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Telephone: +41 22 730 6039
Fax: +41 22 730 5939
E-mail: pressinfo@itu.int
Mr Jose Albuquerque
Director, International Regulatory Affairs
Teledesic Corporation (United States)
The 1995 World Radiocommunication Conference (WRC-95), identified the bands 18.8–19.3 GHz (space-to-Earth) and 28.6–29.1 GHz (Earth-to-space) for use by non-GSO fixed-satellite service (FSS) networks. By so doing, WRC-95 laid the groundwork for an advanced two-way broadband infrastructure that will extend to parts of the world unlikely to get that capability otherwise. Prior to WRC-95, the international regulatory structure assumed essentially that all fixed satellite services would be provided through traditional geostationary satellite orbit (GSO) technology. By making provision for non-GSO FSS technology in at least part of the Ka band, the Conference preserved the option for a new approach that would enable real-time interactive broadband communications on a truly global basis.
As decided by WRC-95, Radio Regulation S22.2, which requires non-GSO networks to cease transmission if a GSO FSS network receives unacceptable interference, was made inapplicable in the upper 400 MHz of the above bands as of 18 November 1995, and coordination under Resolution 46 was required in these frequencies. The lower 100 MHz of the two 500 MHz sub-bands was reserved, pending action by WRC-97.
Resolution 118 (WRC-95) contains these determinations and requested the ITU–R to conduct studies on sharing between non-GSO FSS, GSO FSS, and the terrestrial fixed-service (FS). These sharing matters are essential to the decisions to be taken at WRC-97, where, in addition to the technical aspects addressed in the extensive ITU–R studies performed during the past two years, important policy issues have to be taken into account. This article examines these sharing issues and provides a framework for the decision making process that will take place at WRC-97.
Sharing between GSO and non-GSO FSS systems
No one disputes that there are methods, or combinations of methods, that could be employed to make it theoretically possible for GSO and non-GSO FSS networks to share the same frequencies. However, any approach to sharing imposes significant burdens on one or both of the services, often raising the cost and complexity, degrading the quality of service, and limiting flexibility and innovation. The 1997 Conference Preparatory Meeting (CPM-97) concluded that "in general it is considered that satisfactory ways of co-frequency sharing by non-GSO FSS and GSO FSS networks can be found where the burden is placed on either the GSO or non-GSO network. This will have to be evaluated on a case-by-case basis". Thus, the question is not whether sharing is possible in some narrow technical sense. The questions raised are instead policy questions, namely "Who should bear the sharing burden? How much will it cost? And will the resulting system provide a service anyone wants or needs at an affordable price?".
None of the interference mitigation techniques identified by CPM-97 allows for co-frequency sharing between GSO and non-GSO FSS with approximately equal burden on both systems, mainly because FSS operators in these frequencies will provide service to large numbers of small, ubiquitously deployed user terminals. In this high-density environment , the only available mitigation techniques (e.g. satellite diversity and "restricted elevation angles") place the burden of sharing squarely upon one service or the other.
Regardless of whether the burdened system uses GSO or non-GSO architecture, the use of satellite diversity generally requires a system designer to double the number of satellites, lower the elevation angle, or both, in order to provide redundant coverage. It also results in a dramatic increase in the cost and size of the user terminals and increases the complexity of the network. Reduced elevation angles result in decreased availability due to rain attenuation and also make it much more difficult to share with terrestrial fixed services, as well as to site user terminals sufficiently clear of man-made and natural obstructions.
Under the "restricted elevation angles" approach, the burdened system must observe a maximum elevation angle while the protected system operates above a minimum elevation angle. Like satellite diversity, this technique can be employed by either GSO or non-GSO systems. However, as explained above, setting a maximum elevation angle severely degrades the burdened system.
Given that the burden of sharing must fall predominantly on one type of system or the other, simple fairness requires that the FSS sharing burden be equitably shared between GSO and non-GSO. In bands such as the C and Ku bands, GSO systems are already fully deployed, and it therefore makes sense to impose the sharing burden on non-GSO systems. It does not follow, however, that non-GSO systems should bear the sharing burden in all FSS bands. In fact, given the many benefits that non-GSO architecture can provide, especially to users in remote, rural, or less developed areas, there is no possible policy justification for extending the absolute priority of GSO FSS systems to all parts of the Ka band. The more sensible way of dealing with the vast and relatively "clean" Ka band is to seize the opportunity to create at least one small portion of the band in which non-GSO FSS systems can be freed of the sharing burden imposed on them in all other FSS bands.
That is exactly what WRC-95 did. In the bands specified by Resolution 118, non-GSO FSS systems need not protect GSO systems except as a matter of bilateral coordination. In the greater part of the Ka band, S22.2 still applies and GSO systems continue to enjoy the same priority they enjoy in all other FSS bands. And in the 500 MHz identified for non-GSO FSS, GSO systems will still be permitted to operate as long as they do not claim the same kind of protection they are entitled to in all other FSS bands.
Sharing between non-GSO FSS systems
In response to Resolution 118 (WRC-95), a number of countries have submitted studies to the ITU–R confirming that the Teledesic architecture makes it possible for multiple non-GSO FSS systems to share the same spectrum in the 18.8–19.3 GHz and 28.6–29.1 GHz bands. CPM-97 likewise concluded that "co-directional co-frequency sharing between several homogeneous non-GSO FSS networks appears to be feasible through the use of spatial separation". Therefore, the prospects for sharing between or among non-GSO FSS networks in the frequencies identified for them by Resolution 118 appear to be quite good, subject to coordination with each other under Resolution 46.
Sharing with terrestrial services
Both GSO and non-GSO FSS systems can successfuly coordinate with most point-to-point FS systems, primarily due to the difference in the operational elevation angles of the two types of systems and the relatively small number of FS receivers. As indicated in the CPM-97 report, "coordination distances between FS and non-GSO FSS earth stations are comparable to the coordination distances between FSS earth stations". Therefore, the issue of sharing between FSS systems and FS systems is not impacted by the decision to enable non-GSO FSS technology in the bands identified in Resolution 118.
The CPM-97 report also noted that "as the minimum operational elevation angle of non-GSO FSS systems is lowered, the required separation distances between the non-GSO FSS earth stations and FS stations is increased". An important consequence of this fact is that implementation of satellite diversity, which leads to lower elevation angles, renders sharing between non-GSO FSS and the FS more difficult.
Co-frequency sharing between high density fixed service (HDFS) systems, such as the local multipoint distribution service (LMDS) point-to-multipoint terrestrial system, and FSS is not feasible. As noted in the CPM-97 report, co-frequency operation would lead to restrictions that "may constrain the use of one or both services in the same area". By dedicating appropriate portions of the frequency spectrum available to future desired FSS systems, the flexibility of providing both types of systems can be maintained. Not allowing for these dedicated allocations at an early stage of the development of these systems can prove to be an unwise policy because the process of subsequently reallocating systems to different frequency bands can be quite costly and involve long delays.
Power limits are not the answer
Power limits in the form of severe pfd/e.i.r.p. constraints on non-GSO systems, have been proposed as a means to enable the harmonious development of GSO and non-GSO systems in the same band. Power limits, if they worked, would provide a way to shoehorn at most one non-GSO FSS system into any of the FSS bands currently occupied by GSO satellites. They do not in any way facilitate sharing between or among multiple non-GSO systems. These limitations on non-GSO emissions are designed to protect GSO systems, and no single set of limits would protect other non-GSO systems. Thus, if power limits were imposed and a non-GSO FSS system launched into the Ku band where GSOs are currently operating, it would not be the case that a second non-GSO FSS system could launch into those same frequencies merely by observing the same power limits.
The agenda for WRC-97 gives the nations of the world an opportunity to create a regulatory environment that will enable the development of non-GSO FSS technology. The growing interest in non-GSO FSS systems and the unique benefits they can offer make it essential that WRC-97 follow through on the important work begun at WRC-95. Placing severe power limits on non-GSO FSS operations in the one small portion of the Ka band where WRC-95 mandated that they be able to operate free of the burden that prevails in all other bands, is clearly not the answer. Rather, the true potential of non-GSO FSS systems can be realized by waiving S22.2 and applying Resolution 46 in the remaining 100 MHz of the 500 MHz WRC-95 identified for non-GSO FSS use.
This text is an extract from ITU News 8/97