proposed amendments to draft cpm text on heo systems

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					UK ITU-R SG4 Draft


Document 4A/ … Subject: WRC-03 Agenda Item 1.37

United Kingdom



Rationale for amendments

It is considered that section 3.7.8 of the draft CPM text in document 4A/TEMP/162 – i.e. Attachment 9 to document 4A/136(Rev.1) – requires amendment in two respects, and that it is desirable to shorten the text on HEO systems overall. Section 3.7.8 is headed “HEO FSS, MSS and BSS systems in frequency bands above 3.4 GHz where Article S22.2 applies and epfd limits are given in Article S22”. Other than brevity, the two bases for amendment are as follows: 1.1 Frequency sharing between dissimilar HEO systems

In a contribution to the October 2001 meeting of WP4A (document 4A/317) the UK described simulations which showed that instances of main beam-to-main beam interference coupling would occur between HEO systems with dissimilar orbit characteristics if the longitude spans of the ground-tracks of their active arcs overlapped. It showed also that the ground-tracks of the active arcs of various currently planned HEO systems will overlap in longitude, probably because of a common requirement for coverage of the continental land masses. The study involved computation of the statistics of interference between three dissimilar HEO systems designed include gateway up-links at 18 GHz and user downlinks at 12.5 GHz. The results indicated that, in the absence of techniques to avoid direct coupling of interference between the main beams of satellites in any one of the systems and of earth stations in either of the other systems, the C/I ratios in four out of six reference links would fall substantially below levels corresponding to demodulator loss of synchronisation for small percentages of time. The results included C/I values down to –10 dB for short durations. The most effective techniques for mitigating such short-term effects, often termed „inline‟ interference, are the various forms of satellite diversity, but these might not be practicable for sharing between HEO systems owing to the relatively small numbers of satellites involved. This is likely to increase the importance of other means of mitigation (e.g. link balancing, which would be unlikely to fully solve the problem), and it is considered that the CPM text should draw attention to this potential difficulty of sharing between dissimilar HEO systems.



Doubts regarding the concept of multiple homogeneous HEO systems

Various contributions to WP4A in the recent past have shown that, by appropriate interleaving of the orbital configurations of HEO systems of the same type (i.e. “homogeneous” HEO systems), “in-line” interference could be avoided and thus the number of systems which could potentially share a band is considerably greater than in the case of inhomogeneous HEO systems. (Also, earlier contributions showed this to be true of homogeneous non-GSO systems employing circular orbits.) As might be expected these studies have confirmed that if, in a given pair of frequency bands and covering the same territories, all the HEO systems to be deployed were of the same type, the HEO FSS capacity of the spectrum concerned would be considerably enhanced. Furthermore, since other studies have shown that interference between a typical HEO system and GSO systems would be very small, maximising the HEO capacity in this way could be achieved without significantly constraining the use of the spectrum for GSO FSS networks. However, the enhanced spectral capacity could be realised only if all operators of cofrequency, co-coverage HEO FSS systems used the same configuration of constellation (or very similar configurations). The systems would all have to employ orbit planes of the same eccentricity, the same apogee height and latitude, and the same inclination, and the ascending nodes of the sets of planes would have to be interleaved. Furthermore, the scope for the transmission characteristics to differ between the systems would be small. If even one system should comprise significantly different orbit characteristics, “in-line” interference to and from it and all the other systems would occur, and it would not be practicable for that system to implement satellite diversity with respect to all the other systems. Given the attractiveness of enhancing spectrum capacity it is desirable to answer the question “what steps would need to be taken to ensure that all HEO systems using the same pair of bands would be of the same type ?” Clearly the first step would be a thorough study to identify the optimum system configuration. A key objective of such a study would be to maximise the capacity, taking into account both the per-system capacity and the number of homogeneous systems capable of sharing spectrum, but there would be other criteria. And the baseline characteristics such as the types of service to be provided, the earth station antenna sizes, the link budgets etc. would have to be agreed in advance. For the results of the study to receive the necessary global acceptance it is believed that it would have to be carried out within the ITU-R study groups. There would be a natural tendency for any organisation which had already designed a HEO system to do its best to persuade others that the homogeneity should be achieved by the construction of multiple systems identical to its design (except for the nodes of the orbit planes). Given that the ITU has already received filings for a variety of different HEO system designs, it is likely that agreement on the optimum configuration would be difficult to achieve. If and when such an agreement was reached, for example by the approval of an appropriate ITU-R Recommendation, the next step would be to ensure that all systems

3 would adhere to that design, as it would only take one non-adhering system to confound the strategy. Since adherence would inevitably involve major changes for some, if not most of those HEO system designs which currently exist (but have not yet led to actual construction), it is considered that a WRC decision is the only way to ensure it. If a proposal for such a decision was made to a Conference, and there is reason to doubt that it would be, it is believed that an outcome mandating homogeneity would be very difficult to achieve. Against this background it is proposed that references to multiple HEO system s should not appear in the CPM text for Item 1.37. 2 2.1 Changes to the draft CPM text in section 3.7.8 of Attachment 9 to 4A/136 Sub-section (a) is based on a single study which investigated the ability of up to 10 homogeneous HEO FSS systems to meet the epfd limits. Earlier studies by others have shown that HEO systems in general can meet the epfd limits, at least in Ku-band and the lower Ka-band, primarily because in the active parts of their orbits the satellites are always at large geocentric angles with respect to the GSO. Therefore it is felt that the essential points in this subsection can be made more briefly, without mentioning either “one study” or “up to 10 non-GSO FSS systems deployed uniformly in longitude” as at present. The following replacement text is suggested: Assessment of interference caused by FSS satellite systems using highlyelliptical orbits to GSO FSS networks


In Ku-band and in the lower Ka-band studies have shown that HEO systems having orbital inclinations greater than about 30° are generally able to meet the single-entry epfd limits in RR S22.5C, albeit using satellite EIRP levels somewhat below the levels corresponding to the pfd limits in RR S21 where necessary. However it would be difficult for typical HEO systems to meet the significantly more stringent epfd limits which currently apply in the upper Ka-band (19.7-20.2 GHz). In the case where the orbital inclination of a system is less than about 30°, further study is required on the conditions necessary to make frequency sharing possible.” 2.2 Sub-section (c) “Results of sharing studies” contains two paragraphs. The first describes the potential benefits of multiple HEO systems of standardised design, and for the reasons in 1.2 above it is proposed that this paragraph should be deleted. The second paragraph, which points out that most HEO systems intrinsically protect GSO systems without the use of satellite diversity, is supported by the UK, except that in the last line it is considered preferable to replace the words “virtual geostationary orbit systems” by the words “HEO systems with inclination greater than about 30°”. 2.3 Under the heading “Method 2” in section the first sentence currently reads “In the bands where S22.2 applies it may be necessary …. to quantify the levels of interference acceptable to the GSO.” Since epfd limits were

4 developed to constrain interference from non-GSO systems to acceptable limits, and in view of section of the text which states “The highlyelliptical orbits are taken to be a subset of non-geostationary orbits”, it is proposed that the above sentence should be amended to read “In the bands where S22.2 applies but no epfd limits are given in Article S22 it may be necessary …. to quantify the levels of interference acceptable to the GSO.” Similarly the underlined words should be added to the end of the penultimate sentence of the first paragraph of section 2.4 In the interests of brevity it is proposed that Appendix 2 to Attachment 9 in document 4A/346(Rev.1) should not be included in the CPM text.