Satellite operators challenge mobiles’ use of C-band
JosÚ Albuquerque, Intelsat
JosÚ Albuquerque, Senior Director,
Spectrum Engineering, Intelsat
The World Radiocommunication Conference (WRC-07) will address matters
related to the identification of radio-frequency spectrum for IMT-2000 and
systems beyond. (International Mobile
Telecommunications-2000, or IMT-2000, is ITU’s global standard for third
generation — 3G — wireless communications). In the opinion of many satellite
operators, the frequency bands 3 400–4 200 MHz and 4 500–4 800 MHz (known as
C-band) are not suitable for this purpose.
The frequency ranges 3 400–4 200 MHz and 4 500–4 800 MHz are in the list of
candidate bands that emerged from studies conducted within the ITU
Radiocommunication Sector (ITU–R) in connection with this conference agenda
item. Most existing C-band satellites use the first of these ranges for their
downlink transmissions. The band 4 500–4 800 MHz is associated with the
downlinks of the fixed-satellite service (FSS) Plan, and is intended to preserve
orbit and spectrum resources for future use on an equitable basis by all
Currently, there are some 160 satellites in the geostationary orbit using
C-band frequencies for their downlink transmissions (see Figure 1). This is the
equivalent of more than 3000 satellite transponders with a 36 MHz bandwidth with
the potential for transmitting about 180 Gbit/s at any given instant. This
infrastructure represents an investment in excess of USD 30 billion in
spacecraft and launch costs alone, without taking into account investment in the
ground segment made by users and satellite operators.
Figure 1 — Geostationary satellites currently in orbit using the band
3 400–4 200 MHz
Deployment of IMT systems in these frequencies would drastically reduce the
benefits that these resources have brought to users around the world, because
fixed-satellite services and IMT systems cannot share frequencies in the same
Critical services delivered via C-band
C-band frequencies are used for downlink satellite transmissions that provide
a wide range of services in developed and developing countries, including
critical applications such as distance learning, telemedicine and universal
access services; backhaul services (telephony, Internet); very small aperture
terminal (VSAT) data links such as bank transactions or corporate networks;
distribution of television programmes; mobile-satellite service feeder links,
and emergency links, including disaster recovery services and meteorological
tracking. These services require the high reliability and broad geographic
coverage that can only be delivered in the C-band.
C-band is effective for smaller markets
The satellite beams in C-band cover large geographic areas and facilitate
intercontinental and global communications. In higher frequencies, such as
around 12 GHz (Ku-band) or 20 GHz (Ka-band), beams are more focused towards
smaller areas to overcome the more severe signal attenuation due to atmospheric
effects. This is illustrated by the downlink footprints of a satellite currently
operating at 180║E, which are shown in Figure 2 for C-band and in Figure 3 for
Due to their broad geographic reach, C-band beams allow for economically
viable coverage of smaller markets and regions with low population density. In
region-wide coverage can be provided with high availability, irrespective of
rain zones, because rain fade effects are almost negligible in these
frequencies. On the other hand, the severe rain fading effects on Ku-band or
Ka-band signals require operators to create smaller beams focused on areas of
high demand and population density, in order to maintain the required quality of
service in an economically viable manner.
IMT cannot share with FSS
Figure 3 — Ku-band spot beam of a satellite at 180║E
Figure 2 — C-band footprints of a satellite at 180║E
It is not feasible to undertake co-frequency operation of FSS receiving Earth
stations and transmitting fixed or mobile stations in IMT systems. ITU–R studies
have concluded that separation distances of between tens of kilometres and a few
hundred are required to ensure protection of FSS Earth stations. Considering
that a typical city covers an area with radius of between 15 and 30 km, sharing
between IMT systems and FSS receiving Earth stations is not realistic.
In addition, IMT transmitters can also interfere with FSS Earth stations
operating in adjacent bands. Unwanted emissions generated by IMT transmitters
falling within the FSS desired signal cannot be filtered and will therefore
generate interference. Furthermore, signals generated by an IMT transmitter can
be strong enough to saturate the low-noise amplifier (LNA) of the FSS receiver.
In view of the significant difference between the levels of the desired signal
(originating at the satellite transmitter about 36 000 km away) and the
interfering signal (originating at the IMT transmitter only a few kilometres
away), filtering the IMT signal to the required levels might become unfeasible.
The adjacent-band interference effects described above highlight the fact
that identification of a portion of C-band frequencies for IMT systems, while
keeping another contiguous portion for FSS use, is not free of interference
problems and does not constitute a desirable approach.
Mitigation techniques have been proposed in this context. However, given the
order of magnitude of the separation distances required to reduce interference
to acceptable values and the location of Earth stations in high density areas,
application of mitigation techniques is not a realistic option.
In particular, spectrum management techniques are not feasible because the
weak signal coming from the satellite cannot be monitored by the IMT
transmitter, and development of a database with information on the signals
received by FSS Earth stations is unrealistic. For much the same reasons, site
shielding is not practical either. The number of Earth stations involved is too
large and such a solution, even if it could be implemented, would become too
Number of Earth stations
It is very difficult to make an accurate estimate of the number of C-band
Earth stations around the world. Only a small fraction of those operating are
individually notified to ITU. At the national level, data are also incomplete,
especially because, in the vast majority of cases, receive-only (RO) Earth
stations are not required to be registered (and actually are not registered)
with the telecommunication authorities of each country.
Figure 4 — C-band Earth stations: Incomplete count from a single satellite
As an example of this situation, in
August 2006, approximately 6500 Earth stations deployed in the United States
were in the database of its Federal Communications Commission (FCC), while it
was known that there were more than 11 000 RO Earth stations operating in that
country as cable head-ends.
Another illustration is seen in Figure 4, which presents a map of Earth
station locations of a single satellite operator. As explained, even for this
single operator the map represents a substantial undercount of the actual
C-band is not suitable for IMT systems
It should be noted that C-band frequencies are not the most appropriate for
IMT systems. In areas where population density is high, cell diameters will be
based on usage requirements. In areas where population density is low, however,
cell diameters depend on how far signals can reach.
The characteristics of C-band will significantly increase costs as compared to
deployment in lower frequency bands, because C-band signals, in addition to not
being able to penetrate buildings, lose energy over distance much more than
lower frequency bands. This would mean that IMT deployment to rural areas using
C-band would be much more expensive.
Furthermore, alternative bands are available for IMT. The World
Administrative Radio Conference (WARC) in 1992 and the World Radiocommunication
Conference in 2000 identified a band of around 750 MHz for IMT systems. Several
other IMT candidate bands will be considered by WRC-07, mostly below 3 GHz.
Given all the above circumstances, satellite operators are of the view that
the frequency bands 3 400–4 200 MHz and 4 500–4 800 MHz (C-band) should not be
identified for use by IMT systems, either globally or regionally.