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Figure 3.19
Mobile Satellite Service segments.
Figure 3.20
Basic arrangement of a Satellite Digital Audio Radio Service (S-DARS).
offsets that sequentially provide coverage of the United States at elevation angles
greater than 60 degrees. The latter is illustrated with the 24-hour timeline to the
right of the orbit diagram. In its second generation, Sirius has elected to launch a
GEO satellite with substantially more power than the three HEO satellites, the
intention to provide backup and to improve coverage to users who wish to receive
a consistent signal through the window of a home or other building.
3.2
Point-to-Multipoint (Broadcast) Networks
103
Having two satellites in view in the sky at one time provides a higher assurance
of an unobstructed path. Propagation studies have shown that this is equivalent
to approximately 10 dB of additional fade margin, which would not otherwise
have been practical. Service into urban areas is further enhanced through the use
of terrestrial repeaters that retransmit the same broadcast at much higher power
than would be possible from space. Two approaches are used to backhaul the
broadcast to the terrestrial repeaters: downlinking directly from the GEO satellites
to the repeater, or in the case of HEO, using an independent FSS satellite trans-
mission to low-cost dishes that transfer the signal to the repeater.
The content for S-DARS is assembled at the broadcast facility that houses many
sound studios, downlinks for talk and news programming, and servers containing
recorded music. The management of the system is automated to the point that a
disc jockey need only show up at the appointed time in the proper studio and
from there compile a broadcast. Business management functions for S-DARS are
essentially the same as for DBS, building upon that model of providing service and
distributing equipment to consumers. In addition, the S-DARS operators were
successful in signing up the automobile manufacturers who preinstall the receivers
in vehicles.
3.2.4.3
Ancillary Terrestrial Component
The previous two mobile satellite communications applications would appear to
be different in terms of their target markets. MSS is aimed at users of two-communi-
cations services, including mobile voice and data, and S-DARS is for one-way
broadcasting of music and other content. Both are valuable in their own right;
however, the potential for marrying features of one with the other has become of
great interest in the United States and Europe. Thus was born the ancillary terrestrial
component (ATC). The basic principle of ATC is to offer two-way communications
like MSS and to use terrestrial base stations to extend service into suburban and
urban areas, like the terrestrial repeaters of S-DARS. Figure 3.21 provides an
illustration of one such arrangement of a hybrid terrestrial/satellite wireless network
employing two GEO satellites along with large terrestrial cell sites similar to what
is found in a 3G cellular network. The key is to employ the same spectrum, L-band
in this example, and a common technical ‘‘air interface’’ so that the user devices
are inexpensive and easy to use.
The U.S. Federal Communications Commission issued its Second Report and
Order, laying out the objectives for ATC:
In deciding to permit MSS operators to integrate ATC into their MSS systems, we
stated that ‘permitting MSS licensees to enhance spectrum efficiency through ATC
represents a superior choice to continuing with the regulatory status quo.’ Further,
we found that MSS/ATC would expand the consumer market MSS is capable of
serving, leading to economies of scale and lower prices for consumers. Granting
authority for integrated MSS/ATC would also allow MSS operators to offer the
capability of receiving calls via both satellite and terrestrial radio links with a single
dual-mode handset and a single telephone number, whereas current dual-capability
MSS/Commercial Mobile Radio Service (CMRS) handsets require separate numbers
for MSS and CMRS services. Moreover, MSS/ATC would eliminate operational
104
Satellite Network Architectures
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