 |
 |
 |
|
|
||
|
Telephone: +41 22 730 6039
Fax: +41 22 730 5939
E-mail: pressinfo@itu.int
by Vishnu Sahay, Manager, International Standards and Regulations
and Young Kim, Principal Engineer, Frequency Management Department,
International Telecommunication Satellite Organization (INTELSAT)
(Washington, DC, United States)
The 1997 World Radiocommunication Conference (WRC-97) will make many far-reaching decisions relating to the use of the spectrum/orbit resource. In many cases, it will reflect the work done following requests by WRC-95 through its recommendations and resolutions. For the fixed-satellite service (FSS) community, one of the most interesting issues is how to provide adequate spectrum for non-geostationary orbit (non-GSO) usage and yet maintain access to the GSO.
With the enormous growth in personal communications, Internet, data transfer, direct-to-home (DTH) television and even basic voice communications, the demands being placed on the radio spectrum and the GSO are leading to ever more creative ways of exploiting it, and to the opening up of higher frequencies including Ka band which up until now have not yet been tapped. The growing congestion in the traditional frequency bands used by INTELSAT and others for provision of global international services coupled with the new communications service offerings and the rapid developments in technology led INTELSAT to plan new systems capable of using the 30/20 GHz bands.
New non-GSO systems have been designed which would provide a wide range of services to their customers while continuing to provide protection for the GSO through satellite diversity techniques. At WRC-97, measures need to be taken which would permit both types of systems to share the spectrum.
Regulatory situation
The frequency bands 17.7–21.2 and 27.5–31 GHz have been allocated by the International Telecommunication Union (ITU) for the use of systems in the FSS as well as for other space and terrestrial services on a shared basis. Like most bands of the frequency spectrum, the Ka band is shared between various services defined by the ITU. Within the Ka band allocated to the FSS, different sub-bands are shared with different services, leading to constraints in system design and operation.
Sharing between satellite systems requires detailed coordination between neighbouring systems and includes consideration not only of the placement of terminals, but also of the frequencies used by the radio carriers in the various systems which might be affected.
Of the 2.5 GHz downlink bands (17.7–20.2 GHz) planned for use in the INTELSAT system, the bands 18.4–18.6 and 18.8–19.7 GHz are restricted by terrestrial system constraints alone and do not require inter-network coordination with systems of other space services such as broadcasting satellite, mobile satellite and the earth exploration satellite. They are therefore of particular value for the FSS. However, in much of these bands, WRC-95 introduced the possibility of using non-GSO systems on an equal priority with GSO systems. This introduces a new kind of constraint which, in some cases, requires coordination which may be difficult due to the time-varying nature of the non-GSO interference.
Issues at WRC-97
WRC-97 will consider several issues at Ka band which were identified at WRC-95 in Resolutions 118 (Use of the bands 18.8–19.3 GHz and 28.6–29.1 GHz by non-gepstationary fixed satellite service systems) and 120 (Use of the bands 19.3–19.7 GHz and 29.1–29.5 GHz by feeder links for non-GSO MSS networks).
There exists a Radio Regulation (RR), RR 2613 (S22.2), which protects the GSO FSS systems from non-GSO systems. WRC-95 removed the applicability of this regulation in certain bands. ITU studies have shown that the two types of systems could share the same spectrum if appropriate interference mitigation techniques were adopted in designing the systems.
In the absence of such measures, sharing would be difficult and in essence non-GSO FSS systems could preclude the entry of new GSO FSS networks as well as changes to existing networks. As a result, administrations may experience difficulties in using a fundamental international spectrum resource, a difficulty which can be prevented by judicious regulatory measures, as identified above.
WRC-95 considered but did not designate an additional 100 MHz in the uplink and in the downlink for non-GSO FSS in the bands 28.6–28.7 and 18.8–18.9 GHz. Resolution 118 resolved that this matter be considered at a future competent conference. In the intervening two years, little has changed which warrants the extra spectrum and the matter may be postponed until a future competent conference as and when the demand develops.
If the matter of the additional 100 MHz in each direction is in fact raised at the Conference, the relevant question is whether it should be designated for primary use by non-GSO systems or maintained with GSO priority. If the use of the mitigation technique were not made mandatory, then it would be necessary to consider the relative merits of the two types of system.
There are several advantages for the international telecommunications community that GSO systems offer in comparison to non-GSO systems. Among them are tailoring of coverage to land masses, greater spectral efficiency and the availability of the resource to many different operators.
Wide ownership of the resource
The studies done in preparation for the Conference Preparatory Meeting (CPM) show that only a limited number of non-GSO FSS systems could share the same spectrum/orbit resource. However, several GSO FSS systems could share the use of the resource through spatial isolation. Thus, the benefits of the resource could be made available to operators from many different countries rather than a few. Furthermore, studies of the Radiocommunication Sector (ITU–R) on non-GSO FSS networks’ ability to share spectrum amongst themselves have been based on sharing between two or more non-GSO systems with the same network configuration and architecture. Unless a variety of non-GSO FSS systems can share spectrum, the number of non-GSO operators would also be restricted. Such sharing would be difficult if each system is complex and incompatible with the others.
Spectral efficiency
The spectrum is more efficiently utilized by GSO systems than by non-GSO systems. A comparison study by INTELSAT submitted to the CPM showed, based on GSO systems using the system architecture of a non-GSO system as reported to the ITU–R (i.e., a matrix of cells and supercells), that 20 GSO systems, each providing the same worldwide service could co-exist. However, only a very limited number of non-GSO systems of identical architecture could co-exist due to requirements of maintaining a minimum separation between low-Earth orbital planes.
On a throughput basis also, the non-GSO system has an overall capacity of 20.3 (bit/s)/MHz per km2 on the earth’s surface. In comparison, each INTELSAT-type worldwide network would have a throughput capability of 5.339 (bit/s)/MHz per km2. Using this architecture, 43 such systems could co-exist, assuming 3° spacing, and hence the total GSO capacity is 232 (bit/s)/MHz per km2, that is, 11.5 times that of the non-GSO system. Extrapolating from ITU–R studies, which are based on two identical non-GSO systems, this large number of systems would not be able to co-exist and still meet short-term interference criteria.
From the viewpoint of international cooperative use of the common worldwide resource, GSO FSS systems offer many advantages. Spectrum allocations for non-GSO systems must be done in a manner which does not deprive ready access to the GSO in that spectrum. As discussed before, such access could be assured if a designation for non-GSO usage is accompanied by regulatory measures which would cause such systems to avoid main beam to main beam coupling during events when the satellite is between the earth station and the corresponding satellite on the GSO by practising satellite diversity. At least two systems are known to be designed in this manner, namely FSATMULTI-1A and Celestri.
This text is an extract from ITU News 8/97