The global satellite communications sector, together with a growing number of international user groups, has launched a campaign to heighten public and governmental awareness of a trend that threatens current and future access to satellite telecommunications, broadcasting and other critical services in the C-band frequency range.

A growing international consensus of the satellite communications sector is urging nations around the world to recognize the potential for massive disruption of C-band satellite communications, radar systems and domestic microwave links if spectrum is inappropriately allocated to, and frequencies inappropriately assigned for, terrestrial wireless applications in the C-band.

These organizations include the Global VSAT (very small aperture terminal) Forum, Europe’s Satellite Action Plan-Regulatory Working Group, the European Satellite Operators Association, the Cable & Satellite Broadcasting Association of Asia, the Asia Pacific Satellite Communications Council, the World Teleport Association, the Satellite Users Interference Reduction Group and the U.S. Satellite Industry Association. The following represents the unified position of these industry associations.

Interference in the C-Band

Satellite communications technology in the C-band in the range of 3.4 gigahertz to 4.2 gigahertz, is used for broadcasting television signals, Internet delivery, data communication, voice telephony and aviation systems. The satellite systems that operate in the C-band are suffering substantial interference, to the point of system failure, in places where national administrations are allowing Broadband Wireless Access systems to share the same spectrum. The same will happen if 3G and the planned 4G mobile systems, also referred to as third and fourth generation or International Mobile Telecommunication (IMT) systems, are allowed to use C-band frequencies for satellite downlink services as is being contemplated by some administrations.

To eliminate this harmful interference, the operators of satellite Earth stations and the users of satellite communications services have united to communicate their position on the issue — and the relevant technical requirements — to national and international telecommunications regulators.

Regulators and radio frequency managers need to allocate spectrum in ways that recognize the reality of harmful interference and validate the right of incumbent operators to operate, and their customers to enjoy their services, without disruption by new users.

C-band satellite and the Broadband Wireless Access and IMT mobile services are all important services, and there are ways to find suitable spectrum for all of them to operate.

Several national administrations have designated portions of the C-band frequency for terrestrial wireless applications such as Broadband Wireless Access and future mobile services, despite the fact that C-band is already used by satellite services, radar systems and domestic microwave links.

In such cases, there already have been massive interruptions of satellite services, and interference with radars and microwave links is likely. Satellite operations in places including Australia, Bolivia, Fiji, Hong Kong, Pakistan and Indonesia already have been negatively affected.

Other national administrations can and should avoid repeating this costly mistake. Alternative approaches are available.

Importance of the C-band

Use of the C-band for satellite communications is widespread throughout the Americas and is particularly vital for many developing countries, particularly in South and Central America, southern Asia, and equatorial Africa because of its resilience in the presence of heavy rain.

C-band-based services were vital in facilitating clean up and recovery after the 2004 Asian tsunami disaster. In addition, the supporting equipment is relatively inexpensive and the signals easily cover large areas. Such services are well adapted to provide voice, data services and Internet connectivity in remote areas underserved by other communications means — an essential component to bridge the “digital divide” between the developed and developing world.

They facilitate intercontinental and global communications, and provide a critical applications in developing countries such as distance learning, telemedicine, universal access, disaster recovery and television transmission in many tropical regions. C-band, both standard and extended, frequencies have been assigned for satellite downlinks since the industry was inaugurated more than 40 years ago.

Sensitive antennas

Antennas that receive satellite downlink signals in the C-band are by necessity extremely sensitive devices. They are designed to receive a low-power signal emitted by small transmitters located in orbit 36,000 kilometers above the equator. In the C-band, satellite services have co-existed with domestic microwave links and radar for many years, because the latter systems operate via tightly focused beams from fixed points, and de-confliction can take place when necessary.

By contrast, terrestrial wireless applications are by definition ubiquitous and increasingly mobile, or nomadic. Mobile and base stations for terrestrial wireless applications emit signals from many locations in all directions simultaneously. These signals are powerful enough to saturate the sensitive C-band satellite receiving systems, potentially causing a total loss of C-band service.

The sensitivity of C-band satellite receiving systems also means that they may be disrupted by mobile terrestrial use of frequencies in immediately adjacent bands. Field tests by the Office of the Telecommunications Authority in Hong Kong concluded that use of frequencies for terrestrial wireless services in the extended and standard C-bands was not practical.

Governments that assigned broadband wireless frequencies in the extended C-band thought the problem could be limited by frequency segmentation. This has proven to be ineffective in real-world tests. Large-scale disruptions of services operating in non-overlapping frequency bands have taken place in several countries, and as a result, governments, intergovernmental bodies and the satellite industry have begun to recognize the threat that ill-considered assignment of standard and extended C-band frequencies to terrestrial wireless services poses. Some examples of this are:

   The Hong Kong Telecommunications Authority Working Group conducted an extensive series of field tests, concluding that Broadband Wireless Access “equipment within an area of several kilometers around existing licensed Earth station operating in the same frequencies may cause interference to the latter … protection by separation distance is only meaningful for fixed access but not for mobile access.”

   The Bolivian Superintendencia de Comunicaciones (SITTEL) approved the use of the 3.4- to 3.8- gigahertz band for telecommunication as the primary allocation for use of the WiFi industry. But during the short testing period before the May 2006 roll-out, satellite signals carrying television channels in Bolivia were severely interrupted — viewers were missing World Cup games.

SITTEL suspended wireless access system deployments in the 3.7- to 3.8-gigahertz band in all Bolivian territory for 90 days, so that SITTEL could adopt measures to solve this matter.

   The Asia-Pacific Telecommunity in a report from the APT Wireless Forum has warned that “… to avoid interference in non-overlapping frequency bands … a minimum separation distance of 2 km needs to be ensured with respect to all receivers for [fixed satellite services] FSS , even where [Broadband Wireless Access] and FSS operate on different non-overlapping frequencies.”

   In Europe, studies have shown that to meet all relevant interference criteria, for a representative FSS Earth station, the maximum distances required for Broadband Wireless Access central stations are between 270 kilometers and 320 kilometers . These distances are referred to as “mitigation distances” in the report, to indicate that smaller distances may be achievable through coordination of each Broadband Wireless Access central station. However, even with coordination it is clear that the necessary separation distances are at least tens of kilometres and may be hundreds of kilometres. The feasibility of the use of mitigation techniques by Broadband Wireless Access systems to reduce the separation distances has not been demonstrated.

   The Asia-Pacific Broadcasting Union has warned that Broadband Wireless Access ” is a promising technology. However, if implemented in the same frequency bands as the satellite downlinks, it will have an ad verse impact … and may make satellite operation in the entire C-band impracticable. These bands are by far the most important frequency bands for satellite communication in Asia.”

   Sharing studies conducted by International Telecommunication Union (ITU)-R Working Party 8F have shown that a minimum distance separation of approximately 35 kilometers to 75 kilometers must be maintained between an IMT transmitter (a 4G mobile system) and an FSS receiver. There is no practical way to maintain such large separations between these two systems. Moreover, given the large number of FSS receive stations currently receiving in the 3.4- to 4.2-gigahertz , it is highly unlikely that the requisite separation can be maintained with respect to all of these stations.

It is important to understand that satellite transmissions in the 3.4- to 4.2-gigahertz band are received by a very large number of stations worldwide. Many of these stations are “receive only,” and are therefore not registered at the ITU — or generally even with the local administrations — since such registration is not required. Co-frequency operation of Broadband Wireless Access systems would severely disrupt reception of satellite transmissions.

Fortunately, this is not an insoluble problem. Many other candidate bands have been identified during the course of ITU studies. The merits of these have been documented at length and the alternatives will be presented to the ITU WRC-07 in the report from the WRC Conference Preparatory Meeting that convenes in February

It is critical that governments and spectrum management authorities recognize the very real damage caused, and tremendous threat posed, to satellite services by use of the standard and extended C-bands for terrestrial wireless systems.

David Hartshorn is secretary general of the Global VSAT Forum.