Figure 1.28
The potential for direct RFI from a satellite into a terrestrial microwave receiver where
the frequency band is shared.
34
Fundamentals of Satellite Systems
the same coordination and siting difficulties as C-band. This particular part of
Ku-band is referred to as 14/11 GHz, where the uplink range is 14.00 to 14.500
GHz, and the downlink range is 10.95 to 11.7 GHz (minus a gap of 0.25 GHz in
the center). Only the downlink part of the allocation is actually subject to sharing.
A portion of the Ku allocation for FSS that is not shared with terrestrial services
is referred to as 14/12 GHz (the uplink range again is 14.00 to 14.50 GHz, and
the downlink range is 11.70 to 12.20 GHz). The availability of 14/12 GHz is
limited to Region 2, which comprises North America and South America, and can
be used only for domestic communications services. North America, in particular,
has seen wide use of 14/12 GHz since about 1982; services in this band were first
introduced in South America in 1992. Power levels from these satellites are not
subject to the same restrictions as at C-band, although there is an upper limit, to
minimize interference between satellites. Ku-band satellite operations in the rest of
the world (i.e., Regions 1 and 3) are restricted to the 14/11 GHz shared allocation.
In some instances, a Region 1 (Europe and Africa) or Region 3 (Asia) country can
make 14/11 GHz appear like 14/12 GHz simply by precluding domestic terrestrial
services from that band. However, terrestrial radio services in adjacent countries
are not under their control, so international coordination still must be dealt with
in border areas.
A third segment of Ku-band, referred to as 18/12 GHz, is allocated strictly to
BSS. As with the 14/12 FSS band in Region 2, the BSS band is not shared with
terrestrial services. Its intended purpose is to allow television broadcasting and
other DTH transmissions from the satellite. There are two regulatory features of
this band that make direct broadcasting to small antennas feasible. Because the
band is not shared with terrestrial radio, the satellite power level can be set at the
highest possible level. Adjacent satellite interference could be a problem but is
precluded by the second feature: BSS satellites are to be spaced a comfortable 6
or 9 degrees apart, depending on the region. In comparison, while there is no
mandated separation between FSS satellites, a 2-degree spacing has become the
standard in crowded orbital arc segments such as North America, Europe, and
East Asia.
The operational advantages of 14/12 and 18/12 GHz lie with the simplicity of
locating Earth station sites (without regard to terrestrial radio stations) and the
higher satellite downlink power levels permitted. The latter results in smaller ground
antenna diameters than at C-band, all other things being equal. However, as
discussed earlier in this section, Ku-band is subject to higher rain attentuation,
which can increase the incidents and duration of loss of an acceptable signal. Figure
1.29 indicates the relative amounts of extra downlink power, measured in decibels,
needed to reduce the outage time to four hours a month (on average). Ka-band
(30/20 GHz) is included for completeness and is discussed in Section 1.4.5. The
decibel (dB) is a logarithmic scale (like the Richter scale used to measure earthquake
intensity) and is discussed in Chapter 4.
As shown in Figure 1.29, the amount of decibels to overcome a fade is also a
strong function of the elevation angle from the Earth station to the satellite in
orbit. Rain cells, which are large atmospheric volumes (wider than they are high),
produce the greatest attenuation. Because of the flattened shape, radio waves that
take off at a low elevation angle relative to the ground must pass through a greater
1.4
Frequency Spectrum Allocations
35
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