Satellite Radio: The Next Communications
Revolution?
David Bultman, Student Member, IEEE
Abstract – Various aspects of the emerging satellite radio
technology are presented. To set the stage, the desirability of
satellite radio and the governmental rulings are presented.
The various challenges to broadcasting from satellites which
each service provider must surmount are presented in detail.
In depth discussions of the XM Radio and Sirius Radio
systems follow as a means of evaluating the efficiency and
effectiveness of each.
I. WHY SATELLITE RADIO?
Radio is one of the few services in the nation which
resisted the digital revolution. Television, wireless phones,
and other devices have embraced digital communications
technologies as a means to further bolster the robustness of
their systems.
Today, radio generally falls into two categories. AM
(Amplitude Modulation) radio broadcasts fairly low
frequencies. AM radio is very susceptible to interference and
propagation variability. These problems allow AM radio to
cover a fairly wide area but prevent Am radio from providing
a reliable service over that area. The narrowband nature of
AM stations prevents their broadcasts from containing any
significant amount of audio quality.
FM (Frequency Modulation) was the response to the
problems of AM radio. FM radio uses higher frequencies and
more bandwidth per station. The result is better audio quality
with somewhat reduced susceptibility to interference. FM
radio, however, has less range than AM radio.
Clearly a modern digital alternative to AM and FM radio is
needed. This new digital service should provide high quality
audio to a wide geographical area using a very robust system
if it to compete effectively with current radio and other audio
sources such as CDs and digital cassettes.
II. GOVERNMENT ACTION
The Federal Communications Commission (FCC) set aside
in the Code of Federal Regulations a section defining the
necessary qualifications to be licensed for its new Satellite
Digital Audio Radio Service (SDARS) classification. The
qualified applicants had to “describe in detail the proposed
satellite digital audio radio system, setting forth all pertinent
technical and operational aspects of the system, and the
technical, legal, and financial qualifications of the
applicant.”[9]
The applicants also had to give specifications for receivers,
compression rates, and meet various milestones. At the end
of one year, from the date of authorization, the applicant has
to complete contracting for construction of its first satellite or
begin the actual construction. At the end of two years the
second satellite, if applicable, must be contracted or in
construction. At the end of four years at least one satellite
must be in orbit and operational. At the end of six years the
entire satellite system must be in full operation. The
applicants are also forced to service, at least, the 48
contiguous states of the United States.
At the time, four companies were qualified to compete for
the new SDARS licenses. The companies are: Satellite CD
Radio, Inc. (now Sirius Satellite Radio), Primosphere Limited
Partnership, Digital Satellite Broadcasting Corporation, and
American Mobile Radio Corporation (now XM Satellite
Radio).
The original proposed frequency band extended from 2310
MHz to 2360 MHz. Congress changed the band November
of 1996 by deciding one half of the proposed band be
auctioned for terrestrial services instead. The new SDARS
band then extended from just 2320 MHz to 2345 MHz. The
25 MHz left in the SDARS band was not enough for four
SDARS operators. Congress had effectively allowed only
two of the four qualified applicants to be licensed for use in
the band.
The result was an FCC auction of the remaining bandwidth
in April of 1997. Satellite CD Radio, Inc. won one of the
licenses with a bid of $83,346,000 and a down payment of
$13,669,200[8]. American Mobile Radio Corporation won
the other license with a bid of $89,888,888 and a down
payment of $14,977,777[8].
American Mobile Radio
Corporation (XM Radio from now on) uses the upper half of
the spectrum from 2332.5 MHz to 2345 MHz. Satellite CD
Radio, Inc. (Sirius Radio from now on) uses the lower half of
the spectrum from 2320 MHz to 2332.5 MHz. The frequency
distribution can be seen below in Figure 1.
Figure 1. The SDARS Spectrum[18]
III. TECHNICAL ISSUES WITH SATELLITE
BROADCASTS
A. The Problems to Overcome
The SDARS’ operators target audience is consumers in a
mobile platform (essentially automobiles). This raises a
unique problem. The antenna used for reception of can not
be overly expensive or else consumers will not want the
service. XM Radio and Sirius Radio are forced to abandon
special high gain antennas and complicated satellite tracking
mechanisms to keep the receiving antenna pointed at the
satellite. The providers are forced to use omni directional
antennas very similar to the antennas used for mobile phones.
Ruling out special antennas gives rise to the importance of
schemes for mitigating the three basic reception problems of
satellite communications.
As the mobile receiving platform moves, from time to time
the broadcast signal from a given satellite will be blocked
completely.
This affect depends upon the vehicle’s
movement, the satellite’s movement, and the satellite’s angle
of elevation. The faster a vehicle moves the move possible
blockages it comes into contact with but the length of those
blockages decreases. For non-geostationary satellites, the
path which the broadcast travels to the mobile receiver
changes over time which further complicates the possibilities
for blockages. The movement of the satellite is much less of
a problem than the movement of the vehicle, however. As a
satellite’s angle of elevation increases with respect to the
mobile receiver the satellite becomes closer to being directly
overhead of the mobile receiver. It is obvious that a signal
which travels essentially vertically (as in the case of a very
high angle of elevation) to the receiver is much harder to
block in some ways. Conversely, a low angle of elevation
will allow the broadcast signal to reach into tunnels and under
overpasses better.
The broadcast signal can also be attenuated by foliage.
Foliage acts, to an extent, in the same way complete
blockages work with respect to the motion of the mobile
receiver, the motion of the satellite, and the angle of elevation
of the satellite. However, foliage is unique in that the amount
of attenuation can depend on the frequency of the signal and
the angle at which the signal strikes the foliage. Foliage can
also reflect a signal with a random phase shift. This is related
to multipath interference. Multipath interference occurs
when the broadcast signal bounces off of any variety of
objects with varying degrees of attenuation and phase
shifting. The receiver picks up these other paths as well as
the original broadcast signal and must determine how to
interpret the multiple signals as shown in Figure 2. The
reflected signals can even tend to cancel the legitimate
broadcast signal.
Figure 2: An Example of Multipath Interference [1]
Research has been conducted into the effect of foliage.
“The transmission margin necessary to overcome
multipath fading or attenuation from foliage has been
both measured and estimated by experts to be in the
range of about 9 to 12 dB for satellite radio broadcast
systems operating at UHF frequencies with reception at
mobile platforms having roughly a 20 degree or more
elevation angle to the satellites. Fortunately, multipath
and attenuation from foliage seldom occur
simultaneously.
However, the need for 9-12 dB
transmission margin means that satellite transmitter
power must be increased by a factor of 8 to 12 over its
initially high level.”[1]
B. Solutions to the Problems
There are a number of very inventive ways to alleviate the
problems associated with satellite broadcasts. The first
solution is called spatial diversity. Implementing spatial
diversity entails having more than one legitimate broadcast
signal path which the receiver can draw on. The simplest
way to accomplish this is to have more than one satellite as
shown below in Figure 3.
Figure 3: Implementing Spatial Diversity [1]
Spatial diversity helps to solve each of the three basic
reception problems. As seen in Figure 3, the probability of
completely blocking or attenuating multiple broadcast signals
at the same time, as the mobile receiver moves, is fairly
unlikely. Spatial diversity also helps reduce the effect of
multipath inference. By providing two legitimate broadcast
signals the receiver, in some cases, is able to ignore one of
the signals will all of its reflections in favor of a much
cleaner second signal.
Another solution is called frequency diversity. In a
frequency diverse system the single broadcast signal is sent
via two or more frequencies (preferably spatially diverse) to
the receiver. This solution is primarily aimed at stopping
multipath interference which, as stated before, can be
frequency selective. An example of frequency diversity is
shown in Figure 4.
In the time diverse system, the longer the broadcast delay
between the signals is the longer either of the signals can be
completely blocked from reception at the receiver. The
broadcast delay is typically between 0.5 seconds and 5
minutes [2].
The last solution is a repeater station. There are some
places (generally within cites or within very mountainous
territory) where even the previous 3 solutions can not ensure
a reliable signal fro the receiver. Repeater stations are set up
to receive the satellite broadcast and then rebroadcast at an
entirely new frequency (which further adds to the frequency
diversity of the overall system). Recall from Figure 1 that
some of the SDARS spectrum was reserved by both XM
Radio and Sirius Radio for repeater stations. The operation
of repeater stations is shown below in Figure 6.
Figure 4: An Example of Frequency Diversity [1]
The third solution is called time diversity. Time diversity
is employed is systems already having spatial diversity and
frequency diversity. In a time diverse system, one of the two
broadcast signals is delayed in time before it is broadcast. So
while one signal is at second X in the program the other
signal is at second X – C. Time diversity allows the receiver
to experience fairly severe blockages of both signals at the
same time. The concept of time diversity is demonstrated in
Figure 5.
Figure 5: An Example of Time Diversity [2]
Figure 6: The Compete System with Repeater Stations [2]
IV. ACTUAL IMPLEMENTAIONS
A. XM Radio’s System
XM Radio owns three Boeing HS 702 satellites of which
two are deployed in a geostationary orbit (about 22223 miles
above the earth). One satellite is at 85 degrees west
longitude and the other is at 115 degrees west longitude. An
artist’s rendering of the XM Radio system is shown in Figure
7.
Figure 7: The XM Radio System [16]
Each satellite in XM Radio’s system essentially covers one
coast of the United States. By spreading the satellites over
such a wide distance XM Radio is implementing spatial
diversity.
XM Radio claims the Boeing HS 702 satellites it owns
transmit an effective energy of 10 MW. By using very high
power satellites XM Radio is compensating for multipath and
attenuation effects. The satellites have a 45 degree angle of
elevation. Recall from the previous discussion how the angle
of elevation affects reception. Although XM Radio chose a
typical angle of elevation, the angle is such that a significant
number of repeaters are necessary. In fact, XM Radio has
approximately 1500 repeaters covering about 70 markets.
These repeaters are mostly 2 kW stations. These are not very
powerful repeaters compared to a standard AM or FM station
which can run up to 100kW.
Frequency diversity is implemented in the XM Radio
system by giving each of the satellites and the repeater
stations a different broadcast frequency. Again recall Figure
1. XM Radio also employs a time diversity scheme although
the exact timing is undisclosed.
XM Radio is headquartered in Washington, D.C. At its
headquarters it has 82 studios. The channel line up includes
100 channels, 60 of which are commercial free. There are 6
channels dedicated to decades, 5 to country music, 15 to hits,
10 to rock, 7 to urban, 6 to jazz and blues, 4 to dance, 5 to
latin, 6 to world, 4 to classical, 2 to kids, 12 to news, 5 to
sports, 3 to comedy, and 10 to talk and variety channels.
Currently Sony, Pioneer, Alpine, and Audiovox produce
XM Radio ready receivers. The receivers range from $100 to
$2100 with antennas costing in the neighborhood of $100.
GM, Isuzu, Nissan, Volkswagen, and Honda have deals with
XM Radio to install satellite radio ready receivers in select
models. The XM Radio service runs $9.95 per month.
To be profitable, XM Radio estimates it will need 4 million
subscribers. At the end of March 2002 XM Radio has signed
up about 76,000 people which is ahead of schedule. By the
end of the year they hope to have 350,000 subscribers.
B. Sirius Radio’s System
Sirius Radio owns 4 SS/L-1300 satellites of which 3 are
deployed in an inclined elliptical constellation. In the
constellation each satellite spends about 16 hours of every
day over the continental United States. The Sirius Radio
system is shown in Figure 8. Although the Sirius satellites
are constantly on the move they can be considered to be at
100 degrees west longitude while they are over the United
States. The Sirius satellites tend to be over the center of the
United States as opposed to the coastal approach of XM
Radio. Sirius is implementing spatial diversity but to a lesser
extend than XM Radio. The Sirius satellites have an angle of
elevation of 60 degrees which is much better than XM Radio.
It appears Sirius traded off some of the spatial diversity for a
better angle of elevation which hopefully provides equal
quality service.
The Sirius satellites are of lower power than the XM
satellites. This is partially made up for by the angle of
elevation. Sirius Radio, like XM Radio, requires repeaters.
Sirius Radio has about 105 repeaters serving 46 markets.
These repeaters are significantly more powerful than XM
Radio’s. The Sirius repeaters are up to 40 kW. Sirius
implements frequency diversity (as in Figure 1) and a time
diversity scheme like XM Radio.
Figure 8: The Sirius Radio System [17]
Sirius Radio is headquartered in New York City. Sirius
Radio has 75 studios. The channel line up also includes 100
channels, all of which are commercial free. There are 8 pop,
11 rock, 6 country, 8 R&B or Urban, 4 dance, 7 jazz, 5 latin,
3 classical, 8 variety, 14 news, 5 sports, 5 hispanic, 9
entertainment, and 7 lifestyle channels.
Currently Kenwood and Clarion produce Sirius Radio
ready receivers. The receivers range from $180 to $2800
with antennas costing the same as XM Radio’s. Sirius Radio
has deals with Ford, DaimlerChrysler, BMW, Mercedes,
Mazda, Jaguar, and Volvo to install satellite radio ready
receivers in select models. The Sirius Radio service runs
$12.95 per month. Sirius Radio is not fully covering the
United States at this time but estimates the full service will be
launched in July of 2002.
V. ARE THEY WORTH IT?
From all indications, the services live up to their claims of
nonstop high quality radio. Reviewers thus far, have noted
that signal losses are hard to generate and fairly short in
length. The biggest complaints are from very urban areas
like New York where the immense urban jungle has not been
pierced by repeater towers as of yet.
Perhaps a better question is whether consumers will be
willing to pay to listen to the radio when terrestrial radio
stations go digital was well as they hope to do in the next few
years. In any event, satellite radio will spark the digital
revolution in radio industry which has been so long overdue.
VI. REFERENCES
[1] Robert D. Briskman, John M. Seavey, and Paul Medeiros,
“Radio frequency broadcasting systems and methods using
two low-cost geosynchronous satellites and hemispherical
coverage antennas,” U.S. Patent 5485485, January 16, 1996.
[14] Yahoo!, Inc., “Satellite and digital radio news stories,”
dailynews.yahoo.com/fc/Tech/Satellite_and_Digital_Radio/.
[15] Kevin Bonsor, “How satellite radio works,”
www.howstuffworks.com/satellite-radio.htm.
[16] XM Satellite Radio, www.xmradio.com/.
[17] Sirius Satellite Radio, www.siriusradio.com/.
[2] Robert D. Briskman, “Mobile radio receivers using time
diversity to avoid service outages in multichannel broadcast
transmission systems,” U.S. Patent 5592741, January 7,
1997.
[3] Daniel Lieberman, “Satellite multiple access system with
distortion cancellation and compression compensation,” U.S.
Patent 5720039, February 17, 1998.
[4] Daniel Lieberman, “Satellite multiple access system with
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[5] Robert D. Briskman, “Satellite broadcast system
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[6] Robert D. Briskman, “Digital radio satellite and terrestrial
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[8] Federal Communications Commission Report No. AUC
97-05, Auction No. 15, “FCC announces auction winners for
digital audio radio service,”
wireless.fcc.gov/auctions/15/releases/da970656.pdf, April 2,
1997
[9] Code of Federal Communications 47CFR25.144,
“Licensing provisions for the 2.3 GHz satellite digital audio
radio service,” October 1, 2001.
[10] FCC Auctions Division, “2.3 GHz spectrum band
allocation,”
http://wireless.fcc.gov/auctions/data/bandplans/wcsband.pdf.
[11] Leslie Taylor Associates, Inc., “Regulatory update:
satellite digital audio radio service (SDARS),”
www.lta.com/sdars.html
[12] Jim Wagner, “Wireless providers in ‘Sirius’ Trouble,”
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March 15, 2002.
[13] Renae Merle, “XM Satellite Radio reaches 30,000
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8, 2002.
[18] David H. Layer, “Digital radio takes to the road,”
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dig.pdf.