital. Once in the digital
realm, they are analyzed for quality, and then processed and combined to
produce the best digital "image" of the sound. The receiver also decrypts the
signals and finally converts them back to analog audio, which can be sent to
the radio' s speakers so one can hear it.
The receiver connects to your existing car radio through a device
called an FM modulator that puts the signal on an unused portion of the FM
band. Or you can buy a car radio -- called a "head unit" by industry insiders
-- that's "satellite ready" to make a direct wired connection for maximum
audio quality.
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On the open road, the receivers pick up a signal from orbiting
satellites. Sirius and XM have also built repeater stations on the ground in
major metropolitan areas to maintain reception when the satellites are
blocked by buildings or other large structures.
One receiver utilizes a vehicles existing FM radio. A small flat 2"
disk antenna is attached to the outside of the vehicle, a processing unit is
placed in the trunk or dashboard and a display and control screen mounted
next to the vehicle's FM radio. The display screen indicates the selectedchannel number, channel name, song title and artist.
Each receiver contains a proprietary chipset. XM began delivering
chipsets to its XM radio-manufacturing partners in October 2000. The
chipset consists of two custom integrated circuits designed by ST
Microelectronics. XM has partnered with Pioneer. Alpine, Clarion, Delphi
Deleo, Sony and Motorola to manufacture XM car radios. Each satellite
radio receiver uses a small, car-phone-sized antenna to receive the XM
signal. General Motors has invested about $100 million in XM, and Honda
has also signed an agreement to use XM radios in its cars. OM began
installing XM satellite radio receivers in selected models in early 2001.
WorldSpace satellite receivers are capable of receiving data at a rate
of 128 kilobits per second (Kbps). The receivers use the proprietary
StarMan chip set, manufactured by STMicroelectronics, to receive digital
signals from the satellites
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3. TRANSMISSION AND RECEPTION
Digital radio works by combining two digital technologies to
produce an efficient and reliable radio broadcast system.
Firstly, an audio compression system, called MPEG, reduces the vast
amount of digital information required to be broadcast. It does this by
discarding sounds that will not be perceived by the listener - for example,
very quiet sounds that are masked by other louder sounds - and hence not
required to be broadcast, and efficiently packages together the remaining
information.
The second technology, COFDM (Coded Orthogonal Frequency
Division Multiplex) ensures that signals are received reliably and robustly,
even in environments normally prone to interference. Using a precise
mathematical relationship, the digital data signal is split across 1,536
different carrier frequencies, and also across time. This process ensures that
even if some of the carrier frequencies are affected by interference. or the
signal disturbed for a short period of time, the receiver is still able to
recover the original sound.
The interference which disturbs FM reception, caused by radiosignals "bouncing" off buildings and hills (multipath) is eliminated by
COFDM technology. It also means that the same frequency can be used
across the entire country, so no re-tuning of sets is necessary when
traveling, or taking a portable receiver to a different area.
Instead of having a different frequency for each radio station, digital
radio combines several services together in what is called a multiplex.
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The multiplex has a gross capacity of 2,300,000 bits. which are used for
carrying audio, data and an in-built protection system against transmission
errors. Of these about half the bits are used for the audio and data services.
Throughout the day, the data capacity allocated to each service can be
varied by the broadcaster.
The UK Government has allocated seven multiplexes on the radio
spectrum 217.5 230.0 MHz, which will be used for BBC and Commercial
Radio for national. regional and local services. Each multiplex can carry a
mixture of stereo and mono audio Services and data services too; the
number of each dependent on the quality required.
3.1 GENERATION OF THE DAB SIGNAL
How each service signal is coded individually at source level,
error protected and time interleaved in the channel coder is shown in
Figure 3.1. Then the services are multiplexed in the Main Service
Channel (MSC), according to a pre-determined, but adjustable, multiplex
configuration. The multiplexer output is combined with Multiplex Control
and Service information, which travel in the fast Information Channel
(FIC), to form the transmission frames in the Transmission Multiplexer. Fig
3.1 Finally, Orthogonal Frequency Division Multiplexing (OFDM) isapplied to shape the DAB signal, which consists of a large number of
carriers. The signal is then transposed to the appropriate radio frequency
band, amplified and transmitted.
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Fig 4 Generation of DAB signal
3.2 RECEPTION OF A DAB SIGNAL
Figure 3.2 demonstrates a DAB receiver. The DAB ensemble is
selected in the analogue tuner, the digitized output of which is fed to the
OFDM demodulator and channel decoder to eliminate transmission errors.
The information contained in the FIC is passed to the user interface for
service selection and is used to setup the receiver appropriately. The MSC
data is further processed in an audio decoder to produce the left and right
audio signals or in a data decoder (packet Demux) as appropriate.
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User Interface
Fig. 5 DAB receiver
3.3 FREQUENCY OF OPERATION
Digital radio is operated in a frequency range of between 215 - 230
MHz (Mega Hertz). This part of the radio spectrum is sometimes called
Band III, or VHF, and was previously used for some television
transmissions and by the military. The central frequency for the BBC
National Multiplex is 225.648MHz.
3.4 MULTI PATH INTERFERENCE
Multipath interference occurs when radio waves bounce off
buildings, hills, or other obstacles. This means the waves reach the set at
different times, causing interference. This is a particular problem in the car.
Digital radio sets have processors which filter out interference and correct
errors, such as those caused by multipath, so no interference. In fact, digital
radio is designed to use multipath to its advantage.
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4. ADVANTAGES OVER ANALOG RADIO
Conventional analog radio cannot meet this standard, simply
because of the technology used and the transmission environment in which
it is broadcast.
As well - unlike AM and FM - digital radio reception is virtually
immune to interference, which means there are no static growls or 'multi
path' echoes (caused by signal reflections off buildings or topographical
features) to make listening unpleasant. at home, or in the car, In short,
digital radio eliminates the noise that creeps into analog radio transmission
and reception
The reason digital radio is so reliable is because it employs a 'smart'
receiver. Inside each digital radio receiver there is a tiny computer: a
computer capable of sorting through the myriad of reflected and
atmospherically distorted transmissions and reconstructing a solid, usable
signal for the set to process.
In contrast, an un-intelligent analog receiver cannot differentiate the
useful information from the useless noise. It reproduces the entirety of
whatever signal it is tuned to: static, 'multipath' echoes, and all.
The XM satellites have been allocated 12.5 MHz of frequency
spectrum-over sixty times the bandwidth of a single FM radio station. In
contrast, a commercial FM radio station has only 200 kHz of bandwidth.
Also, in FM radio, the modulation signal is limited to frequencies below
15.000 Hz, whereas the satellite radio audio signal is able to extend to
above 20,000 Hz.
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5. CONCLUSION
For the listener, digital radio will be more than just 'the best sound
on the airwaves', it will be an intelligent communications device that will
offer more services and conveniences than can be provided by
conventional analog technology.
For the broadcaster, digital radio is not just a way to stay
competitive with other forms of digital sound, but one that offers
numerous new business opportunities as well.
It is a bright future for listeners and broadcasters alike: a future that
truly promises to provide 'the best sound on the airwaves' for the world.
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6. BIBLIOGRAPHY
1. D. Prabakaran, “WORLD SPACE- Satellite digital audio broadcast
service”. Electronics For You. Nov 2001, Volume 33, No:11.
2. www.xmradio.com
3. www.siriusradio.com
4. www.worldspace.com
5. www.howstuffworks.com
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ABSTRACT
Satellites are one of the greatest achievements of mankind. They
have been used for various applications like communication, military
application, weather forecasting and so on. They play a big role in the case
of television channels and other entertainment networks. One of the latest
applications of satellites is the satellite radio.
Satellite radio is a subscriber based radio service that broadcast
directly from satellites. It is an advanced form of mobile radio service
where one can receive compact disc quality music and other entertainment
channels. Even if the person is miles away from the radio station, the
quality of the program is not affected. The paper deals with the basic
structure of the satellite radio and its transmission and reception procedures.
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CONTENTS
1 INTRODUCTION 1
2 BASIC COMPONENTS OF SATELLITE RADIO 3
SATELLITES 4
2.11 XM Satellite radio 4
2.12 Sirius Satellite radio 5
2.13 World space Satellite radio 6
GROUND REPEATERS 8
RADIO RECEIVETRS 9
3 TRANSMISSION AND RECEPTION 12
3.1 Generation of DAB signal 13
3.2 Reception of DAB signal 143.3 Frequency of operation 15
3.4 Multipath interference 15
4 ADVANTAGES OVER ANALOG RADIO 16
5 CONCLUSION 17
6 BIBLIOGRAPHY 18
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