Industry coalition rejects C-Band re-allocation after WiMAX interference during tests

Details: Category: Satellite Networks; 31 October 2007; 7247 views

Reports of severe WiMAX interference during tests conducted simultaneously with satellite tests have led a global industry coalition to argue against re-allocating C-Band spectrum... Reports of severe WiMAX interference during tests conducted simultaneously with satellite tests have led a global industry coalition to argue against re-allocating C-Band spectrum. Here, Developing Telecoms is publishing a background document published jointly by this coalition. Implications for developing markets are included.

Testing conducted earlier this month has confirmed what a growing number of telecom service providers already suspected: WiMAX systems cause severe interference to satellite services when operated in an adjacent frequency band. The test results, which were generated by an international coalition of the global satellite communications sector in co-ordination with the WiMAX industry, were reported as delegations from every country in of the world are preparing to travel to Geneva for ITU's World Radiocommunication Conference (WRC), where they will make crucial spectrum-allocation decisions.

In a joint statement issued by eight non-profit industry associations, the issue was described as having a major impact on every region: "Throughout the world, satellite services are being severely disrupted by interference from new Broadband Wireless Access (BWA) networks operating in the "extended" C-band frequencies. Further, there is an effort underway by the terrestrial-wireless industry for the ITU to replace satellite services with future mobile phone networks like IMT advanced and 4G services in "standard" C-band frequencies."

The groups organising the response comprise: the Cable and Satellite Broadcasting Association of Asia (CASBAA), the Global VSAT Forum (GVF), the Asia-Pacific Satellite Communications Council (APSCC), the Satellite Users Interference Reduction Group (SUIRG), the European Satellite Operators' Association (ESOA), European Satellite Action Plan Regulatory Group (SAP-REG), (US) Satellite Industry Association (SIA), World Teleport Association (WTA), and other international associations of the satellite industry. Their declaration commenced: "Even as the market demands complementary satellite services for extending the reach of GSM, CDMA and, potentially, WiMAX services, next-generation terrestrial-wireless interests will be attempting to persuade national delegations during WRC to co-locate WiMAX and satellite services - a move that would result in unreasonable levels of interference not only to terrestrial-wireless backhaul services but also to mission-critical applications including broadcasting, emergency management, offshore communications, air traffic control, distance education, and much more.

The joint statement continued: "It is essential that the telecommunications sector come to a better understanding of the commercial and technological implications of this development.In particular, national administrations should recognise the potential for massive disruptions to 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 (specifically 3.4 - 4.2 GHz).

The test and its implications

The co-ordinated effort to confirm whether WiMAX systems would cause severe interference to satellite systems operating in an adjacent frequency band began during an Emergency C-band Summit held in Washington DC earlier this year. During discussions involving the WiMAX Forum, GVF, and the Satellite Users Interference Reduction Group (SUIRG) it was agreed that a side-by-side test would be conducted before WRC to inform national administrations' decision-making process.

Reports from operators and governments throughout the world support the most recent test results. The satellite systems that operate in the 3.4-4.2 GHz band (C-band) are suffering substantial interference, to the point of system failure, in places where national administrations are allowing Broadband Wireless Access systems like WiMAX to share the same spectrum bands already being used to provide satellite services. The same will happen if 3G and the planned 4G mobile systems (also referred to as IMT systems) are allowed to use the frequencies used in the C-band for satellite downlink services as is being contemplated by some administrations in the context of WRC-07 agenda item 1.4.

To eliminate this harmful interference, operators of satellite earth stations and users of satellite communications services have united to communicate their positions and technical requirements to national and international telecommunications regulators. Regulators and radio frequency managers need to allocate spectrum in ways that recognise 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 (BWA) and IMT mobile services are all important services, and there are ways to find suitable spectrum for all of them to operate.

The problem

Several national administrations have designated portions of the frequency band 3.4-4.2 GHz for terrestrial wireless applications such as BWA and future mobile services (IMT advanced - beyond 3G, 4G).This band is already in use by satellite services, radar systems, and domestic microwave links. This band is commonly referred to as the C-band.

In places where administrations have allowed BWA services to use the C-band, there have been massive interruptions of satellite services. Interference with radars and microwave links, which also operate in these frequencies, is likely. Satellite operations in places including Australia, Bolivia, Fiji, Hong Kong, Indonesia, Pakistan and sub-Saharan Africa have already been negatively affected. Other national administrations can and should avoid repeating this costly mistake. Alternative approaches are available.

The importance of C-band

Use of C-band for satellite communications is widespread throughout the world. It is particularly vital for many developing countries, particularly in South and Central America, Asia, and equatorial Africa because of its resilience in the presence of heavy rain. C-band earth stations are also used extensively in many developed countries. C-band ("Standard C-Band" and "Extended C band") frequencies have been assigned for satellite downlinks since the industry was inaugurated more than 40 years ago.

C-band services cover large areas. They facilitate intercontinental and global communications, and provide a wide range of services in developing countries. Services in this band now provide critical applications such as distance learning, telemedicine, universal access, disaster recovery and television transmission in many tropical regions.

Technical explanation

Antennae which 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 C-band satellite services have co-existed with domestic microwave links and radars 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/nomadic. Mobile and base stations for terrestrial wireless applications emit signals from many locations in all directions, simultaneously powerful enough to saturate the sensitive C-band satellite receiving systems, thereby causing potential for total loss of service in C-band. Recent operating experience in Australia, Fiji and Indonesia, and field trials in Hong Kong have confirmed this interference(in the Hong Kong experiments, television signals feeding 300,000,000 households throughout Asia were inadvertently knocked off the air!)

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 the use of frequencies for terrestrial wireless services in the Extended C and Standard C bands was not practical.

A particular problem for developing countries

C-band services are especially important for developing countries. 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. They are an essential component in the ITU's push to bridge the "Digital Divide" between the developed and developing world. Because they cover wide areas with minimal susceptibility to rain fade, they have proven to be exceptionally useful for disaster recovery in tropical areas - for example, C- band services were vital in facilitating clean-up and recovery after the 2004 Asian tsunami disaster. Other growing applications in developing countries' use include distance learning and telemedicine.

Growing recognition of the problem

It was thought by those governments that assigned broadband wireless frequencies in the extended C-band that 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 - particularly in Asia, which is most dependent on these frequencies - have begun to recognise the threat that ill-considered assignment of standard C-band and extended C-band frequencies to terrestrial wireless services poses.

Even in the case where BWA and satellite earth stations operate on different frequencies in the same portion of the C band, geographic separation is necessary. The Hong Kong Telecommunications Authority Working Group conducted an extensive series of field tests, concluding that " BWA equipment within an area of several kilometres around existing licensed earth stations 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....Based on the assessment in this paper, there are interference problems caused by the proposed allocation of BWA in the 3.4-3.6GHz band to the reception of satellite signals by FSS systems in the 3.4-4.2GHz band. For the coexistence of the two services in the same territory, some technical constraints must be observed. The technical constraints would imply significant costs to be incurred by both BWA operators and FSS users and they may make it difficult for a wide and cost-effective deployment of BWA systems in a dense urban environment."

In South America, the Bolivian Superintendencia de Comunicaciones (SITTEL) approved the usage of the 3.4-3.8GHz band for telecommunication as the primary allocation for usage for the WiFi industry. During the short testing period prior to the planned May 2006 roll-out, satellite signals carrying television channels in Bolivia were severely interrupted and major interference was reported. Viewers were missing World Cup games. SITTEL issued an administrative resolution mandating that wireless access system deployments in the 3.7-3.8GHz band be suspended in the entire territory of Bolivia for a period of 90 days so that SITTEL could adopt measures to solve this matter. The resolution also instructs the spectrum planning department of SITTEL to propose a new norm for channels in the 3.4-3.8GHz band. More recently, SITTEL has indicated that it intends to accommodate the BWA operators in the band 3.4-3.5GHz and had initiated the required procedures to finalise such arrangement.

In another context, the Asia-Pacific Telecommunity (APT - a regional intergovernmental organisation) warned in a report from its APT Wireless Forum (AWF): "...BWA systems within several kilometres of an FSS receiver earth station operating in the same frequency bands but on a non-co-channel basis would need to carefully conduct coordination on a case-by-case basis. Moreover, to avoid interference in non-overlapping frequency bands...a minimum separation distance of 2 km needs to be ensured with respect to all FSS receivers, even where BWA and FSS operate on different non-overlapping frequencies. This distance can be reduced to about 0.5 km if an LNB band pass filter is fitted at all FSS receivers, the BWA base station has additional filtering of spurious emissions and outdoor BWA user terminals are prohibited. The effectiveness of any mitigation technique is dependent on its application to individual site situations and can be applied only when FSS earth stations are confined to a limited number of specific known locations.

In Europe CEPT has prepared a new ECC Report on Compatibility Studies In The Band 3400-3800MHz Between Broadband Wireless Access (BWA) Systems And Other Services (Draft ECC Report 100). The studies have shown that to meet all relevant interference criteria for a representative FSS earth station, the maximum distances required for BWA central stations are between 270 km and 320 km. These distances are referred to as "mitigation distances" in the report, to indicate that smaller distances may be achievable through coordination of each BWA 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 BWA systems to reduce the separation distances has not been demonstrated.

The Asia-Pacific Broadcasting Union (ABU - a regional organisation grouping government and non-government entities) has warned that: "BWA is a promising technology. However, if implemented in the same frequency bands as the satellite downlinks, it will have an adverse 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 ITU-R Working Party 8F have shown that a minimum distance separation of approximately 35-75 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 receiver stations currently receiving in 3.4-4.2 GHz, 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 - 4.2 GHz band are received by 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 BWA systems would severely disrupt reception of satellite transmissions.

Alternatives to C-band

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 form of the report from the WRC Conference Preparatory Meeting that convened in February 2007.It is critical that governments and spectrum management authorities recognise the very real damage caused, and tremendous threat posed, to satellite services by use of the Standard C and Extended C-bands for terrestrial wireless systems.

A real effort to use alternatives must begin immediately.

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